Personalize your drink coasters with this excellent themed craft. Your kids can customize them for special occasions like New Year, Christmas, Thanksgiving, Hanukkah, and more.
What you'll need:
How to make it:
- Open the graphics software and choose Avery Kids #03111 Square Stickers as the format.
- Design the coaster to match the celebration that you are having.
- Add photos to the coasters if you would like a personalized coaster for each guest.
- Print out the stickers, and let dry.
- Peel off stickers, and apply them to cardboard.
- Cut around stickers.
- Apply Con-Tact paper on the top and the bottom of the coasters. Be sure to leave enough Con-Tact paper around the coaster to seal it from spills.
- Set out and enjoy the compliments.
Chemistry
All things in the universe consist of a combination of elements in specific orders and following certain "natural laws". Rocks are no exceptions. They consist of arrangements of elements in specific chemical groups, forming neutral charged compounds. To understand rocks, a basic understanding of chemistry is needed. The following is a very generalized and simplified summary of chemistry to understand geologic concepts.
Matter comes in three forms (solids, liquids and gases) that can be broken down into smaller particles known as atoms. An atom is the smallest particle of an element that retain all its chemical properties.
Atoms are composed of a nucleus surrounded by constantly moving electrons. The nucleus is composed of protons and neutrons. The protons have a positive (+) charge. (Elements of different types will have different numbers of protons.) The neutrons have a neutral charge and add "weight" and "stability" to the atomic structure.
The outer portion of the atom contains various levels that are filled by the moving electrons. These electrons have a negative (-) charge. For an atom to have a neutral charged, it must have the same number of electrons in the outer "shells" as there are protons in the nucleus. But the electrons' energy levels may not fit this "configuration". Often atoms loss or gain electrons to fill outer shells and stabilize the energy levels. This produces a charged atom called an ion. In geology, the "magic" configuration is eight electrons in the outer shell and the atoms associated with minerals will often have charges based on this configuration.
There are over 100 known elements, only a few of which are important in introductory geology. This includes the eight most common elements on the earth's crust:
Element | Symbol | Charge | |
---|---|---|---|
Oxygen | O | -2 | (the only negatively charged element in this group) |
Silicon | Si | +4 | |
Aluminum | Al | +3 | |
Iron | Fe | +2, +3 | |
Calcium | Ca | +2 | (Let's consider the positively |
Sodium | Na | +1 | charged elements as "metals" |
Magnesium | Mg | +2 | to simplify further chemistry.) |
Potassium | K | +1 |
Elements are represented by one, two, or three-letter abbreviations. The symbol consists of a capital letter, followed by small letters (when appropriate). The symbol is unique for each element. Symbols are surrounded by four positions which represent important properties of that element to a chemist.
mmm | cccc |
Xxx | |
---|---|
nnnn | aaaa |
The number at:
- --(mmm) represents the atoms mass (protons plus neutrons);
--(nnnn) represents the atomic number (number of protons);
--(aaaa) is the number of that atom present within any given chemical formula; and
--(cccc) is the charge of that atom as an ion (negative symbol must be supplied).
Examples:
C=Carbon
O=Oxygen
CO=Carbon and Oxygen (one of each) and does not equal Co (Cobalt)
Atoms combine with other atoms to form molecules and compounds through various chemical means. This combination, known as bonding, can occur in several ways. Geologist are concerned with only four varieties: Ionic bonds, covalent bonds, metallic bonds and Van der Waals bonds. The resulting compound in geology must have a neutral charge.
In ionic bonds, an atom that loses electrons (positive charge) joins with an atom that gains electrons (negative charge).
Ex.: Sodium Chloride (Geology Name: Halite)
- Na (+1 charge) + Cl (- 1 charge)=NaCl
Atoms that can either gain or lose electrons will often bond covalently. This is where similar atoms share electrons between themselves.
Ex.: Carbon (Geology Names: Graphite and Diamond)
Metallic bonds occur with "pure" metallic elements. The electrons are not confined to a single atom and "roam" freely within the piece of metal. This allows special properties such as magnetism, malleability, and conductivity.
Ex.: Iron, Aluminum, Gold, Silver
Van der Waals bond is a weak bond that occurs in several mineral "families". This bond occurs between neutral areas and acts similar to "static electricity", holding the mineral together until something interferes.
Ex.: Micas and Clays
Chemical Groups
Based on availability and chemical reactivity, there are a limited number of combinations, in geology, that occur when the elements bond together. These combinations can be lumped into groups that behave in chemically similar ways. There are seven common chemical groups in geology.
Elemental Chemical Group
Minerals consisting of pure elements are grouped into a single chemical group. This group can be divided into two areas: metallic and nonmetallic types. Metallic types include gold (Au), silver (Ag), copper (Cu) and other economic ore minerals. Nonmetallics include sulfur (S), and carbon (in the form of graphite and diamonds).
Oxide Chemical Group
Oxide minerals consist of negative oxygen ions bonded to one or more positive "metallic" ions.
Ex.: Aluminum Oxide (Geologic Name: Corundum); Iron Oxides (Geologic Names: Hematite and Magnetite)
Sulfide Chemical Group
Minerals containing negative sulfur ions bonded to one or more positive "metallic" ions are known as sulfides.
Ex.: Iron Sulfide (Geologic Name: Pyrite)
Halide Chemical Group
Halides are a chemical group that include chlorides, bromides and iodides as a base. We will consider them any mineral that contains "salts" or soluble compounds.
Ex.: Sodium Chloride (Geologic Name: Halite) and Potassium Chloride, "salt substitute" (Geologic Name: Sylvite).
Three important chemical groups in geology are built around a "radical". A radical is a group of bonded atoms that act as a single atom in a chemical reaction (think of it as a "click" or "gang"). In geology, these radicals are built around covalently bonded oxygen.
Sulfate Chemical Group
Sulfate minerals contain the radical, SO4, bonded to one or more positive "metallic" ions.
Ex.: Calcium Sulfate (Geologic Name: Gypsum)
Carbonate Chemical Group
Minerals containing the radical, CO3, bonded to one or more positive "metallic" ions are known as carbonates.
Ex.: Calcium Carbonate (Geologic Name: Calcite) and Calcium Magnesium Carbonate (Geologic Name: Dolomite)
Silicate Chemical Group
The most abundant group of minerals on the earth contain the radical, SiO4, bonded to one or more positive "metallic" ions. When bonded together, SiO4 forms a single charged silica tetrahedron. The crystal structure of individual silicate minerals varies because of the ability of the oxygens in the tetrahedron to covalently bond in one or more directions around the silica tetrahedron. Silicate crystal structures range from single tetrahedrons, long chains, double chains or rings, to sheets or complex frameworks. Because there are so many silicate minerals, they are commonly grouped into "families" with similar crystal structures and/or physical properties. For example, sheet silicates include the family of micas (Ex.: Biotite, Muscovite, Chlorite) and clays (Kaolinite). Feldspars, a family that includes orthoclase and the various plagioclases, are one of the most common silicate minerals.
Geology 101 - Gale Martin - Class Notes
The surface of the earth is constantly being modified by two sets of forces. The internal forces of the earth, tectonics, deform and alter rock producing massive mountains. The external forces, denudation, wear away the mountains through weathering and erosion.
These opposing forces create the topography we see: from majestic mountains to rolling hills, flat plains and deep ocean basins.
Gravity and the hydrologic cycle are major contributors to denudation. As water is recycled on the earth's surface, it assists chemical reactions and moves material to new locations. This is accomplished through gravity driven forces. There are many forms of erosional agents which can be observed: mass wastage, fluvial, groundwater, glacial, eolian, lacustrine, littoral processes and others. This course will present a general overview of only the first few.
Erosion occurs when there is sufficient energy to move sediment. Any moving force, wind, waves, streams, etc., can accomplish this. They are all driven by gravity (Ex.: water flowing from high mountains to lower elevations). Deposition of sediment occurs when there is a decrease in available energy.
Erosion can occur in three ways. Sediment can be picked by through the hydraulic action in the moving medium. Image the forces of currents under your feet in a fast moving stream, the pounding waves on a beach, or the push of wind against your back. Sediment is picked up and carried along with the downhill movement. The grains bounce and hit one anther as they shift downward, producing abrasion on the grain's surfaces or along exposed rock outcrops. This "sand blasting effect" loosens more sediment to be picked up and carried along. Lastly, if the rock is chemically soluble, dissolution will occur as rock is dissolved and remove by the water.
Each erosional agent reacts differently with the rocks they traverse. Glaciers carry masses of rock that have been torn from the outcrops through frost wedging and pulverized by continuous abrasion. Streams roll and push sediment in a continuous "conveyor belt" of sediment to the oceans. Groundwater efficiently carve long networks of caves along fissures in limestone. Each environment will produce unique characteristics that, when preserved in the rock as sedimentary structures, will tell the tales of how that rock formed.
Mass Wastage or Mass Movement
When rock and loose sediment move down slope due to gravity, the process is referred to as mass wastage or mass movement. This process can be sudden and catastrophic in nature or slow and gradual over years time. Many factors control whether sediment or rock remains stable or shifts by mass movement. These include the gradient, water, amount of vegetation and rock characteristics. (It's all a matter of physics. Which is greater: gravity forces? or the cohesive nature of the rock outcrop?)
One of the single most important factors in the stability of a rock outcrop is the gradient of the outcrop. Loose rock material cannot form vertical cliffs. The rock slides down into a conical slope no steeper than the angle of repose (approximately 30). As rock is forced upward into mountains by tectonics, the loose material slides off the ridges into talus slopes.
Several characteristics of the rock itself influence its cohesive nature. Rocks that are well consolidated or crystalline can form steep outcrops that are stable in nature. These include many igneous rocks, nonfoliated metamorphics and silica cemented sedimentary rocks. (Ask any climber, they can tell you.) Rock layers that are oriented opposite the slope direction, i.e. dip into the hill, are more stable than rocks whose beds or foliation planes slope in the same direction as the hill side. Finally the shear weight of the rock or thickness of the bed can be a deciding factor in control of mass wastage.
Water can be important. Though a small amount of water actually increases the cohesiveness of rock material, excessive water allows slope failure. Water reduces friction and acts as a lubricate between grain surfaces in a rock mass. It results in a buoyancy factor, lifting and supporting individual grains. The addition of water to an outcrop also contributes an added weight factor, causing previously stable slopes to fail and move.
Vegetation when present, slows mass wasting by holding and stabilizing the ground with the massive root structures. Fires and land development in steep terrain often lead to unstable slopes that slide with the slightest incentive.
Sometimes rocks can remain in an unbalanced region for long periods of time and only move when another event triggers greater instability. Such triggers include earthquakes, heavy rains, volcanic eruptions, seasonal fires or careless land development.
Classification
Though most mass movement events reported in the news are quick catastrophic events, many slower and less traumatic forms of mass wastage are possible. Classification of mass movement is commonly based on the consolidation of the rock material, the amount of water present and the rate at which the movement occurs. The classifications are divided into three general categories: flows, slides and falls.
Flows are characterized by a mass of loose sediment or rock that acts as a unit but contains individual particle motion. Water is a common factor and acts as the medium by which the grains move. Usually the more saturated the flow material is the more rapidly the mass moves. Slow forms include creep. Here the sediment or soil along the surface slowly moves downhill as individual grains are lifted during a freeze and inch downhill with the following thaw. Creep acts at rates which are not visible to an untrained eye. It can be perceived by curved tree trunks, tilted fence posts and bulking of roads in areas prone to creep. Solifluction, also slow in nature, occurs in areas of permafrost: a thin veneer of saturated soil slipping along a frozen surface in high tundra regions. Faster forms of flows include earthflows, debris flows and mudflows. They may result from a heavy rain that produces failure in an accumulation of loose debris on steeper slopes. These flows commonly act as a thick viscous fluid that moves downhill along a previous low lying area. The resulting landforms are hummocky in nature and may appear as "tongues" which extend from mountain valleys into flat lying reaches.
Slides are mass movements that occur as a single consolidated mass along a plane of weakness. They consist of two general styles of movement based on the shape of the failed surface. A slump is a mass of rock that fails along a curved surface. When the rock mass moves it commonly rotates (like a person slipping on a banana peel and falling on their backside). In areas prone to slumps it is common to find curved scrapes along the upper portion of the slump zone. Slides, such as landslides and rock slides, fail along a flat surface (Ex.: a bedding plane). Water may seep into the crack and lubricate the surface to act as a "slide" for the rock to slip across. Trees and plants on the surface of a slide can remain intact during the event and continue to grow after the movement.
A fall is individual rocks that tumble down a steep slope or cliff. The rock bounces and plummets down the slope at great speeds usually free falling through the air. They accumulate at the base of a cliff as talus piles.
The above classification is a generalization. There are many forms of mass movement that can be considered a mixture of all three types. (For example: an avalanche: falling off the mountain ridge, slipping down the slope and cascading over a ridge!) Anywhere there are slopes, either steep or very low, gravity comes into play and mass wastage occurs.
Geology 101 - Gale Martin - Class Notes
Elastic Rebound Theory
Rocks along faults rarely glide along in a smooth, frictionless fashion. Movement along a fault is sporadic and unpredictable. Behavior along the fault is modeled using the Elastic Rebound Theory. The faults tend to 'lock-up' and resist motion. Stress being applied stretches, twists and distorts the rock resulting in a build up of energy between the grinding rocks. When the fault finally slips, energy is released when rock bounces back to its original shape. As the Earth is jerked and jarred by the shifting fault, vibrations radiate from the break producing an earthquake.
The point along the fault where the movement occurs is the focus of the earthquake. Directly above this is the area of most intense damage along the surface. This area is known as the epicenter. The destruction that occurs on the surface is dependent on several factors. This includes the type of rock (bedrock versus valley fill), the depth of the movement and the type of building structures that are present on the land surface.
When a movement occurs, only a small portion of the fault shifts. This does not 'relieve' the built up energy along the entire fault plane. Regions where the rock remains 'locked' may develop even more stress. The movement often propagates beyond the initial break as energy is released in other areas along the fault plane. These smaller shifts are referred to as aftershocks. (A similar even might be an accident on the expressway -- traffic gets tied up behind it and more accidents occur as drivers 'jockey' for the best way out.)
Seismic Waves
The vibrations produced by the energy release are known as seismic waves. The waves radiate from the focus in all directions, passing through the Earth's interior and along the crust. Several different waves are produced. The behavior of each wave is based on the velocity and the way the wave propagates through the rock.
Body Waves
Body waves can travel through the interior of the Earth; bouncing, bending and reflecting around for long periods of time (hours, even!). They consist of two general types: P- and S-waves. P-waves, or primary waves, are the fastest seismic waves. They can travel at several kilometers per second but the speed will vary depending on the rock type and depth (pressure). As a P-wave moves through rock, it compresses and expands material parallel to the wave motion. This is known as a compressional, or longitudinal, style of wave motion (ex.: a 'slinky'). S-waves, known as shear waves or secondary waves, travel at about half the speed of a P-wave. Rock affected by an S-wave will shift back-and-forth, or in a transverse style, as the wave travels through (ex.: a 'cracked' rope or sidewinder snake). S-waves can only travel through solid material. When they enter a region that is 'liquid', the S-wave will cancel out.
Surface Waves
Several types of waves travel only along the outer crust of the Earth. These waves are referred to as surface waves. The motion of surface waves are complex. For example, one of the surface waves travels similar to ocean waves, rolling in an up and down motion. (Hence people often report a 'rolling' feeling during an earthquake.) Surface waves are much slower in speed and larger in amplitude (size). The energy released by a surface wave quickly dissipates and the motion is felt in a limited region around the epicenter.
Measuring Earthquakes
There are commonly two ways to report the size of an earthquake. The Mercalli Scale is used to determine the intensity of an earthquake (i.e., how it was felt and the amount of damage). The Mercalli Scale is a subjective scale set up to record the effects of the earthquake in the region around the epicenter. A survey is performed and a regional map created with areas labeled using 12 divisions (I - XII). The levels are descriptive in nature; ranging from effects not being felt, to objects falling from cupboards, to the highest being total devastation. (See your text for the division descriptions.)
To compare the size of an earthquake with other global readings, the Richter or Magnitude Scale is used. This type of scale reports the magnitude, or amount of energy released, during an earthquake. A network of instruments, known as seismographs, are set up to record the presence of seismic activity. (The instrument is firmly attached to 'bedrock'. When the ground moves, a spring (or small arm) remains quietly suspended. The movement is recorded as a graph known as a seismogram.) The magnitude scales report the size on a logarithmic scale from 1 to 10. (Each number on the scale is 10x the magnitude of the number before it. The amount of energy can be shown, through mathematical calculations, to be over 30x greater between numbers.) Earthquakes of magnitudes less that 4.0 are common events in regions prone to movement; magnitudes of 6.0 or greater usually cause serious destruction to a region.
Epicenter Location
Based on arrival times between P- and S-waves, seismic readings are used to locate an earthquake's epicenter. Think of it as a race between a sports car (P-waves), a slow sedan (S-waves) and an old junker (surface waves). When the race begins, all the vehicles (seismic waves) start at the same point. A few miles from the starting line, the difference in speed results in gaps between the cars. As the race continues, the old junker's engine dies out (energy loss in surface waves) and the faster sports car continues to gain distance on the slower sedan. The further the seismograph is from the epicenter (race start), the larger the gap in time between the P- and S-wave arrival (finishing times between the remaining cars).
Though a single seismogram can give the distance to an earthquake's epicenter, it cannot provide the direction in which it occurred. Several seismograms for an individual event are plotted on a single map. Each seismograph is plotted at the center of a circle (i.e., multiple unknown directions). The radius represents the distance from the seismograph to the earthquake. The area where the circles intersect is the location of the earthquake. A minimum of three widely spaced seismographs are needed to 'triangulate' the location.
The Earth's Interior
Seismic activity can be used to interpret the configuration of the Earth's interior. The velocity of the body waves varies with different rock types and densities. As the waves penetrate the Earth, changes in rock layers are recorded by changes in body wave velocities and directions. (See your textbook for wave property explanations.) Several 'layers' have been identified in the Earth's interior, including the crust, the upper and lower mantle and the outer and inner cores. This class will cover a very simple model of the outer most layers of the Earth.
The crust consists of two distinctive types: oceanic and continental. The two crusts are firmly attached to one another but do not overlap. Oceanic crust is thin (10 kilometers or less) and consists mostly of fine grained basalt. The high density of basalt means the oceanic crust tends to 'sink' and form 'low lying' regions on the Earth's surface. Water often floods the low lying regions creating oceans. Continental crust is thicker (20 to 40 kilometers, or more) and is composed of silica-rich rocks, such as granite and shale. The 'granitic' continental crust tends to 'float' on the surface with deep 'roots' below large mountain ranges (similar to ice cubes in a glass of water).
The upper mantle lies beneath the crust and can be divided into several layers. Each layer is determined by a change in rock type, density or other characteristics (ex.: plastic nature) that result in shifts in seismic wave velocity. Directly below the crust is a rigid portion of the upper mantle approximately 70 km in thickness. The thickness varies, especially near mid-ocean ridges, but it occurs even under extensive mountain roots. The rigid layer is very brittle and is firmly attached to the overlying crust. This unit (crust and rigid upper mantle) is known as the lithosphere. Below the lithosphere is a region known as the asthenosphere. This portion of the upper mantle is partially molten and acts as the source of magma for igneous activity. The region is plastic in nature; 'giving' and shifting, accommodating mountain roots and filling in regions near thinner lithospheric areas. Deep seated earthquake foci occur to a depth of approximately 700 km, usually considered to be the lower limit of the asthenosphere.
This configuration of the Earth's interior is the basis for the theory of plate tectonics. The movement of broken fragments of lithosphere along the flexible asthenosphere is used to explain the complex geology that occurs in long linear features along the Earth's crust.
Geology 101 - Gale Martin - Class Notes
Fluvial Systems, or rivers, are by far the most abundant and important erosional agent on the surface of the earth. The information covered here is but a small portion of that needed to understand the dynamics of fluvial systems. One can approach rivers as components of evolving landscapes, means of sediment transport/deposition and agents of destruction (via floods). They are constantly changing. Rivers are one of the few agents of erosion that man can actually notice the forces at work during his or her life span. Yet few people are aware of the longer, geologic life spans of rivers and how man interferes (or at least tries to interfere!) with the shape of this "living", dynamic force.
Fluvial systems are produced by the collection of surface runoff in topographic lows. The water is under the force of gravity; draining from higher elevations where precipitation has occurred (rain, snow, sleet, etc.) toward lower elevations where it collects in BIG puddles (lakes, oceans), etc.). These "lows" are referred to as channels when they become modified by the erosive nature of the flowing water. The moving water has many names (rivers, streams, rills, rios, creeks, etc.) but is known to a geologist as a fluvial system. (Let's use the terms "river" or "stream" for this class.)
The head of a river consists of a network of smaller streams, tributaries, the at drain a large area. The collection area is referred to as its drainage basin and consists of all the area between any topographic "highs", or drainage divides, that surround and separate it from other river systems. As it drains the land it collects water and sediment; carving and modifying the surface. When it reaches the ocean, it releases the water through a system of distributaries, which branch out and form the river's mouth.
The water in a river is seeking the lowest possible level to which it can flow (base level). As it winds over its course, a river flows for many miles and attempts to reach an equilibrium with the surrounding environment (graded stream). Many changes occur: water is contributed through precipitation and tributaries, rock outcrops get in it's way, and low lying basins temporarily slow it down. But every river eventually flows to the lowest reaches on the surface of the earth: the oceans, or ultimate base level.
Factors that Influence Erosion and Deposition
As the water flows over miles of rock and soil it greatly alters the shape of the surrounding land. Moving water has energy and is able to do work. (Mankind will often use it to drive turbines that generate hydroelectric power.) This energy is used by the river to move sediment. The faster the river flows - the more erosional its nature. Rivers can erode or deposited tons of sediment along it's course depending on several factors.
The slope of a river channel greatly affects it's flow. A stream's gradient, or slope, is a measure of the drop in elevation that a river has over the distance that it travels. Rivers in mountainous regions usually have high gradients and fast flows. Sediment in these regions is quickly removed and transported to areas of lower gradients, such as flat lying basins.
The stream's velocity is a measurement of rate at which the water flows. Though rivers with steep gradient have high velocities, the velocity in any given river varies depending on the river path, the shape of the channel and roughness of the rock that forms the river bed. Friction in a river channel slows the flow and causes irregular patterns or turbulent flow. In a meander, the velocity of the stream will be concentrated on the outside of the curve, where the water is deflected by the bank. Any increase in velocity within a stream will result in erosion.
The volume of water that a stream carries can greatly influence its erosional behavior. For most rivers, the volume of water carried will increase from it's head to the river's mouth. This is due to the number of tributaries along its course. The discharge of a river, or the volume of the stream at a given time, is affected by the amount of precipitation that occurs. During spring rains/winter melt, a river must carry a greater volume of water downstream in a short span of time. This increase in discharge results in flooding, or overbank discharge. Within an individual river system, an increase in discharge results in erosion.
The stream load also is a factor that determines if erosion occurs. The stream has a limit to the amount of sediment it can carry (capacity) and the size of particles that it can move (competence). These limits are dependent on stream discharge and velocity. Generally speaking, the greater the flow of a stream, the higher the competence and the greater the capacity. If an "outside force" disturbs the river's equilibrium, erosion or deposition will occur. (Mankind has a tendency to do this quite often. Building a dam to block a river results in changes that induce deposition behind the dam and erosion at the spill gates.)
Erosion/Transportation/Deposition
Erosion in a stream channel results in extension of the river system throughout its drainage basin. A river can erode channels by lifting and carrying sediment, abrading the river banks or dissolving any soluble rock (see Denudation). Erosion of the stream bed resulting in downcutting and deepening of the channel. This steepens the sides and results in unstable slopes. Mass movement of debris into the channel brings the slopes back to the angle of repose and produces a "V" shaped valley configuration. Along meandering streams, the outside curves, or cutbanks, become undercut by the force of the stream's velocity and lateral erosion occurs. The valley widens as the meander erodes and extends itself. A stream increases its length by headward erosion on the uphill end of its tributaries. Here the valley grows longer with additions of small gullies and rills which mark the outer reaches of its drainage basin.
Not all sediment sizes are eroded at the same flow or stream velocity. Obviously, heavier particles require greater velocities. But clay size particles are also harder to erode. Clay particles are flat and closer to the stream bed and may be "sticky" in character. Higher energy is needed to pick up a clay grain than those needed for silts and sands.
Once a grain has been eroded, a stream can carry it in a variety of ways. The bed load consists of larger sediment grains. Most gravels and pebbles are too heavy to be carried all the time. They move down stream by traction, i.e., rolling and sliding along the bed. Sand particles typically move through a process called saltation. They bounce and leap along the bottom, ricocheting and hitting other grains on the river bed. The suspended load usually consists of clays and silt size particles. These grains are light enough to move in the water column. Rivers which flow through regions containing weathered silicates are typically "muddy" looking due to the suspended clays in the water. Most rivers also carry a dissolved load. This consists of the ions picked up during chemical weathering (dissolution) of rocks in the drainage basin. Rivers that flow through carbonate dominated terrain (Florida, Bahamas, Aruba, etc.) contain only dissolved loads and appear clean and clear.
Deposition of sediment occurs whenever the river looses enough energy that it can no longer carry all of it's load. Any decrease in stream flow will result in deposition. Examples include: --where a mountain stream enters a flat basin area the drop in gradient results in a decreased stream flow; --a sudden decrease in velocity occurs when a stream enters the ocean or any standing body of water; and --sediment is deposited as a river's discharge drops with receding floodwaters. Heavier materials, silts and sands, are the first to be deposited. Clays can remain suspended for long periods of time even after the stream's velocity has stopped.
Common Features
(See your text for pictures/figures. Figures remain the best method of landforms affected by rivers.) Rivers produce different erosional and depositional features based on climate and tectonics of the region. Let's generalize and group them into similar styles.
Mountain streams are usually erosional by nature due to high gradients and fast flows. Sediment is quickly removed and carried downstream. Waterfalls and steep V-shaped valleys predominate. Resistant layers result in differential erosion forming ridges and overhangs that water cascades over. The pounding falls (hydraulic action) results in undercutting at it's base. The overhang eventually collapses and a steep set of rapids remains. Potholes are commonly formed as rocks swirl and grind against the bare bedrock in the stream.
As a stream leaves steep mountainous regions a drastic drop in gradient results in deposition of sediment. In arid environments, alluvial fans are produced at the base of the mountain as streams shift to avoid the accumulating pile of sediment. Given sufficient time neighboring fans coalesce to form bajadas. The streams that exit the mountains commonly become intermittent in nature and often percolate into the porous gravels. The dissolved load may even be precipitated along the base of the fan complex. Enclosed basins may contain playa lakes; dry and alkaline during most of the year, they fill during flash floods and rain storms.
In regions were water is more abundant, a stream that exits mountainous regions must still contend with excess sediment loads. The coarse material is deposited in sand bars that quickly fill the channel. The stream spreads out and produces a flat broad valley with shifting bars and diverting streams. This braided stream complex is common around mountain ranges with plentiful glacial melt or high spring run off.
After a river settles into a main channel complex it typically begins to meander and produce features common to flood plains. The discharge of a river varies greatly between seasons and over the years. It typically overflows its banks and deposits sediment along its sides to produce a flat broad area called it's flood plain. As the river overflows a large pile of coarser sediment, known as a levee, accumulates next to the main channel. The levee increases the depth of the channel and can alleviate minor floods. If the flood plain remains flooded for major portions of the year, vegetation suitable to watery environments begins to grow and a swamp can develop.
The main channel of a river is not a permanent feature. Sand bars are commonly deposited throughout the channel and the stream's flow shifts within the flood plain through erosion of cutbanks and deposition of point bars. Through time meanders become elongate and exaggerated. With an increase in stream flow a meander can develop a cutoff. This shortens the route the river must take to traverse its course. Sand bars eventually isolate the meander and produce an oxbow lake. As sediment fills the oxbow, a meander scar develops. (Flood plains, produced by lateral erosion and deposition (due to flooding) are broad, flat fertile areas which have attracted farmers for thousands of years. It's still a part of a dynamic system -- flooding is part of a river's cycle. Mankind can't stop it.)
Whenever a river enters a standing body of water, be it a lake or an ocean, the velocity of the water drops quickly. Deposition of sediment blocks the river's mouth and it divides into a complex set of distributaries. A large wedge of sediment called a delta, eventually forms at the river mouth. The delta's shape is determined by many factors. The sediment input from the river is often reworked by tidal forces, waves and longshore currents. The delta can be stabilized by growth of vegetation in the form of swamps and marshes.
The appearance of a river and the ensuing landforms depend on numerous complex and interacting factors. Tectonics (uplift or mountain building events) and climate changes can readily alter the pattern of erosion/deposition and modify it's drainage pattern. Most of the landforms and surface topography of the world are developed by the dynamic nature of running water both from the present and the recent geologic past.
abrasion - physical wearing away of rock by grinding and "sanding" of the surface through impacts with other rocks.
absolute age - giving the age of a rock formation in years; commonly based on radiometric ages.
acidic - igneous composition that is high in silica and rich in potassium, aluminum and sodium.
aftershock - an smaller earthquake that lies on the same fault as a previous larger event.
alluvial fan - a fan shaped wedge of sediment at the base of mountains in drier climate regions; produced when streams flow from high gradient areas to low gradient basins.
amphibolite - a nonfoliated metamorphic rock with abundant amphiboles (and sometimes garnets).
andesite - an aphanetic, intermediate igneous rock.
andesitic - igneous composition that is intermediate between felsic/acidic and mafic/basic.
angle of repose -the maximum slope at which loose sediment remains still.
anticline - a linear fold with older rock formations within the core. The limbs dip away from the central axis.
aphanetic - igneous texture that has microscopic crystals.
aquiclude - a rock formation that has insufficient flow for groundwater use.
aquifer - a rock formation that has sufficient flow for groundwater use.
artesian - groundwater system under sufficient pressure to flow above the surface of the well. Usually requires a confined aquifer system.
ash - glass shards and igneous rock fragments formed as lava explodes into the air. Measured by size (see text).
assimilation - changing of magma by melting of country rock into a magma chamber.
asthenosphere - the plastic layer of the upper mantle which lies beneath the lithosphere. The layer occur to about 700 km in depth and is the source of magma.
aureole - the bands of altered rock around igneous intrusions produced by increased temperatures in the rock.
axis - a line around which the limbs bend around the center of a fold.
bajada - area of multiple alluvial fans that have overlapped forming a long "apron" along the mountain front.
bank - the sides of a stream channel or lake edge.
basalt - an aphanetic, mafic igneous rock.
basaltic - a silicate magma or lava that has a basic or mafic composition.
base level - the lowest level to which a fluvial system can erode.
basic - igneous composition that is low in silica and rich in magnesium, iron and calcium.
basin - a circular or elliptical fold with younger rock formations in the center and all sides tilted toward the center region.
batholith - intrusive igneous rock body that is over 100 kilometers square when exposed. A solidified magma chamber.
bed load - sediment that is rolled and bounced along the bottom of a stream; usually gravels and sands that are too heavy to carry in suspension.
bedding - layers in sedimentary rock representing a single depositional event.
Benioff Zone - a set of seismic activity that ranges from shallow near an oceanic trench dipping into the Earth's interior under either a volcanic island arc or continental volcanic arc. It is believed to represent the movement of a descending oceanic slab.
biochemical - sedimentary rock texture consisting of fossil fragments lithified into rock. Also known as bioclastic.
bioclastic - See biochemical.
body wave - a seismic wave that is capable of moving through the Earth's interior.
Bowen's Reaction Series - the order of mineral crystallization from an igneous melt based on temperatures of crystallization. There are two series: discontinuous (mafic minerals) and continuous (plagioclase feldspars).
braided stream - a stream which flows in several shallow channels that switch back and forth due to the large amount of sediment in the system.
breccia - sedimentary rock with clastic texture of angular gains larger than 2 mm in size, usually poorly sorted in character.
brittle - a response to stress that results in the object breaking or fracturing.
calcareous - modifier for clastic sedimentary rocks that contain calcite cements.
caldera - a deep depression where a volcano use to be present; created either through collapse of the volcano into an empty magma chamber or a violent explosion due to magmatic gasese.
caliche - hardpan; a deposit of carbonates or soluble minerals in deserts that precipitates when water evaporates from surface sediment.
capacity - the volume of sediment that a flowing current (air or water) can carry.
capillary action - water in soils that are held as a film around grains by the cohesive nature of the liquid.
carbonate chemical group - minerals containing the radical "CO3" bonded to one or more positive metallic ions.
cave - a natural opening beneath the Earth's surface produced by dissolution of a soluble rock.
cavern - an extensive network of natural openings beneath the Earth's surface produced by dissolution of soluble rock.
cementation - growth of minerals, usually silica or calcite, between sediment grains to form a solid rock.
Cenozoic - (kainos - new, recent; zoe - life) The most recent era of the Phanerozoic Eon ranging from approximately 66 mybp to present; "Age of Mammals".
central eruption - extrusive igneous activity that is associated with a small, opening in the earth's crust.
channel (main channel) - a low lying region through which the main portion of surface run-off flows.
chemical precipitate - sedimentary rock texture produced by the growth of crystals (usually microscopic) from a solution.
chemical weathering - weathering of rock through chemical means that alters the original minerals into minerals stable on the earth's surface.
chert - a) a mineral composed of cryptocrystalline quartz, usually gray in color; b) a chemical precipitate composed of various forms of cryptocrystalline quartz including chert, flint and jasper.
cinder - a fragment of igneous rock that is vesicular in texture.
cinder cone - a central eruption of cinders, usually basaltic in composition. Commonly produced when lava reacts with a source of underground water.
clastic - sedimentary rock texture formed from the accumulation of sediments produced by weathering of previous rocks. Also called detrital.
clay - a) sedimentary grain size of less than 1/256 mm. b) a mineral composed of a aluminum silicates bonded to water in a sheet-like crystalline structure, usually formed by the hydrolysis of feldspars.
cleavage - the ability of a mineral to break along a smooth surface.
coal - a bioclastic sedimentary rock composed of compressed plant fragments. Classified by grades of compaction; lignite (low grade) - bituminous (medium grade) - anthracite (high grade).
columnar joints - cracks in lava that are perpendicular to the surface, extending down into the flow. They are created by the shrinking of the lava as it cools.
compaction - reduction of space between grains due to overlying weight of sediment.
competence - the largest size of sediment that a flowing current can carry.
composite cone - stratovolcano.
composition - the types of minerals present in a rock.
compressional stress - forces that push objects together. The object is typically shortened in length.
concordant - lying between rock layers.
cone of depression - a region where the water table has dropped due to the withdrawal of water from a well; typically conical in shape around the well casing.
confining bed - a rock formation that in impermeable in nature, thereby restricting water movement.
confining pressure - pressures developed due to deep burial; the pressure is applied in all directions and does not distort the general shape of the rock.
conglomerate - sedimentary rock with clastic texture of rounded grains larger than 2 mm in size, commonly poorly sorted.
consolidation -- any process that causes loose rock material to become solid; includes lithification of sediment and crystallization of minerals (ex.: magma).
contact metamorphism - alteration of country rock created by the increase in temperatures are igneous intrusions.
continental accretion - the idea the continents grow by the addition of material onto an original nucleus.
continental crust - the crust that exists within the continents and is 20-40 km in depth with deeper roots below extensive mountain regions. It is of a low density (2.6-2.7 g/cc) and felsic in composition.
continental drift - a hypothesis summarized by Alfred Wegener using paleontological, climatological and geological evidence that assumed that the continents were once one large continent that later drifted apart.
convection cell - the movement of material through the mantle; driven by the density differences between hot and cold material.
Convergent Plate Boundary - Plate Tectonic Theory: areas on the Earth's surface where plates are being pushed together.
correlation - determining the age relationships of rock over large areas.
country rock - the general term used to describe any rock surrounding a magmatic intrusion.
craton - stable part of a continent's interior.
creep - slow downhill movement of loose material by individual grains that are lifted during a freeze and dropped a few millimeters downhill with a thaw.
crossbedding - inclined orientations within beds produced as sediment is moved by currents; sediment slides down slip faces on front of moving sand "pile".
cross-cutting relationships - processes that alter/deform rock must be younger than the rock it deforms or cuts.
cross-section - a diagram showing the view of the Earth from the side; an exposure of the interior of the Earth by a road-cut or outcrop.
crust - the outer most layer of the earth. Consists of oceanic or continental varieties.
cryptocrystalline - microscopic crystals.
crystal - a solid with a regular repeated patterns of elements bonded together.
crystal fractionation - the settling of crystal in a magma chamber resulting in magmatic differentiation.
crystal habit - the shape in which a mineral grows. It is often based on the crystal structure and the environment in which the mineral grows.
crystal structure (or crystal form)- the geometric shape of a crystal. There are limited shapes in geology including: cubic, tetrahedrons, hexagonal, and rhombic.
cutbank - the side of a channel where erosion is greatest. It is commonly located on the outside of a meander resulting in the deflection of a stream's velocity.
cutoff - a shorter channel produced when a stream abandons a long meander loop.
decomposition - another name for chemical weathering.
deformation - change in rock due to application of tectonic forces.
degrees - a form of angular measurement. A full circle is 360°; North is 0° ; the degrees increase in a clockwise direction.
delta - a wedge shaped accumulation of sediment at the mouth of a stream; produced when a stream enters a standing body of water, looses energy and deposits it's load.
denudation - All the processes that flatten the surface of the earth through erosional means.
deposition - the accumulation of sediment when the erosional agent loses energy. depositional environment - the type of area in which sediment has accumulated. Ex.: river, ocean, shoreline, etc.
detrital - see clastic.
dike - intrusive igneous rock body that is tabular and discordant in nature.
diorite - a phaneritic, intermediate igneous rock.
dip - the angle at which a rock bed is tilted.
directed pressure - pressures developed during tectonic/orogenic events; the pressure is usually concentrated in opposing directions and cause foliation of in rocks.
discharge - volume of water that flows past a given point in a specific time period. Used for flow from an aquifer system and in fluvial run-off.
discordant - cutting across rock layers.disintegration - another name for mechanical weathering.
dissolution - chemical weathering process which breaks the mineral into the original ions that remain in solution.
dissolved load - sediment that is carried as ions in solution.
distributary - a branch of the river that empties water out of the system.
Divergent Plate Boundary - Plate Tectonic Theory: areas on the Earth's surface where plates are being pulled apart.
dolomite - a) a mineral consisting of magnesium, calcium carbonate; b) a sedimentary rock composed of the mineral dolomite.
dome - a circular or elliptical fold with older rock formations in the center and all sides tilted away from the center region.
drainage basin - the area that collects all the water drained by a stream and its smaller branchs.
drainage divide - the high area located between drainage basins.
earthquakes - vibrations on the surface of the earth usually caused by movement along a fault.
effluent - a stream that is feed by ground water discharge.
Elastic Rebound Theory - the idea that movement along a fault releases built up energy causing vibrations in the Earth.
elastic strain - deformation that is not permanent. The object returns to it's original shape.
elemental chemical group - minerals consisting of pure elements.
eolian (aolian) - wind driven erosion or deposition.
eon - the largest division of time in the geologic time scale.
epicenter - the region above an earthquake focus where the greatest destruction occurs.
era - division of time for the geologic time scale (usually applied to the Phanerozoic era), each including several smaller divisions (periods).
erosion - the movement of material to a new site.
evaporite - precipitation of minerals due to the evaporation of water.
extrusive - igneous activity that occurs on the earth's surface.
facies- Characteristics of a rock which describe how it was formed.
fall - fastest form of mass wastage; produced by rocks cascading down a steep slope, usually due to free fall.
fault - fractures in rock along which movement has occurred.
fault plane - the surface of a fault along which movement occurs.
fault scrap - a slope or cliff produced by movement along a fault.
faunal succession - sequence of development in fossils over time; it is results in a predictable pattern in rocks over large areas.
felsic - igneous composition that is high in silica and rich in aluminum, potassium and sodium.
fissile - very thinly bedded in nature, usually compacted clays.
fissure eruption - extrusive igneous activity that occurs along a long crack in the earth's crust; usually basaltic in composition.
flood plain - the regions beside a stream system that are covered by water overflowing it's bank.
flooding - overbank discharge of a stream.
flow - a form of mass wastage where the loose material moves as a single mass of viscous liquid as the individual grains move separately.
flowstone - carbonate growth in caves due to precipitation of crystals as water flows, drips or drizzles along a cave surface.
fluvial - refers to river and stream systems.
focus - the area along a fault where movement produces an earthquake.
fold - bends in rock.
foliation/foliated - metamorphic rock texture where minerals are aligned in a single plane; commonly developed in directed pressure environments.
footwall - the piece of rock located beneath the fault plane.
formation - a layer(s) of rock with characteristics that make it easy to distinguish from adjacent rock layers.
fossil - evidence of previous life forms, including original parts, casts, molds, and impressions.
fossil assemblage - a group of fossils that represent a single environment of deposition.
fossiliferous - modifier for sedimentary rocks that are abundant in fossils.
fracture - an irregular break in a mineral.
frost wedging - mechanical weathering of rock caused by the increase in volume of water during freezing.
frosting - etching of grains due to abrasion.
gabbro - a phaneritic, mafic igneous rock.
geologic time scale - an arrangement of geologic events and rock ages based by fossils in stratigraphic units. See text.
geothermal gradient - the increase of temperature with depth in the earth's interior. This varies with many factors but is estimated to be approximately 25°C per kilometer.
geyser - a hot spring system that erupts due to pressure produced when water turns into steam beneath the surface.
glacier - a mass of ice, deformed under pressure, that is heavy enough to move under it's own weight.
glassy - igneous texture where no crystals have developed.
gneiss - foliated metamorphic rock with gneissic texture; usually high grade.
gneissic - foliated metamorphic rock texture with visible minerals that are separated into distinctive bands of felsic versus mafic minerals.
graben - a down dropped block of rock between two normal faults.
graded bedding - within a single bed: a fining upward sequence of grains by size or density.
graded stream - a stream whose gradient is in equilibrium with the dynamics of the region; i.e. the stream flow is sufficient to carry any sediment supplied.
gradient - the slope of a stream or land surface; based on the drop in elevation over the length measured for the system (ft/mile or m/km).
granite - a phaneritic, felsic igneous rock.
granite pegmatite - a pegmatitic, felsic igneous rock.
granitic - silicate magma that in acidic or felsic in composition.
gravity fault - a fault where the hanging wall drops with respect to the footwall.
greenstone - a nonfoliated metamorphic rock that contains fine grained green mafic minerals, including chlorite.
groundmass - in a porphyritic rock: the fine(r) grained crystals; due to the second, quicker stage of cooling.
groundwater - subsurface water that has collected in the pores of rock and flows beneath the surface to the oceans.
gully - a small depression in the land surface, "so deep that it cannot be crossed by a wheeled vehicle..." (A.G.I. Dictionary of Geological Terms) where surface run-off collects.
half-life - the amount of time is takes for one half of a sample of radioactive isotope to decay to the final daughter product. This occurs at a constant rate unique to each isotope.
halide chemical group - a chemical group that include chlorides, bromides, iodides as a basic building block.
hanging wall - the piece of rock located above the fault plane.
hardness - the ability of a mineral to resist being scratched or abraded.
hardpan - see caliche
head - pressure of a fluid due to height/depth.
headward erosion - the extension of a stream system at the upper reaches of a tributary by rills and gullies.
hematitic - a modifier for clastic sedimentary rocks that are red in coloration; they contain hematite.
hornfels - a fine grained, nonfoliated metamorphic rock containing microscopic mafic minerals.
horst - an 'uplifted' block of rock between normal faults.
hot spring - groundwater that is above normal temperature.
hummocky topography - an irregular, rolling surface commonly due to mass movement.
hydraulic action - the physical force of a flowing medium (such as flowing water) that can loosen and remove rock.
hydrologic cycle - the movement of water on the earth's surface in a continuous cycle: progressing from the ocean to the atmosphere, the land surface and back.
hydrolysis -- (not a definition) chemical weathering processes that produces new minerals that incorporate water into the new crystals.
hydrothermal metamorphism - metamorphic alteration due to hot fluids, especially water, that may escape from magma chambers; usually resulting in precipitation of ores and metallic deposits.
igneous petrology - the study of igneous rock and the processes necessary for their formation.
igneous rock - rocks that are formed from the solidification of molten material either above or below the surface.
immiscibility - when referring in magma: the inability of different compositions in the magma to mix well.
index fossil - a fossil of wide distribution but short duration of time; used to date a strata that contains it.
index mineral - metamorphic minerals that represent specific ranges of temperatures and pressures during alteration.
influent - a stream whose water sinks into it's bed, infiltrating into a ground water system.
intensity - the measure of the way an earthquake affected a given region.
intermediate - igneous composition between felsic and mafic.
intermittent stream - a stream that flows only during precipiation or seasonal run-off.
intrusive - igneous activity that remains beneath the earth's surface.
ionic substitution - when an ion of similar size and/or charge is replaced for the common ion as a mineral crystallizes.
isograd - zones representing pressures associated with regional metamorphism.
joint - a crack in rock along which no movement has occurred.
karst topography - a type of topography produced in areas with underlying soluble rock (limestone, marble or gypsum, etc.) that has been dissolved by groundwater.
laccolith - intrusive igneous rock body that is concordant in nature but domed the overlying rock into a hill.
lacustrine - refers to lake systems.
land subsidence - the compaction of an aquifer due to the overlying weight after the groundwater has been drastically withdrawn on a regional scale.
landslide - a type of mass movement which is produced by rapid movement along a flat plane of failure, commonly a bedding plane.
lateral erosion - the cutting away of a stream bank, resulting in mass wasting of steepen side and extension of the meander into the curve.
lateral fault - a fault with side-to-side motion. Also known as a strike-slip fault.
lava - magma that has reached the earth's surface.
lava dome - a large mound of lava that solidifies above a central eruption; usually rhyolitic in composition. (For this class: similar to a volcanic plug).
lenticular bed - a pinched out bed that shows the location of a buried abandoned river channel.
levee - a natural or man made "ridge" along a river bank that confines a stream to it's channel during lower flood stages.
limb - the side of a fold.
limestone - a sedimentary rock composed of calcite; either chemical or biochemical in nature.
limonitic - a modifier for clastic sedimentary rocks that are yellow in coloration; they contain limonite.
lithification - compaction and cementation of sediments to form a sedimentary rock.
lithosphere - the upper portion of the earth consisting of the crust and rigid upper mantle.
littoral - refers to coastal processes.
load - the volume of sediment carried by an erosional agent.
longshore current - currents along shorelines produced by angled approach of waves.
luster - the appearance of a mineral in reflected light.
mafic - igneous composition that is low in silica and rich in magnesium, iron and calcium. Also refers to minerals that are rich in magnesium and iron.
magma - molten material in the earth's mantle that usually consists of melted rock, gases and water vapor.
magma chamber - a large body of magma that has accumulated in the lithosphere.
magmatic differentiation - alteration of magma composition by several processes.
magnitude - the amount of energy released by an earthquake.
Magnitude Scale - a measurement scale, similar to the Richter Scale, used to report the level of energy released during an earthquake.
marble - nonfoliated metamorphic rock composed of carbonate minerals; its parent rock is either limestone or dolomite.
marker bed - a distinctive rock formation used as a 'tag' for reference.
mass wasting(mass movement)- movement of rock or loose debris downhill due to gravity.
meander - the curving loops in a river.
meander scar - curved cuts in a floodplain developed by the movement of meanders due to lateral erosion.
mechanical weathering - weathering or breakdown of rock material into smaller pieces through physical means. The mineral composition of the rock remains unaltered chemically. Also known as disintegration.
Mercalli Scale - the scale used to report the intensity of an earthquake.
Mesozoic - (mesos - middle; zoe - life) The middle era of the Phanerozoic Eon ranging from approximately 245 mybp to 66 mybp; "Age of Reptiles".
metaconglomerate - a nonfoliated metamorphic rock produced by the alteration of a conglomerate or breccia.
metamorphic facies - groups of rocks with specific index minerals that represent a defined temperature and pressure regime.
metamorphic grades - a description of the amount of alteration of a metamorphic rock from it's original parent rock. Low grades are minimal alteration; high grades are intense alteration.
metamorphic rock - rocks with textures and compositions altered by increased temperature, pressures or by chemical means while in the solid state.
metamorphism - the alteration of rock, in the solid state, due to increased temperatures and pressures present during tectonics.
micaceous - platy silicate minerals grouped into the "family" of micas; include biotite, muscovite, etc.
mid-oceanic ridges- long cracks, present along the ocean floor, from which basaltic lava is extruded. Considered the site of sea-floor spreading.
mineral - a naturally occurring, inorganic, crystalline solid with a limited chemical composition.
mineral stability - A mineral is at equilibrium only in the environment in which is created. (Equilibrium referring to persistent or enduring; will not alter.)
monocline - a linear fold where one limb dips down.
mouth - the area where a river empties into a standing body of water.
mudcracks - hexagonal patterns of cracks in sediment created by the drying and shrinking of muds in an arid environment.
mudflow - a type of mass movement; a rapid flow that consists of water saturated mud that moves as a viscous liquid.
nonfoliated - metamorphic rock texture that has random crystal orientation.
normal fault - a fault where the hanging wall drops with respect to the footwall.
nuee ardente - a hot cloud of ash, gas and steam that rolls down the side of a volcano during an eruption.
oceanic crust - the crust that makes up the ocean floor; the thickness ranges from 7-10 km. It is basaltic composition and high density (3.0 g/cc).
oceanic trench - a deep linear depression in the ocean floor, usually located along an island arc or volcanic arc system.
original horizontality - loose rock material and sediment are deposited in horizontal layers as they settle out.
orogeny - mountain building events.
outcrop - rock formations that are exposed on the Earth's surface.
oxbow lake - a curved lake formed by the abandonment of a river meander.
oxidation - chemical weathering process that results in the release of metals which recombine with free oxygen to form oxides.
oxide chemical group - minerals containing free oxygen bonded onto any "metal".
paleomagnetism - the study of the Earth's ancient magnetic field preserved in rock.
Paleozoic - (palaios - ancient; zoe - life) The oldest era of the Phanerozoic Eon ranging from approximately 570 mybp to 245 mybp; spanning from the "Age of Invertebrates" to the "Age of Fish" followed by "Age of Amphibians".
Pangaea - the single large continent present in the late Paleozoic to early Mesozoic. Named by Alfred Wegener.
parent rock - the rock from which a metamorphic rock is created; it greatly effects the final composition of the metamorphic rock.
peat - a bioclastic sedimentary rock composed of plant fragments. See coal.
perched aquifer - a small aquifer sitting atop an impermeable rock bed that is located above a larger aquifer system.
peridotite - a phaneritic, ultramafic igneous rock.
period - division of geologic time used to divide eras into smaller units.
permeability - the ability of fluids to flow through rock; based on the size and shape of the pores and interconnections of the rock.
Phanerozoic - (phaneros - evident; zoe - life) The most recent eon in the geologic time scale ranging from approximately 570 mybp to present. Originally designated as the time span in which fossils were present; now usually refers to time when "hard parts" in fossils are common.
phenocryst - in a porphyritic rock: a geometrically well shaped crystal of larger proportions; developed in the first, slower stage of cooling.
phyllite - a foliated metamorphic rock with a phyllitic texture; usually lower grade.
phyllitic - foliated metamorphic rock texture with microscopic minerals in parallel alignment. The minerals are sufficient large to allow light to reflect on the cleavages producing a "sheen".
physical continuity - tracing characteristics of a rock formation over large areas.
pillow basalts - accumulation of flatten, bulbous masses of basaltic lava; usually produced by eruption of lava onto the ocean floor.
plastic strain - deformation that is permanent in nature. The object remains deformed after the stress is released.
plate - a fragment of the lithosphere.
Plate Tectonics - the theory which states that the earth's outer layers are broken into plates that move about creating the geologic structures on the earth's surface. It describes the formation of folds and faults in mountains, the location of earthquakes, volcanoes and distribution of rock through the geologic past.
plateau basalts - large expanses of basalt that usually form by accumulations of basalt flows from fissure eruptions.
playa lake - a shallow lake that forms in the center portion of a desert basin when it rains.
plunging - the tilt of a fold's axis.
pluton - igneous rock body that has crystallized within the earth's crust.
plutonic - igneous rock that has solidified at depth.
point bar - sediment deposited on the inside of a meander.
pores - voids or spaces in rock not filled by solid material.
porosity - percentage of open voids to the solid portion of a rock.
porphyritic - igneous texture with two or more crystal sizes (due to several stages of cooling history).
porphyry - an igneous rock that has a porphyritic texture; usually modified with the groundmass composition. Ex.: Rhyolite porphyry
pothole - a hole at the base of waterfalls or rapid produced by the abrasive action of a swirling rock fragment.
Precambrian - The most largest and oldest division in the geologic time scale ranging from the Earth's origin (approx. 4.6 bybp) to approximately 570 mybp. Originally referred to as the Proterozoic; now commonly divided into three or more eons.
precipitation- the growth of crystals from a solution.
Proterozoic - (proteros - fore; zoe - life) The eon just prior to the Phanerozoic Eon in the geologic time scale ranging from approximately 2.5 bybp to 570 mybp;. Originally the oldest time span containing rock with no fossils present.
P-wave (primary waves) - compressional, or longitudinal, body waves produced by earthquakes. They are the fastest seismic waves.
pyroclastic - (pyro = fire; clast = piece) igneous rock fragments produced by being violently erupted from a volcano. Pyroclastic textures refer to igneous textures composed from the fine fragmented glass shards.
quartzite - a nonfoliated metamorphic rock composed of quartz; its parent rock was sandstone.
radioactive decay - the natural breakdown of unstable parent isotopes to a stable daughter isotope at a specific rate; may involve intermediate isotopes that progress through a predictable pattern.
radiometric date - number of years a rock/mineral has been in a particular state (ign., meta., etc.); calculated through the use of radioactive decay.
raindrop impression - small, round depressions, usually in arid environments, produced by rain impacts in muds.
rapids - a rough portion of a stream where the velocity is irregular.
recharge - water flowing into an aquifer system.
recrystallization - regrowth of minerals in a sedimentary rock, usually due to deep burial.
regional metamorphism - metamorphism that is the result of pressures being applied to rock; usually associated with tectonic activity.
regolith - a layer of loose, broken and partially weathered rock material commonly found between bedrock and soil zones.
relative age - chronological sequencing of geologic events based on neighboring rocks.
reverse fault - a fault in which the hanging wall has been pushed up with respect to the footwall.
rhyolite - an aphanetic, felsic igneous rock.
Richter Scale - a scale used to report the magnitude, or amount of energy released during an earthquake.
rift zones - a region of normal faults, down dropped valleys and basaltic volcanism.
rill - a small trickle of water that flows into finer tributary regions.
ripple - a small scale, undulating surface in sediment produced by current activity.
rock - An aggregate of minerals.
rock cycle - the cycling of rock material through the earth's crust through erosional and tectonic processes. The material is altered into igneous, sedimentary and metamorphic rock types as it adjusts to the current physical environment in which it currently exists.
rock gypsum - a chemical precipitate composed of the mineral gypsum, usually produced as an evaporite.
rock salt - a chemical precipitate composed of the mineral halite; usually produced as an evaporite.
rounding - the smoothing off of edges on a sediment grain.
runoff - water that flows along the surface, usually in stream systems.
salt water encroachment - mixing of denser salty water into a fresh water aquifer. Occurs when the fresh water is withdrawn and salt water flows into the aquifer to replace the depleted water.
saltation - the bouncing of grains along a flowing medium (water, wind, etc.) when grains are too heavy to stay suspended but too little to remain on the surface.
sand - sediment grain size measuring between 1/16 and 2 mm in size.
sandstone - clastic sedimentary rock with sand size particles, usually composed of quartz but may contain feldspars, clays and micaceous material.
scarp - (for landslides) an arc shaped, steep, crack in rock or soil produced by rock pulling away from the zone of failure in a slump.
schist - foliated metamorphic rock with a schistose texture; usually medium grade.
schistosity - foliated metamorphic texture with visible minerals in parallel alignment. Usually micaceous minerals are prominent in the rock.
sea floor spreading - the idea that the ocean is separating along the mid-oceanic ridges, creating new crust along the ridge and destroying crust at the trenches.
sediment - loose material created by the weathering of previous rocks.
sedimentary rock - rocks that are formed by accumulation and consolidation of sediment, precipitation of minerals from solution or by growth of minerals by plants or animals.
sedimentary structures - features in a rock created during erosion/depential stress.
shield volcano - a flat, broad shaped volcano created by build-up of thin layers of basaltic lava around a central eruption.
silica - combination of silicon and oxygen.
silica tetrahedron - the building radical in silicates that consists of one silicon and four oxygens bonded in the shape of a tetrahedron.
silicate chemical group - minerals containing the radical "SiO4" bonded to one or more positive metallic ions.
sill - intrusive igneous rock body that is tabular and concordant in nature.
silt - sediment grain size measuring 1/256 to 1/64 mm in size.
siltstone - clastic sedimentary rock composed of silt sized sediment. similarity of rock types - using unique facies or sequences of rocks to compare rock over large areas.
sinkhole - a depression in the Earth's surface produced by the collapse of an underlying cave system.
slate - foliated metamorphic rock with slaty cleavage; usually low grade. slaty cleavage - foliated metamorphic texture with microscopic minerals aligned in parallel sheets.
slide - a type of mass movement where the rock acts as a consolidated unit that moves along a zone of weakness.
slump - a type of mass movement; the rock as a unit that fails along a curved surface.
soil - a mixture of weathered rock and organic material that can support living organisms.
solifluction - movement of water saturated soil along top of permafrost.
solubility - ability of a mineral to dissolve in a liquid.
solution valley - a long, steep sided valley produced by the connection of several collapsed sinkholes.
sorting - separation of grains by size. Well sorted = one grain size; poorly sorted = many grain sizes.
specific gravity - The ratio of the weight of a particular substance to the weight of an equal volume of pure water.
spreading center - another name for a mid-oceanic ridge system.
spring - an area where the groundwater table intersects the Earth's surface.
stalactites - a carbonate deposit produced by water dripping from the ceiling of a cave forming a long piece of flowstone hanging from the ceiling.
stalagmites - a carbonate deposit formed by water dripping from a cave ceiling producing a pile of flowstone below the site of dropping water.
stock - intrusive igneous rock body that is less than 100 kilometers square when exposed.
strain - alteration of shape or size produced when a stress is applied to an object.
strata - (plural) another term for bedding.
stratigraphy - the study and correlation of rock outcrops and their geologic relationships throughout the world.
stratovolcano - a large, steep sided volcano produced by the accumulation of ash and lava layers erupted from a central eruption; usually andesitic or rhyolitic in composition.
streak - the color of a mineral in powdered form.
stream bed - the bottom of a stream channel.
stream load - the sediment carried by a river system.
stress - (in geology) a force applied to an object for a given area.
striation - fine, parallel etchings on flat reflective surfaces of minerals.
strike - the intersection of a tilted rock with the Earth's surface.
strike-slip fault - a fault with a side-to-side motion; also known as a lateral fault.
structure - features, such as folds and faults, on the earth's surface that are the result of stresses applied to rock.
subduction - the descent of a lithospheric slab into the interior of the Earth along a convergent zone.
sulfate chemical group - minerals containing the radical "SO4" bonded to a "metal".
sulfide chemical group - minerals containing sulfur bonded to any "metal".
superposition - younger rock material is deposited on top of older rock materials.
surface wave - a seismic wave that travels only along the surface of the Earth.
suspended load - finer particles that are carried in the stream's flow after being lifted from the bottom.
swamp - low land that is usually water saturated year round, often found along rivers.
S-wave (secondary wave) - shear, or transverse, body waves produced by earthquakes. They travel at approximately half the speed of P-waves and cannot travel though liquids.
syncline - a linear fold with younger rock formations within the core. The limbs dip toward the central axis.
talus - loose rock fragments lying at the base of a cliff.
tectonics - All the processes which act on the earth to deform the outer layers.
tensional stress - forces that pull an object apart. The object typically is lengthen in shape.
texture - the orientation of minerals in a rock.
thermal expansion - mechanical weathering produced by the expansion of individual minerals in a rock at different rates when exposed to heat.
thrust fault - a reverse fault whose fault plane lies at a low angle, producing a long plane of slip.
topography - the shape of the land surface.
traction - the rolling of heavy grains along a stream bed.
Transform Plate Boundary Zones - Plate Tectonic Theory: areas on the Earth's surface where plates are sliding past one another in a side-to-side motion.
travertine - see flowstone.
tributary - a stream that flows into another stream.
tufa - minerals deposited around the rim of springs; typically rich in sulfur bearing minerals when precipitated around hot springs and geysers.
tuff - a pyroclastic igneous rock. The rock is often modified with the name of its aphanetic equivalent. Ex.: felsic tuffs are called "rhyolite tuff".
turbulent flow - a mixing, irregular flow in stream currents.
ultimate base level - the lowest level to which a stream can erode: the ocean.
ultramafic - igneous composition concentrated in Mg and Fe and very low in silica.
unconformity - gaps or missing rock in the rock record; created by erosion or lack of deposition of rock.
uniformitarianism - geologic events and natural laws behave in a similar fashion throughout geologic history. "The present is the key to the past".
upper mantle - the portion of the mantle just below the crust. It is considered partially molten in its characteristics and probably ultramafic in composition.
vent - a small opening in the crust through which extrusive material is erupted.
vesicular - igneous texture with trapped gas bubbles.
viscosity - resistance to flow.
volcanic - igneous rock that has solidified at or very near the surface.
volcano - a centralize structure built by the accumulation of lava, ash and other materials ejected from a vent in the earth's crust.
water table - the upper surface of the zone of saturation.
waterfall - an area of a stream that decends vertically.
wave - an oscillating movement of water producing a rise and fall at the surface.
weathering - the chemical and physical breakdown of rock due to exposure on the earth's surface.
well - a man-made hole in the Earth through which water is pumped or drawn to the surface.
Wentworth Scale - division of sediments into sizes according to a set of standard sieve sizes. (see text)
xenoliths - inclusions of "foreign" rock in an igneous rock. Possibly unmelted country rock that has fallen into a magma chamber.
zone of aeration - an area beneath the surface where the openings within rock contain air and small amounts of water.
zone of saturation - an area beneath the surface where the openings within the rock are filled with water.
Geology 101 - Gale Martin - Class Notes
Only a portion of the water that falls as precipitation returns to the ocean as fluvial runoff. As rain strikes the surface of the Earth, some of it soaks into the ground and seeps into the soil . Here it becomes part of the soil moisture, often used by plants and small organisms. If the water continues deeper into the underlying regolith it can collect in cracks and crevasses in rock or between the pores within the rock itself. This body of water, referred to as groundwater , flows beneath the surface on its return course to the oceans.
Water can be found in any rock that has openings or pores. Cracks and fractures are common in many rocks, but unconsolidated sedimentary rocks have the greatest porosity . Small voids are common between the loose, round grains of a clastic sediment. If water completely fills the pores, the body of rock is referred to as the zone of saturation . Above this is the zone of aeration . Here water may coat individual grains or 'stick' between close grains (known as capillary action ) but the voids are not completely filled. The imaginary "line" the separates the two zones of rock is known as the water table .
The water table is complex and flexible; moving up and down as the volume of water in the pores fluctuates. This is a dynamic balance between recharge by infiltration and discharge by several means throughout the system. The type of rock and the climate of the region can greatly influence the depth at which the water table will occur. Where there is sufficient precipitation, the water table often mimics the topography: elevated under hills and lower, often flowing, into valleys. Streams, lakes and ponds are often surface expressions of where the water table reaches the Earth's surface. Such streams, referred to as effluent in nature, flow year round with groundwater as a primary source. In areas where rain is less frequent, water tables are typically much deeper and more flat in configuration. Here the streams are commonly influent : water drains down through the stream bed toward the water table and stream flow is intermittent in nature.
The flow of water through the subsurface is governed by the permeability of the rock. Not all rock allows groundwater to flow through it. The behavior of groundwater is governed by complex interactions of many factors, including the characteristics of the rock, the influence of gravity and the friction of water as it moves between grains. Calculations, using Darcy's Law, can be used to determine groundwater flow. If the pores within the rock are interconnected by sufficient openings, water easily flows through the rock. Low permeability occurs if the interconnections are too small or if clay particles block openings preventing effective flow. If there are no connections within the rock, permeability can be nonexistent.
Based on it's usefulness to mankind, rock formations are separated into several types of groundwater systems. Zones of saturation that have high permeability are commonly referred to as aquifers . Flow rates are sufficiently high to yield water for use by mankind. If the rate of flow is too low, the rock body is confining beds, allow very little flow to occur. If water encounters such a formation, flow can diverted, blocked, or " perched " above the main water table until an access route can be found around the impermeable bed.
Types of Groundwater Discharge
Groundwater reaches the Earth's surface by many means. Springs and seeps are common in areas where aquifers intersect the surface at the side of a hill or where folded rock is exposed and confining layers eroded away. Faults and cracks act as weaknesses in rock where water can easily flow from underlying aquifers to the surface. Hot springs occur when the groundwater flows through hot rock beneath the Earth's surface, quickly surfacing at temperatures above expected levels. In areas of geologically recent igneous activity, groundwater quickly picks up heat from deep seated magma chambers and becomes "superheated" quickly. This heated groundwater may arise as hot springs or, if the water "flashes" into steam, geysers .
When the water table is relatively close to the surface, a hole, or well , can be dug and water drawn to the surface. Deep wells require pumps to draw the water upward. Under special circumstances, wells can deliver water under pressure. Commonly referred to as artesians , these wells consist of a confined system. The aquifer in an artesian is bounded above and below by confining beds which restrict the direction of flow. The aquifer is tilted; the area of recharge being higher in elevation than the discharge zone. When water infiltrates into the upper reaches of the system it can only flow in one direction. This results in a pressure build up or hydraulic head. If the artesian is breached, either by a well hole or by intersecting the surface naturally, the excess pressure allows the water to flow above the land surface without pumping.
Erosion in Groundwater Systems
Unlike fluvial systems, groundwater is limited in how it can erode rock. Hydraulic erosion is uncommon because flow rates are slow relative to the strength of the rock. The openings through which groundwater moves restrict the size of particles that can be carried by the water, thus limiting the abrasive capability of most groundwater systems. (It is this very reason that well, spring and artesian water is considered so "pure" -- the water appears clear because it lacks suspended sediment loads.) The most common means of eroding rock in groundwater systems is by chemical dissolution of rock through which the water flows. Soluble rocks types are most susceptible to groundwater erosion. Karst topography commonly develops in regions underlain by limestone, marble and evaporite formations.
Rainwater that percolates through soil picks up compounds from decaying organic matter, making groundwater even more acidic than usual. This groundwater quickly dissolves away any soluble rock it encounters. Cracks in limestone and marble enlarge and widen as water flows through them. Large networks of crisscrossing chambers, known as caves and caverns , develop along pre-existing fractures in the rock. As the cave grows larger, the overlying rock may become too heavy to support and the roof may collapse into the empty void. These collapsed structures appear on the Earth's surface as sinkholes . The sinkholes occur in straight lines that follow previous fracture systems. Streams that flow across the surface of karst regions appear erratic in character: they commonly disappear into sinkholes, flow beneath the surface and reappear in springs several miles away.
As the cave system increases in size, the roof fails along most regions of the cave network. Sinkholes become connected and produce solution valleys with flat bottoms and step sides. As the surface network enlarges and connects, tall spires of carbonate rocks are left behind as erosional remnants in the predominantly collapsed karst topography.
Deposition in Groundwater Systems
As the groundwater chemically dissolves more and more carbonate rocks, it becomes "saturated" with calcite ions. If the water flows through a cave system that contains open spaces with air pockets, the water drips off the ceilings and drizzles down the sides of the caverns. The calcite beings to precipitate out as flowstone , or travertine , a layered deposit that accumulates by growth of carbonate crystals. The flowstone takes on many shapes as it drips, splatters and drops, the most common are called stalagmites and stalactites .
The carbonate ions can precipitate out in many environments, including the cement that lithifies sedimentary rock. One form of deposition, known as tufa , occurs around springs, especially hot springs and geysers. When groundwater is heated, its "corrosive" nature increases and it becomes more capable of chemically weathering rock. Upon reaching the surface, the groundwater quickly cools and minerals precipitate near the spring. Pools of warm water create "steps" around most hot spring sites, and numerous colors, produced by organisms sensitive to selective temperature ranges, are often prominent (ex.: Yellowstone National Park). Sulfur rich minerals are common in tufa surrounding hot springs and geysers. The most common minerals, however, are carbonates.
A form of carbonate precipitation, hardpan or caliche , is associated with water infiltration in desert regions. Rain falling on carbonate rich mountains chemically dissolves the rock. When the water runs off into the catchment basin, typically a desert valley, it begins to soak into the ground. The intense heat of the region evaporates the water before it can percolate into the groundwater system. This results in the ions within the water precipitating as crystals in the top layer of sediment in the valley. Through repetition, an accumulation of minerals "cement" the sediment together. This prevents further percolation of water through the layer and creates a solid rock layer (hardpan) only a few feet below the surface.
Misuse of Groundwater Sources
Mankind has found the presence of groundwater to be a convenient water source. They have tapped and used the source, almost at will, with little regard for the limits of the system. There are two general ways that groundwater sources can be misused: quantitatively and qualitatively. Qualitative misuse, or contamination, of groundwater sources is a large field often covered in environmental courses. Dumping of chemicals, locations of landfills, feedlots and waste disposal sites and improper use of toxic and non toxic chemicals should be everyone's regard. Groundwater easily filters out solid waste particles due to the small interstices it flows through. But these opens can become clogged and restricted easily. The greatest danger is that groundwater retains dissolved contaminants for extended periods of time and, therefore, great distances. Remediation of groundwater sources is an expensive and difficult task.
Quantitative misuse of groundwater is an important issue in many areas due to the limited nature of groundwater flow. It takes years for a groundwater system to regard itself; yet mankind approaches many systems as endless sources of water. Let's briefly cover some of the common difficulties encountered with overdrawing groundwater sources.
When a well is sunk, the casing must go deep enough to penetrate into the zone of saturation even during times of lower water table levels, ex. droughts. The rate at which a person draws on the aquifer should always be lower than the rate at which the groundwater flows through the system. As a person draws on a well, the water around the casing can be pulled out of the aquifer faster than the water can flow back into the openings of the sediment. This produces a cone of depression in the water table around the well casing. If you stop pulling water out of the well, the water will eventually flow back into the cone of depression and the water table will recover from this temporary depletion.
If you continue to pull on the well at too fast a rate, the region around the well "goes dry", i.e., the cone of depression completely surrounds the well region. (This is the same effect as when you are drinking a slush cone and you suck on the straw too fast.) This situation can also be temporary. Let the groundwater recover (set the slush cone aside) for a long period of time. The amount of time depends on the flow rate of that system, but the water will eventually flow back into the depleted zone from higher regions in the aquifer.
Unfortunately, the typically response to a well "going dry" is not to "set it aside" and come back after it has recovered. People usually "dig it deeper" or "pump on it faster" to force the well to pull more water from the ground. When a well is continuously overdrawn or multiple wells are overdrawn in a single region, the water table level can drop on a regional basis. Recovering from this regional depression can take years, if it's even possible to do.
Often, when there is a regional drop in water table level, the aquifer permeability can be affected. The openings in the aquifer are supported by the buoyancy of the water between the grains. When the water is withdrawn, the overlying weight of material can compact the aquifer and the openings are collapsed. What was once flowing aquifer system can become an aquiclude as the permeability decreases and flow rates drop. This compaction of the aquifer results in land subsidence of the overlying surface. Several groundwater systems have been restricted in their use or completely determined "off limits" due to the land subsidence resulting from their overuse (others should quickly pay heed!).
When the groundwater system is in areas of carbonate rocks, the withdrawal of water results in unique problems. The dissolution of the carbonates in the system forms caverns in the aquifer. Excessive withdrawal removes the roof support supplied by the groundwater and results in the catastrophic collapse of the aquifer producing sinkholes . (Note: these aquifers are also more prone to contamination: the large cavities do not filter particles as do clastic dominated aquifer systems.)
Regions with multiple formations that contain groundwater may have unique problems involved with excessive withdrawal from one of the systems. The most common of these types, involves salt water encroachment . Fresh water aquifers "float" above denser salt water systems. When the groundwater is removed from the fresh aquifer, salt water easily flows into the aquifer to replace the withdrawn water. This results in brackish and contaminated wells. Though this may be more prominent in coastal regions, "mixing" problems are possible wherever two or more types of groundwater systems occur. If one of the systems is "contaminated", the mixing can raise important issues in groundwater management.
Geology 101 - Gale Martin - Class Notes
Igneous rocks form when molten (rock) material solidifies. Each rock, via its texture and composition, reveals a "story" of its origin when properly "translated". These "clues" provide evidence for the igneous activity, both past and present, occurring within the earth. To understand igneous petrology, this course will review some general characteristics of magmas and how they solidify to form unlimited types of igneous rocks.
(Note: Most igneous processes are driven by Plate Tectonics. It is hard (no, impossible!) to separate the topics (see the textbook). This course attempts to simplify igneous petrology (and its mass of vocabulary) by first covering the formation of igneous rocks (what and how) and leaving the tectonic causes (why) for later.)
Magma Behavior
Mo.st magma originates in the earth's crust and upper mantle. The core is extremely hot. (Core temperatures exceed 7000 C (12,600 F).) With increasing depth into the interior of the earth, the temperatures increase by a rate of approximately 25 C per kilometer. (This is known as the geothermal gradient.) Many factors, including pressure (both burial and tectonic), water content in the rock, and mineral composition, influence at what temperature rock melts (between 600 C to 1200 C (1100 F to 2200 F)). Such temperatures are easily reached in the mantle.
Most magmas are a silicate mixture with gases (water vapor, carbon dioxide, sulfur dioxide and others) and minor amounts of other elements (traces of gold, silver, copper, etc.). The molten material is less dense than the surrounding rock and rises upward toward the surface. It makes its way through cracks, perhaps melting other rock material, and cooling as it rises. The magma's behavior depends on the temperature and composition of the melt. Hot fluid magmas are more likely to reach the surface, producing lava. Cooler, silica-rich magmas are viscous and more likely to remain underground. Let's examine two types of magma.
Imaging cutting open the outer portion of the earth's surface and exposing the mantle. Magma would "ooze" out of the "slush" to fill the crack. This type of magma (let's call it unaltered magma) is basaltic in nature. It's composition consists of silica, with iron (Fe), magnesium (Mg), and calcium (Ca). (The other elements: aluminum (Al), potassium (K), and sodium (Na) are present but not in major amounts.) This magma is very hot and bonding between silica tetrahedron is not common; few minerals have begun to crystallize out. This means that the magma is very fluid in character and can often make its way to the earth's surface along cracks in the crust. It is the most common form of extrusive igneous activity. Any gases that are present easily escape as the magma reaches the surface producing a "quiet", nonviolent eruption.
Several processes alter magma to "change" it and produce another style of magmatic behavior. These processes are collectively called magmatic differentiation. They include crystal fractionation, magma mixing (or immiscibility) and assimilation.
One means of differentiating magma occurs through crystal fractionation. As a magma cools, crystals begin to grow in the melt. Not all minerals crystallize (or melt) at the same temperature. (This is a basic chemical principle. Different materials melt or solidify at different temperatures. Example: Compare water, plastic and steel. Water solidifies at "the freezing point" of 0 C or 32 F; it melts when at room temperature. Plastic melts around the temperature at which water vaporizes; i.e. "the boiling point". Steel doesn't melt until you take a blow torch to it.) This results in a magma that is partially crystallized: containing a mixture of liquid and crystals. The crystals have a higher density than the surrounding liquid and sink to the bottom of the magma chamber. The crystals no longer react with the melt and become "fractionated" or removed from the mixture. The chemical elements incorporated in the crystals are no longer available to the remaining melt. If the molten portion is now moved to another location the "changed" magma has been "depleted" in those elements. (Example: Suppose you buy a dozen donuts: 6 chocolate frosted, 3 maple frosted and 3 plain. Your co-workers eat a half dozen: 5 chocolate and 1 plain. Your donuts are "depleted" in chocolate frosted donuts. When the boss arrives (late), she/he asks why you bought mostly maple frosted donuts!)
Crystal fractionation changes magma in a predictable fashion because the minerals in a silicate mixture will crystallize out at specific temperatures. The order of mineral crystallization from a cooling silicate melt is known as Bowen's Reaction Series (see text for figure). As the melt cools, two series of minerals crystallize simultaneously: the mafic minerals (olivine, pyroxene, amphibole or, lastly, biotite) and plagioclase feldspars (calcium rich to sodium rich, in layered zones). Following the growth of Fe, Mg, and Ca minerals, the Na, K and Al rich minerals crystallize (muscovite and K-spar). The last common mineral to form is quartz. If any liquid is "differentiated" from settling crystals (mafic-rich and Ca-plagioclase) before the magma chamber completely crystallizes, the "extracted" melt will appear K, Al and Na enriched.
A second way the magmatic differentiation occurs is through mixing different magmas or separating out portions of a melt. (These are separate complex processes but for simplicity we'll group and generalize.) Different compositions in magmas don't necessarily like to "mix" and will separate (similar to vinegar and oil in salad dressing). This concept is known as immiscibility. Less common elements (ex.: elemental ores) can "pool" and separate out of the melt. The final "dredges" of the melt will be enriched in these elements. When "squeezed off" into cracks it "reacts" with country rock producing "ore veins". Quartz, containing many inclusions of rarer minerals/elements, is common in these veins. (Remember, its the last silicate mineral to crystallize out: Bowen's Reaction Series!.) Realize this "separation" is not common. If the magma chamber stays mixed the elements will be finely disseminated throughout the magma body.
A third way that magma can be altered as it makes its way to the surface is through the process of assimilation. Magma that is moving upward will be hotter than the surrounding country rock. Any of the surrounding material that drops into the magma as it shifts and "oozes" through openings will be melted and incorporated into the mixture. (Think of ice cubes tossed into warm soda or coffee.) This process cools the magma and alters its composition: the further the magma migrates upward the greater the affect. Xenoliths are considered to be evidence of incomplete assimilation.
Through various processes of magmatic differentiation, magmas are created that are "altered" in their compositions. These magmas are considered "granitic" in composition. They are rich in silica, Al, K and Na (actually, depleted in Fe, Mg and Ca). These magmas are "cooler" in temperature, allowing bonding to occur within the melt. Many crystals of various minerals have begun to form and the mixture is often a "slush" of thick, viscous liquid and crystals. (Think of what happens to syrup when its refrigerated.) This bonding forms a network of atoms within the melt that restrict the movement of gases. The gases become trapped and build to high pressures that are confined only by the pressure of overlying rock. This magma, because of its thick nature, often remains below the surface to solidify as large masses of intrusive igneous rock. If the magma should move close enough to the surface that the gas pressure is released, violent explosions of ash, gas and rock are the result.
Igneous Activity
Igneous activity occurs in two forms: volcanic (extrusive activity) and plutonic (intrusive activity). Extrusive activity can be readily observed and it's behavior is dependent on the composition of the extruding lava. Plutonic rock types are very similar in textures and are not dependent on magmatic composition.
Volcanic Activity
Fissure EruptionsWhen lava extrudes along a large crack in the Earth's crust, it is known as a fissure eruption. The lava is basaltic in composition (highly fluid in nature) and forms thin expansive sheets that cover hundreds of square miles. Successive layers accumulate to produce flat lying broad expanses of dark lava flows known as plateau basalts. (Ex.: Columbia River Plateau Basalts). These basaltic lavas often have distinctive features. Columnar joints form as the lava cools; shrinking and breaking it into hexagonal, vertical wedges. Undersea lava flows produce pillow basalts as lava oozes into the water and accumulates in "blobs" or "pillows" along the ocean floor. Fissure eruptions produce the greatest volume of extrusive eruptions in the world and are associated with mid-oceanic ridges (Ex.: Iceland).
Central Eruptions
If an eruption occurs through a small opening in the crust (vent), the accumulation of lava or ash at that site is known as a volcano. The composition of the lava will determine the shape and appearance of the resulting volcano. Several types of volcanic features may form including shield volcanoes, composite cones or stratovolcanoes, cinder cones and calderas.
--Shield volcanoes
When the lava erupting from a vent is basaltic the volcano produced is a shield volcano. (Ex.: Hawaiian Islands) The fluid nature of the lava produces thin flows that spread out and solidify into a low, broad, gently sloping structure around the vent(s). Hundreds of lava flows accumulate through the life-span of the volcano.
Usually the eruptive styles of a shield volcano are very quiet and non-violent in nature. Explosive eruptions may be produced if the magma comes in contact with a groundwater source. The hot lava is instantly particularize, resulting in cinders and steam being ejected forcefully from the vent. The cinders accumulate as cinder cones that are shaped by prevailing winds. (Ex.: Sunset Crater, AZ)
--Composite cones or stratovolcanoes
Lavas that are more viscous in composition (intermediate or acidic/felsic) produce volcanoes that are known for their violent behaviors. These volcanoes are called stratovolcanoes or composite cones. They are built from alternating layers of pyroclastics and viscous lava. The thick lava has difficulty flowing, usually accumulating in the throat of the volcano as a volcanic plug or lava dome. This plug acts as a "cork" in the volcano; blocking the vent and preventing gases from escaping easily. The gases build up and violently erupt in clouds of hot ash, gas and steam that roll down the side of the volcano (nuee ardente) or are shot into the air. The ash accumulates around the vent and may alternate with occasional lava flows. The results is a very large, steep sided volcano. (They often have snow capped peaks due to their extreme heights. (Ex.: Mt. Rainier, WA (or most of the Cascades); Mt. Fuji, Japan)
--Calderas
Calderas are usually produced in association with a stratovolcano, either through a violent eruption or collapse of the volcano. Explosion calderas form when pressure from magmatic gases builds to extreme levels. The ensuing eruptions results in a massive explosion with the stratovolcano entirely destroyed by the eruption. (Ex.: Mt. St. Helens, WA; Krakatau, Indonesia). Collapse calderas are produced when the weight of the overlying volcano can no longer be supported by the empty magma chamber. The volcano collapses into the void and produces a large depression (Ex.: Crater Lake, OR).
Plutonic Activity
As magma makes its way toward the surface it fills voids and cracks in the country rock. If the molten material solidifies beneath the surface the igneous rock bodies are known as plutons. They are named by their size and orientation in the crust. Plutons include sills, dikes, laccoliths, stocks and batholiths.
Thin cracks that are filled with magma produce formations known as dikes and sills. They are tabular in form (longer/wider than they are high). Dikes may have been feeder cracks to volcanic activity (long since eroded away) or cracks around large magma chambers. They are discordant and cut across several rock layers. Sills usually form between layers of existing rock and are considered concordant in nature. If the sill is large enough to force the overlying rock into a dome shape than it is referred to as a laccolith. Around massive magma chambers, cracks are plentiful and often occur as "swarms" or interconnected cracks.
Large bodies of magma may solidify within the crust, resulting in stocks and batholiths. Later erosion of the rock may expose the ancient magma chambers in the core of the volcanic mountain chains. If the resulting body of exposed rock is small in size (less than 100 km square) it is referred to as a stock. Large massive intrusions are called batholiths. They contain massive crystals due to the long periods of crystallization involved and are usually granitic in composition.
Classification of Igneous Rocks
All rocks are classified by texture and composition. Igneous rock compositions are dependent on the magma from which they solidify. Igneous rock textures are defined by the size of the crystals in the rock.
Igneous Rock Compositions
Four general compositions are used in basic igneous rock classification:
--Acidic or felsic compositions are rich in K, Al, Na and considered the silica rich varieties. Minerals common to felsic rock include feldspars and quartz with only a few dark, mafic minerals.
--Basic or mafic compositions are rich in Fe, Mg, and Ca and lower in silica content. Mafic minerals are common in mafic rocks and include pyroxene, olivine, along with the feldspar, Ca-plagioclase.
--Intermediate compositions are rocks that are between felsic and mafic in composition. Amphiboles and biotite are common and associated with Na-plagioclase.
--Ultramafic compositions are more rare in occurrence. Most ultramafic rocks consist of pure mafic minerals (usually olivine or pyroxene) and occasionally pure Ca-plagioclase. The mantle is assumed to be ultramafic in composition.
Igneous Rock Textures
Crystals growing in an igneous melt form by bonding of additional elements in layers around a crystal nucleus. The longer the time span a crystal has to grow, the larger the crystal will become. Volcanic activity results in very rapid rates of crystallization. The magma is extruded into an environment which is much colder than the lava and it solidifies "quickly". This leaves little to no time for crystals to form. Four basic textures form during volcanic activity:
--Glassy textures form when the rock is "quenched" and no crystal growth has occurred. The volcanic glass developed in known as obsidian.
-- Vesicular form when gases in the lava have time to froth and the bubbles are preserved in the rock texture. Scoria and pumice are two rocks based on vesicular textures. Pumice is light in density and often floats. Scoria is usually basic/mafic in composition and therefore dark in color (Its the stuff you call: lava rock in your barbecue!)
--Pyroclastic textures are developed when lava is solidified and fragmented into fine glass shards as it is blown out of a volcano. When the ash accumulates it forms a rock known as tuff.
--Aphanitic textures are developed when lava has sufficient time to produce microscopic crystals. Because the minerals are only visible under a mircoscope, aphanitic rocks appear "smooth" or in texture and consist of a uniform, single color. These rocks are classified by both texture and composition:
Rhyolite | (acidic) | light in color (pink or gray) |
Andesite | (intermediate) | medium in color (medium gray) |
Basalt | (mafic) | dark in color (black) |
Intrusive activity usually produces larger crystal textures. This is due to the insulating properties of the surrounding rock. It takes long periods of time (hundreds to thousands of years or more) for magma to solidify beneath the surface. Two plutonic textures commonly develop:
--Phaneritic textures contain crystals visible to the human eye and are relatively uniform in size. These crystals are interlocking and irregular in their shape but the can be easily identified in hand sample. The rocks are classified by the minerals they contain (i.e., their composition, see above):
Granite | (acidic) |
Diorite | (intermediate) |
Gabbro | (mafic) |
Peridotite | (ultramafic) |
--Pegmatitic textures contain extremely coarse crystals, at least 2 or 3 cm in length. Pegmatites develop in stocks or batholiths where crystals can grow to extreme sizes (maybe even a meter!) and where rare mineral types are present (e.g. tourmalines, apatite). Granite pegmatite is the most common pegmatitic rock.
--Porphyritic textures are igneous rock with two distinctive crystal sizes. This texture develops when two different stages of cooling history occur in a single rock. The first stage of cooling occurs when the magma is deep in the crust. Large crystals begin in grow in a magma chamber. These crystals, called phenocrysts, are usually geometrically well shaped because they grow in an unrestricted environment (the liquid portion of the magma). Something happens (tectonic activity of some sort) that moves the magma to a new location (usually higher in the crust or onto the surface). The remaining liquid crystallizes at a quicker rate and a groundmass of finer crystals forms.
Porphyritic rocks are named after the groundmass and modified with the word "porphyry" to denote the presence of more than one crystal size. Ex.: Andesite Porphyry or Porphyritic Andesite.
A summary chart of igneous rock classification is available in both the lecture textbook and the lab manual. The three composition pairs: rhyolite-granite; andesite-diorite and basalt-gabbro are the most common rock types associated with igneous petrology.
It is important to realize that all igneous rocks are created in a high temperature environment. The minerals that crystallize from igneous melts, whether produced by a volcanic eruption, or exposed during tectonic events and later erosion, are unstable in low temperature environments that occur on the surface of the earth's crust. This unstability will drive the next potential phase of the rock cycle: weathering.
Geology 101 - Gale Martin - Class Notes
Introduction
This Physical Geology class covers three general areas
--Rock classification and a brief description of the processes necessary to form rock.
--Denudation processes: including fluvial, glacial, eolian, mass wasting and groundwater processes.
--Tectonic processes: including earthquakes, orogenies, plate tectonics and the structures created by these processes.
Rock Classification
There are three categories for rock classification:
--Igneous: created through solidification of magma.
--Sedimentary: formed by accumulation of sediment or crystallization of minerals from solutions, such as water.
--Metamorphic: formed from increases in temperatures, pressures and chemical means while the rock remains in a solid state.
Individual rocks are formed by various geologic events. Different processes, such as volcanic eruptions, floods, landslides, or wind storms, will produce a unique rock type. The "history" of the development of each rock is preserved in the rock's texture and composition and referred to as the rock's facies.
The goal of this course is to review rock textures and compositions and learn basic concepts of the processes to enable students to understand the origins of rocks and the earth's geology.
Geology 101 - Gale Martin - Class Notes
Minerals
The chemical elements bond together in various ways to form the basic building blocks of rocks -- minerals. Minerals, in geologic terms, must fit a very restricted set of rules.
--Naturally occurring:
They must be natural in origin. Synthetic gems and crystals grown in space are not minerals.
--Inorganic:
Minerals are inorganic in character; though they may be produced by plants and animals. Shells, bones and teeth are commonly preserved in rock after the "complex organic molecules" (tissue) has decomposed.
--Crystalline:
Minerals are crystalline in nature. The atoms within a mineral have a definite internal atomic structure that repeats itself in a symmetrical pattern. When a mineral crystallizes or grows in a void (an "unrestricted" environment), the mineral shape reflects the internal structure and a crystal forms. In most instances, however, the mineral is growing in a complex area with other crystals. This causes "interference" of growth; the minerals intergrow and "distort" the outer appearance of the minerals. The internal arrangement remains true but the outer form takes on the "look" of the "restricted" growth and appears as an irregular shape. This shape is commonly called...a rock.
--Chemical composition:
Any given mineral will have a limited chemical composition. If the element in that crystal are changed, it will no longer be the same mineral. Sodium chloride is the mineral halite; potassium chloride is the mineral sylvite. Calcium carbonate is the mineral calcite; calcium sulfate is the mineral gypsum.
There is some minor flexibility, known as ionic substitution. Some of the ions are similar in size and ionic charges (Ex.: Fe +2 and Mg+2). If the mineral is crystallizing out of a liquid that is rich in both elements, the crystal may incorporate some of the "substitute" ions instead of the more appropriate ion.
This substitution is common in the silicate families.
Given the these limitations, mineral characteristics follow predictable patterns. Certain mineral groups are soluble because of their chemical compositions; minerals break in predictable fashions because of weaknesses in their crystal habits; some minerals are more dense (heavier) than others (lead composition verses aluminum). Several physical properties, in combination, can be used to identify common rock forming minerals.
Physical Properties
Each of the following physical properties has a simple test that can be performed "in the field" to narrow the choices for mineral identification. Most "handbooks" on mineral identification will list the major physical properties common to any given mineral. Not all the physical properties apply to all minerals.
Crystal Habit:
If all minerals grew in a pure state (with no inclusions or contaminants) and formed perfect crystals (unrestricted growth) there would be no difficulty identifying minerals. They don't ordinarily come that way -- that's why you spend big bucks at gem shows! A limited number of minerals commonly grow as well formed crystals; use them when available. Most minerals grow into irregular masses of interlocking crystals; other physical properties must be used.
Cleavage:
Weaknesses in bonding in the atomic structure form predictable patterns of breakage in a mineral. If a mineral has cleavage it will always cleave in the same way within a single "crystal". Some minerals will cleave in one direction; others cleave in two, three or more directions at specific angles. Some of the cleavages are perfect (clean, crisp and flat planes); some are imperfect (stepped and jagged surfaces). In the field, cleavage is tested by striking a mineral with a hammer and observing how it breaks. In the lab, look for flat surfaces that reflect light well.
Fracture:
This property is recognized as a random, irregular pattern along the surface of a mineral. All minerals have fracture, but not all minerals have cleavage. Look for duller surfaces and breaks that are non parallel in character. Many "glassy" minerals often have only fracture; note how the breaks occur in random patterns.
Luster:
The way a mineral looks in reflected light is known as luster. Hundreds of terms are used to describe luster, the most important of which are metallic and nonmetallic types. Either metallic or nonmetallic may be shiny or dull in character. Metallics resemble common metal pieces: shiny "foils" of silver, gold, brass, etc. or dull, rusty reds, grays and yellows. Nonmetallics range from glassy colors to greasy or waxy lusters and dull, "earthy", muddy colors. (Black shiny minerals are commonly nonmetallic and glassy in texture! Think patent leather!)
Color:
Though most "handbooks" on mineral identification separate minerals by colors, it is the most unreliable property to use for most common rock forming minerals. Very few minerals have characteristic colors. Most minerals contain inclusions and contaminants of other minerals which mask or discolor the true shade of a mineral. Use caution when attempting to identify a mineral by its color.
Streak:
The true color of mineral becomes apparent when it is crushed to powdered form. This color is usually determined by rubbing the mineral against an unglazed porcelain slab called a "streak plate". This process can only be used for softer minerals; i.e. most non silicates. Because the streak plate is harder than these minerals, rubbing the mineral across the plate leaves a powder residue. When the excess powder is blown away, what remains is the color of the streak. Metallic minerals have the most distinctive streak colors; nonmetallics are often white (and therefore indistinguishable), or, in the case of many silicate minerals, too hard to perform the test.
Hardness:
Minerals are commonly tested to see how well they resist being scratched. Most gemstones are minerals that resist abrasion - they hold up well to being bumped and abused. To determine hardness, minerals are compared to materials of known hardness, referred to as Moh's Scale of Hardness. The materials are ranked from one to ten with one representing the softest mineral (talc) and ten, the hardest (diamond). (See text for figure.) Common field tools used for comparison include fingernails (H=2.5); a copper penny (H=3.5); and glass (H=5.5-6.0).
Specific Gravity:
This is the "heft" of a mineral sample or how heavy it is relative to water. When a sample has a specific gravity of 5, it is 5 times heavier than an equal amount of water. This test can only be properly performed in a lab setting and a "density test" is often used as a substitute for measuring true specific gravity. (See the lab manual for directions on how to measure density). When in the field, use the "this one feels heavier" test.
Other Properties (also referred to as Special Properties):
--Solubility:
Some minerals dissolve when placed in water (halides and some sulfates); others effervesce (dissolve) when placed in acid (carbonates: especially calcite; dolomite will if powdered first).
--Magnetism:
Only one mineral is attracted to a magnet: magnetite; one of the iron oxides.
--Taste:
Minerals that are soluble in water often have a taste; especially the halides.
--Feel:
Check for a slippery (clays when wet), soapy (talc; i.e. baby powder!) or greasy (graphite) feel.
--Odor:
Check for sulfur smells (rotten eggs) in elemental sulfur and sulfides (a light scratch with a little acid will bring out the smell). Clays will smell "earthy" (wet desert) when moistened.
--Striations:
These are fine parallel etchings or "scratches" on flat reflective surfaces of minerals. They are common on some crystal faces (quartz and pyrite, running across the crystal face) and on cleavage planes in plagioclase feldspars (running along the cleavage surface). This is NOT the change in colors present in some feldspars (called laminae); it is visible only in reflected, glassy surfaces.
--Optical Qualities:
Calcite, in the form of "Icelandic spar", will cause "double images" (double-refraction) when placed over letters on a piece of paper.
Keep in mind that not all minerals will have all the above mentioned physical properties. When working with metallic lusters be sure to check streaks and hardness, cleavage is not common in these minerals. Nonmetallic lusters are distinguished by their cleavage, hardness and occasional special properties; their streaks are not considered important properties.
Common Mineral Assemblages:
Igneous Rocks:
- Silicates: nine in particular
- Ferromagnesian silicates:
- *Olivines
*Pyroxenes (Ex.: Augite)
*Amphiboles (Ex.: Hornblende)
*Biotite
Micas:
- (Biotite)
*Muscovite
Feldspars:
- *Orthoclase (K-spar)
*Ca-plagioclase
*Na-plagioclase
*Quartz (all coarse crystalline varieties, Ex.: smoky quartz, milky quartz)
- *Olivines
- Ferromagnesian silicates:
Sedimentary Rocks:
- Silicates:
- rock fragments of all varieties weathered silicates:
- *Quartz (both coarse crystalline and cryptocrystalline varieties, Ex.: *Chert, Jasper, Flint and Agate)
*Clays (Ex.: Kaolinite)
Carbonates:- *Calcite
*Dolomite
Sulfates: *Gypsum
Sulfides: *Pyrite - *Quartz (both coarse crystalline and cryptocrystalline varieties, Ex.: *Chert, Jasper, Flint and Agate)
- rock fragments of all varieties weathered silicates:
Metamorphic Rocks:
- Silicates:
- all igneous silicates except Olivine, Ca-plagioclase in addition:
- *Garnets
Micas: *Chlorite- *Talc
- *Garnets
- *Calcite
*Dolomite
- all igneous silicates except Olivine, Ca-plagioclase in addition:
Economic Minerals:
- Oxides
Sulfides
Elemental
Rock Cycle
Each mineral forms in a very specific environment: the correct chemical composition at the right temperature and right pressure. (It's kind of like cooking: you can make cakes, donuts or breads with the same ingredients. It depends on the mix and how you cook it.) The rock that is created will be "stable" only where it was formed. If it is moved through erosional or tectonic processes it will become "unstable" and alter to new minerals. This concept is known as mineral stability.
The rock cycle (see text for figure) is a summary of some of the processes that rock material can go through as it is altered to new rocks. This class will review one of the many possible cycles that rock material can "follow" as it reacts with various geologic processes. The rock cycle is actually much more complex and the rocks need not make a "complete" cycle. What a rock becomes and when it is altered will depend entirely on its location during Plate Tectonic events. (Side Thought: Remember that the Scientific Method is based on the collection of data (cold, hard facts) and interpretations of that data. The problem with data collection in geology is...the rock cycle. The data (rocks) are continuously destroyed. Geologist must work with incomplete and "tampered" data to interpret the earth's geologic history!)
Geology 101 - Gale Martin - Class Notes
History Behind Plate Tectonics Theory
To understand the theory of plate tectonics, it's best to know the history and development of the idea. The theory was developed through many years of scientific study and 'arguments' (scientific discussions).
Continental Drift
Several geologists, from many different continents, had commented on the similarity of rocks, fossils and structural geology through geologic time. In the early 1900's, Alfred Wegener published a book comparing and summarizing the evidence into one hypothesis called Continental Drift.
Several pieces of evidence support the concept of continental drift. One obvious line of evidence is the external outline of the continents. Over the centuries, many explorers and scientists had commented on the similarity of the coastlines (especially South America and Africa). Wegener placed all the continents together into one large continent, which he called Pangaea. He noted now the scientific evidence of rock and fossils supported a single landmass. Mountain ranges and their structural features matched between South Africa/Argentina and eastern North America/ Greenland/Great Britain and Norway. Late Paleozoic/Early Mesozoic rock types, typically developed in distinctive climatic zones (glacial deposits, coal beds and desert sands), seem randomly situated with the present configurations for the continents. When Pangaea in 'reunited', distinctive climate zones with a single equatorial region is evident. Plant and animal fossils for species of very specific regions (land based or climatic restrictions) also form distinctive patterns within Pangaea. With the breakup of the continent, the fossil patterns diverge and adapt to new climatic zones on separate continents.
Though the evidence collectively pointed to the existence of a single continent, the hypothesis was greatly opposed. Wegener had envisioned the continents breaking apart and pushing along the ocean floor, scraping up mountain ranges along the leading edge of motion. The mechanism for how and why the continents moved caused the greatest opposition. Support for the idea would have to wait for evidence from the ocean itself.
Sea Floor Spreading
During World War II, evidence from oceanographic studies reveled more information than military strategies. It became evident that the ocean floor was not a flat featureless region: there were trenches, long mountain ranges and individual sea mounts scattered throughout the ocean basins. In 1962, Harry Hess published the idea of sea floor spreading. He postulated that the features on the ocean floor were created by upwelling magma released as the crust separated along mid-oceanic ridges. As the new crust is developed along the ridge, old crust is subducted at deep ocean trenches. (Evidence to supported this would later come from seismology in the form of Benioff Zones.) Thus the ocean crust was constantly being consumed and regenerated.
Paleomagnetics
Paleomagnetics, a field developed in the 50's, supported Hess' idea. Mafic lavas, as they cool, preserve the orientation of the Earth's magnetic field. Vine and Matthews noted that there have been reversals of the Earth's magnetic field throughout geologic time. A distinctive pattern of magnetic stripes is evident along the ocean floor. This pattern is centered along ocean ridge systems and evenly reflected on both sides of the ocean basin. The pattern must be created as the crust cracks and splits, pulling apart at the mid-ocean ridge.
In 1968 the Deep Sea Drilling Project (DSDP) began exploring the ocean floor using the ship Glomar Challenger . DSDP supplied evidence that the ocean floor is basaltic in composition (i.e., a volcanic origin). The youngest basalt occurs along the ocean ridge; it becomes progressively older as the distance from the ridge increases. The oldest basalt found, located along the continental edge, was approximately 250 million years old. Overlying sediment confirms the age trend for the basalt; sediment is thicker further away from the ridge system (older the basalt has accumulated more 'dust'). It became evident that sea floor spreading was, in fact, happening.
The process of sea floor spreading supplied an appropriate mechanism for continental movement. The continents did not physically 'push' their way across an ocean floor but, instead, 'hitched a ride' along with the ocean crust as it spread apart. In the late 1960's this idea was coined: Plate Tectonics.
Plate Boundary Configurations
The lithosphere is broken into many pieces referred to as plates. Geology in the interior of the plates is relatively inactive. The edges of the plates, where they interact with one another, is where the major geologic activity occurs. The shifting and sliding of plates causes earthquakes, volcanic activity and various types of faults and mountain building events.
The mechanism for motion is still under study. It is believed that the heat in the mantle causes convection in the plastic asthenosphere. Hot material slowly rises and pushes against the rigid lithosphere, cracking it. The plates are pushed or dragged away as the hot material spreads out when it reaches the lithosphere. When the material cools, it sinks, potentially dragging the plate downward into the mantle. In this fashion, ocean floor is created and destroyed, while continents are geologically altered as they pass over various 'convection cells'.
The styles of tectonics are commonly grouped according to the type of stress found. Where plates are pulled apart they are referred to as divergent in nature. Collisions are produced along converging zones and transform motion in produced in regions of shear. (Refer to your text for drawings of each. This course will remain very basic in nature.)
Divergent Plate Boundary Zones
Divergent Plate Boundaries occur where upwelling mantle physically rips the crust apart. This can begin within a continent (ex.: East Africa, Pangaea) where tensional forces extend and thin the crust. Long linear valleys, known as rift zones, are created as pieces of crust drop along normal faults. Any crack that extends into the asthenosphere acts as a conduit for the hot rising fluids beneath. Thin veneers of mafic rich lavas cover the rift valley floor. As the plates continue to diverge, the crust drops low enough that the ocean eventually floods the region producing a long linear sea (ex.: Red Sea, Gulf of Aden). Given sufficient time, the rift zone will enlarge and form an ocean (ex.: Atlantic Ocean). Along the spreading center, the newly formed basaltic ocean crust is hot and buoyant, resulting in a raised mid-ocean ridge. As the crust pulls away from the ridge, it cools and sinks forming a deep ocean basin. Divergent Plate boundaries are the regions where ocean crust is made.
Geology within rift zones consists of block faulted mountains. Fissure eruptions of basalt are common (ex.: Iceland). When the eruptions occur underwater (ex.: mid-ocean ridges), hydrothermal alteration of the sea floor produces rich mineral deposits. Earthquakes are commonly shallow and volcanic in nature.
Transform Plate Boundary Zones
When the lithosphere cracks along divergent ridges, the break is not smooth and straight. Offsets occur between segments of the ridge system. These areas are known as Transform Plate Boundary Zones. Here the plates slide past one another in a shearing motion. Geology along transform zones is usually restricted to earthquake activity. As the two spreading ridges pull apart, shallow earthquakes occur along the stressed offset zone. Transform Boundary Zones in continental crust (ex.: the San Andreas Fault Zone) produce larger earthquakes due to the length of the fracture and the complexity of the crust it involves.
Convergent Plate Boundary Zones
In regions where the cold convecting material sinks into the mantle, plates collide and may be dragged into the Earth's interior. The types of geology that occur along these Convergent Plate Boundary Zones will depend on the types of crusts involved in the collision. Three combinations can occur: ocean-ocean, ocean-continent and continent-continent.
Ocean-Ocean Convergent Zones
As two plates whose leading edges are oceanic collide, one of the plates gives and is pushed beneath the other plate. This process of subduction is evident by the trace of earthquakes that occurs, known as the Benioff Zone. Earthquake activity is shallow along the deep oceanic trench formed at the site of collision. Foci depth increases at an angle into the interior of the Earth. It is assumed that the trace of foci shows the descending slab of rock being pushed into the asthenosphere. As the slab descends into the subsurface, it is pushed into regions where it begins to partially melt. (Remember: different minerals melt/crystallize at different temperatures. Review Bowen's Reaction Series.) The magma produced is more intermediate/felsic in composition and as it rises may also be altered by assimilation. A line of andesitic/rhyolitic volcanoes, known as an island arc, will be produced on the overlying lithosphere above the deep seated earthquakes (ex.: Japan, Aleutian Islands). Sediments deposited on the ocean crusts will be folded and thrust onto the colliding plate forming complex folded mountains.
Ocean-Continent Convergent Zones
During an ocean-continent collision, the plate with the oceanic leading edge will be subducted. This occurs because ocean crust is denser than continental crust. Ocean crust therefore sinks, while continental crust remains 'floating'. Once produced the only way to 'destroy' continental crust is through erosion. The geology along an ocean-continental collision is similar to an ocean-ocean collision to some degree. With the subduction of the oceanic slab, Benioff Zone earthquakes are produced. The earthquakes occur as a slanted zone that becomes progressively deeper toward the interior of the continent. A rhyolitic/andesitic volcanic arc is produced above the melting slab (ex.: Andes, Cascades). Often the magma is more felsic in composition due to the thickness of the lithosphere it must travel through to reach the surface. Felsic magmas tend to be thicker in character and, therefore, may get 'stuck' beneath the surface. Large granitic batholiths are common along ocean-continent collision zones. The edge of the continent goes through more structural changes: mountains with major thrust faults and complex folds are common (ex.: Rocky Mountains).
Continent-Continent Convergent Zones
Collisions between to plates with continental leading edges produce no subduction. Both plates are buoyant and refuse to be subducted. Earthquakes are shallower in character (no Benioff Zones) and confined to the lithosphere in depth. The collision produces a large complex of folded, faulted and thrusted rock with little, if any, volcanism (ex.: Himalayan and Appalachian Mountains).
Continental Accretion
Plate convergence in often an 'evolutionary' process. With the advent of ocean-ocean subduction, a small island arc is produced on the surface. This land formation is felsic in composition and cannot be subducted. If it becomes involved with a collision, it will act as a small 'micro-continent'. This means that is will either 'suture' itself to another continent (via continent collision) or act as the nucleus to another continent. Continents grow by accretion, the 'suturing' of small pieces through several collisions (ex.: interior of North America). As the continental grows larger it becomes known as a craton. It's interior regions become geologically 'inactive' and only the edges are altered by collisions. When two large continents collide (ex.: Asia with India) the collision results in the end of convergence at that boundary. The convergence will shift to a region along the coast where oceanic crust will 'give' and subduct. This process continues until the next collision occurs or the continent shifts off the convection cell deep in the mantle interior.
Throughout Earth's history the surface of the plate has been altered and changed by the movement, collisions and shifting of the lithospheric plates. The current shape of the continents is only a brief configuration for the present time. The Earth's surface is destined to be changing and evolving as the Earth's internal forces shape the land surface.