Geology - Glossary
Posted On: 2020-08-15

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.



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