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Geology - Igneous Petrology

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

Most 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 Eruptions

When 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 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.

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