Igneous Rocks Of the three principal rock types (igneous, sedimentary, and metamorphic), igneous rocks can be thought of as "primary" rocks because they crystallize from a liquid. Sedimentary and metamorphic rocks, which we will study later, may be thought of as derivative rocks. Igneous rocks are rocks formed from the crystallization of a liquid (molten rock). Igneous rocks may be divided into two categories. Intrusive or plutonic rocks crystallize from magma beneath the earth's surface. Extrusive or volcanic rocks crystallize from lava at the earth's surface. The texture of an igneous rock (fine-grained vs coarse-grained) is dependent on the rate of cooling of the melt: slow cooling allows large crystals to form, fast cooling yields small crystals. Magmas and their resultant plutonic rock bodies cool and crystallize slowly and are characterized by coarse-grained texture, in which the mineral crystals are visible to the unaided eye. On the other hand, lavas cool quickly at the earth's surface and are characterized by fine-grained texture, in which the crystals are too small to be seen by the unaided eye. Very quickly cooled lavas, typically those quenched in water, will have a glassy texture. They cool too quickly to form crystals. Glasses do not have an orderly arrangement of atoms and there are therefore no minerals, in the strict sense, in them. Volcanic glass is called obsidian. In addition to texture, igneous rocks may are classified according to their chemical composition. The most general classification is based on the relative abundance in a rock of felsic (feldspar and silica-quartz) minerals vs mafic (magnesium and ferrum or iron) minerals. Felsic minerals (quartz, K feldspar, etc) are light colored while mafic minerals (hornblende, pyroxenes) are normally dark colored. Felsic minerals have the lowest melting points (600 to 750 °C) and mafic minerals have higher melting points (1000 to 1200 °C). Generalized Cooling and Crystallization of a Magma
 Bowen's Reaction Series can be thought of as an idealized order of crystallization of a cooling magma. However, not all of these minerals will be crystallized together in the same rock. A mafic magma will begin crystallizing olivine and continue with pyroxenes and calcium rich plagioclase feldspar. Some amphiboles may also
crystallize before the melt is used up. Mafic melts don’t have enough silica to crystallize potassium feldspar, quartz, etc. Felsic melts don't have enough iron, magnesium, and calcium to form olivine, pyroxene, or calcium plagioclase. The first-formed minerals in a felsic melt are amphiboles (hornblende) or biotite mica, along with some intermediate or sodium plagioclase. Eventually, as the melt continues to cool and becomes richer in silica (as the metal cations are used up preferentially in
the double chain and sheet silicates) potassium feldspar and quartz crystallize. Classification of Igneous Rocks Igneous rocks may be simply classified according to their chemical/mineral composition as felsic, intermediate, mafic, and ultramafic, and by texture or grain size: intrusive rocks are course grained (all crystals are visible to the naked eye) while extrusive rocks may be fine-grained (microscopic crystals) or glass (no crystalline structure; i.e., no minerals). Volcanic rocks, especially felsic and intermediate, often have a porphyritic texture characterized by visible crystals floating in a fine-grained groundmass. Shown below is a simple classification scheme. Alternative, complex classification schemes take into account finer gradations in composition and especially the varying amounts of the elements potassium, aluminum, sodium, and calcium. Note that felsic rocks are light in color; intermediate rocks range through grays, and mafic rocks are black in color. Ultramafic rocks (peridotite) may range black to olive green (dunite) from the mineral olivine.
Igneous Rocks Igneous rock is formed from the cooling of a magma. The interior of the Earth is hot. Geothermal Gradient: the heat within the Earth steadily increases with depth; the heat is from radioactive decay and remnant heat of the Earth's formation. Magma is molten rock material. As magma cools the elements within the magma combine and crystalize into minerals that form an igneous rock. Magma cools either below the surface or at the surface (magma that reaches the surface is called lava). As magma cools igneous rock is formed. There are two types of igneous rock: 1) Extrusive: magma reaches the surface of the Earth before cooling and the lava cools rapidly. 2) Intrusive: magma cools under the Earth's surface. The magma cools very slowly. As magma cools minerals are formed into an interlocking arrangement producing an igneous rock. As magma cools it undergoes reactions that form minerals. The rate of cooling is very important. If the magma cools slowly then the reactions proceed for a longer period of time and the resulting mineral crystals (grains) become large. If the magma cools rapidly then the reactions are much quicker and the mineral crystals (grains) are smaller. Igneous rocks will have a distinctive appearance (this is referred to as texture) based on the rate of cooling (fast or slow). Textural terms for igneous rocks: 1. Phaneritic (coarse grained): rocks are composed of mineral grains large enough to be seen with the unaided eye. Indicitive of SLOW cooling, usually deep within the Earth. 2. Aphanitic (fine grained): rocks are composed of mineral grains too small to be seen by the unaided eye. Indicitive of faster cooling, usually at or near the Earth's surface. 3. Glassy: magma cooled so quickly that crystal formation could not occur. Indicative of almost instant cooling at the Earth's surface. 4. Porphorytic: large mineral cyrstals surrounded by a fine grained matrix (background). This texture forms when slow cooling begins to form large mineral crystals. Before the magma is completely cooled the magma (and its large mineral grains) moves closer to the surface and finishes cooling at a more rapid pace. Igneous rocks have a range of chemical composition. 1. Sialic (granitic) or felsic: rocks have highest silica, contain feldspar and ferromagnesium minerals. Rocks are light in color. 2. Intermediate (andesitic): rocks have less silica, contain feldspar and ferromagnesium minerals. Rocks are medium gray in color. 3. Mafic (basaltic): rocks have higher amounts of ferromagnesium minerals and less silica and feldspar. Rocks are dark in color, black or dark gray. 4. Ultramafic: highest amount of ferromagnesium minerals and least amount of silica. Rocks are usually a dark green to green-black. Classification of Igneous Rocks Igneous rocks are classified by both texture and composition. You will learn to identify common igneous rocks in lab.
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