chapter2 - Chapter 2 Igneous Rocks Most students find the...

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21 Chapter 2 Igneous Rocks Most students find the definition of a mineral to be rather long and cumbersome. In contrast, the definition of a rock is short and sweet. A rock is any naturally occurring aggregate of 2 or more minerals, and they are classified as either igneous, sedimentary, or metamorphic depending upon their origin. Igneous rocks are formed from molten rock. Sedimentary rocks are made of particles or grains of earlier rock which have been deposited, compacted and ultimately re-cemented into new rock. Other sedimentary rocks precipitate from solutions rich in ions derived from the decomposition of previously formed rock material or through organic activity. Metamorphic rocks are made from any existing rock that has been changed either in surface appearance or mineralogy by the forces of heat, pressure and/or chemically active fluids. The first rocks that you will encounter in GY 111 are those of igneous origin. Igneous rocks tend to be easier to deal with than the others because they are frequently made up of readily identifiable minerals, most of which you have already encountered (e.g., feldspars, quartz, olivine etc.). All molten rock originates deep in the Earth’s interior due to a combination of geothermal gradient, heat generated through radioactive decay of unstable elements like Figure shows a cross-section of Mt. Vesuvius, a stratovolcano circa 1756. This volcano blew up in 76 A.D. with spectacular and tragic consequences. From: LeConte, J., 1905. Elements of Geology. D. Appleton and Co., New York, NY, 667p.
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Chapter 2: Igneous Rocks 22 uranium, and heat and pressure associated with tectonic activity (e.g., divergence or subduction of lithospheric plates). Molten rock below the Earth’s surface is called magma. Molten rock that makes it up to the Earth’s surface via volcanoes is called lava. Igneous rocks form through the process of crystallization as a magma or lava cools and hardens into a solid mass. Between the temperature of 1800°C, above which mineral structures can not exist, and 200°C, below which magmas become completely solidified, individual minerals crystallize over specific temperature ranges. Bowen's Reaction Series is a description of the cooling path that most (but not all) magmas take on the way to becoming igneous rock. The series identifies the minerals that can occur together at specific temperatures and is a tool for predicting the composition (mineral content) of igneous rocks (see Figure 2.1, Table 2.1 and your lecture notes). Magmas which cool and crystallize wholly within the Earth’s interior (e.g., within a magma chamber) generally do so very slowly, possibly over tens of thousands or millions of years. Lavas cool much more quickly, perhaps only hours or days after the eruption. The faster
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This note was uploaded on 02/04/2012 for the course GLY 111 taught by Professor Haywick during the Fall '11 term at S. Alabama.

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chapter2 - Chapter 2 Igneous Rocks Most students find the...

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