21 Chapter 2 Igneous RocksMost students find the definition of a mineral to be rather long and cumbersome. In contrast, the definition of arockis 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 rocksare formed from molten rock. Sedimentary rocksare 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 rocksare 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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