Rocks are naturally formed, consolidated material composed of grains of one or more minerals. Geologists group rocks into three categories depending on their origin: igneous, sedimentary and metamorphic.
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Perhaps the most apparent feature of rocks to the observer is the coloration. Although most rocks have a rather drab appearance, some have very distinctive and, in some cases, beautiful colors. Shades of red, green, gray and brown may be caused by iron-bearing minerals. Very light-colored rocks are generally lacking in iron-bearing minerals. The coloration of sedimentary rocks reflects the environmental conditions that existed during deposition.
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Purple and Red Rocks
Purple, maroon and red rocks are stained by the mineral hematite (iron oxide). Hematite results from the decomposition and oxidation of iron-rich minerals such as magnetite, ilmenite, biotite, hornblende and augite. A rock composed of only several percent hematite may be stained a deep red.
Green sedimentary rocks are typically formed in a reducing environment where oxygen is not available. For sedimentary rocks, this would normally mean deposition in deeper water than red rocks. In a reducing environment, iron combines with silica compounds to form iron silicate minerals. Then lowgrade metamorphism will convert the iron silicates to the green mineral chlorite. Chlorite in sedimentary rocks indicates a deep-water depositional environment. Where chlorite-rich strata alternate with hematite-rich strata, a change in sea level probably occurred.
Higher-grade metamorphism (high heat and pressure) will convert the hematite in red rocks and the chlorite in green rocks to the black minerals magnetite and biotite. An abundance of these minerals will yield a gray to dark gray mineral. Traces of black organic matter will also darken a rock to a gray or dark gray.
Many rocks have a different color on the weathered surface than on a fresh break. Weathering of disseminated pyrite (iron sulfide) in rocks will convert them to brown or yellow iron hydroxide and iron sulfate.
Sedimentary rocks are derived from preexisting igneous, sedimentary and metamorphic rocks. These rocks contain many clues as to their origin and the conditions that existed while they formed. Sedimentary rocks make up 75 percent of the rocks at the earth's surface but only 5 percent of the outer 10 miles of the earth. Sediment, as distinguished from sedimentary rock, is a collective name for loose, solid particles and is generally derived from weathering and erosion of preexisting rock. After formation, sediments are transported by rivers, ocean waves, glaciers, wind or landslides to a basin and deposited. Lithification is the process of converting loose sediment into sedimentary rock and includes the process of cementation, compaction and crystallization.
Sedimentary rock is formed by lithification of sediments, precipitation from solution and consolidation of the remains of plants or animals. Coal is an example of sedimentary rock formed from the compression of plant remains.
Rounding occurs during the transportation process by one or more of the erosional agents. Current and wave action in water are particularly effective in causing particles to hit and scrape against one another or a rock surface. The larger the particle the less distance it needs to travel to become rounded. For example, the boulders of the melon gravel deposited by the Bonneville flood were rounded after 3 to 6 miles of transportation.
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