Rocks and Weathering

Rocks and Weathering - Rocks and Weathering A mineral is a...

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Rocks and Weathering A mineral is a naturally occurring inorganic compound with a characteristic crystal structure. An example is quartz, SiO2, that has one atom of silica for every two of oxygen. Quartz is organized in a hexagon pattern, which is its crystal structure. Ice is also a mineral and has hexagonal crystals (shown in snowflakes). A rock is an group of minerals. There are three types of rock: 1. Igneous Rocks - formed by the cooling of magma , which is molten rock. When magma gets to the surface still in its liquid state, it is called lava . Igneous rocks make up 95% of the earth's crust. Igneous rocks are classified two ways: one is by mineral content where we group them as either felsic (quartz and feldspars) which is most common on the continents or as mafic (magnesium and iron) which is most common in oceans. The other way of classifying igneous rocks is by texture , referring to the size of the crystals- coarse or fine? The mineral content is determined by what is available in the magma and the texture is determined by the rate of cooling. Temperature generally increases away from the surface. As magma moves toward the surface it cools. If it cools enough, it solidifies and becomes rock. If it solidifies below the surface it is intrusive igneous rock and if it cools at the surface it is extrusive igneous rock. Intrusive rocks cool slowly, so there is enough time for large crystals to form (like granite) and extrusive rocks cool so quickly the crystals are small (like obsidian). 2. Sedimentary Rocks form when material accumulates in an area and is turned into a rock. The material can be from other rocks, plant life or animal life. There are three kinds: a. Clastic sedimentary rocks- made of particles of broken rock or remains of organisms. Clastic rocks are categorized by the size of the particles making them up: shale - clay (<0.0039 mm) mudstone - mixed silt and clay siltstone -silt (0.0039-0.0625 mm) sandstone -sand (0.0625-2 mm) conglomerate - pebbles and larger (>2 mm) b. Chemical sedimentary rocks form when material that was chemically dissolved in water settles out. If more of a substance exists in seawater than the water can hold, some will precipitate out and collect at the bottom. Another type of limestone forms when calcium carbonate, the same material as in skeletal remains, precipitates out of seawater. In arid areas, evaporating water can leave salt deposits, known as evaporites , like rock salt and gypsum. c. Organic sedimentary rocks are made of formerly living things. Coal , for example, is formed when vegetation accumulates in swamps. It first becomes peat , then further compaction turns it into coal. Shell and coral limestone can be thought of as both clastic and organic.
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3. Metamorphic Rocks form when a rock is altered by heat and pressure. If a rock is exposed to high temperature and pressure, it can be altered to the point of having new mineral assemblages and different crystals. In other words, the rock metamorphosed into another kind of rock. There are two general ways to
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This document was uploaded on 10/27/2011 for the course GEOG 1401 at Texas Tech.

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Rocks and Weathering - Rocks and Weathering A mineral is a...

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