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Rocks and Minerals Reading Plate Tectonics, Rocks, and Minerals Earth’s plates consist of rocks composed of many minerals, plus a thin covering of soils. Developing an understanding of the relationship between the most obvious aspects of geology—minerals and rocks—and the unifying theory of plate tectonics may seem difficult at first. Plate tectonics describes how, and by what mechanisms, Earth’s surface has been and is being transformed on a large scale. We often think of continental movement, mountain building, volcanoes, and earthquakes as those aspects of geology explained through plate tectonics. However, rock and mineral types, locations of mineral deposits, and rock formation and transformation also can be understood when studied in the context of plate tectonics. Plate tectonics provides a link between rock and mineral specimens that can be studied in the classroom, and the geologic history that led to their development. Minerals A mineral is a naturally occurring, inorganic crystalline material with a unique chemical combination. Different combinations and atomic arrangements of elements give each mineral its characteristic properties, such as color, hardness, shape, density, and cleavage. Although about 3,000 different minerals have been identified, most are extremely rare. About 95% of Earth’s crust is comprised of only 10 to 15 different minerals or mineral groups. The following relatively short list of minerals and mineral groups comprises the primary ingredients of rocks: Feldspar—a group of similar minerals; they make up roughly 60% of Earth’s crust Quartz—approximately 10% of Earth’s crust, found in many different rocks; sandstones can be nearly all quartz Pyroxenes—mineral group with many members; common dark-colored minerals Amphiboles—another mineral group with many members; common dark-colored minerals Mica—mineral group; includes biotite and muscovite, common in igneous and metamorphic rocks Garnet—another mineral group; diverse in occurrence and color Clay—mineral group; sediments and sedimentary rocks; produced by weathering of feldspars Calcite—major constituent of sedimentary rock limestone, and metamorphic rock marble Dolomite—similar in occurrence to calcite, but sedimentary rock is called dolostone Olivine—uncommon at Earth’s surface because it weathers easily; common within Earth Project Earth Science: Geology, Revised 2nd Edition Copyright © 2011 NSTA. All rights reserved. For more information, go to www.nsta.org/permissions. 221 Reading 4 This website from the Smithsonian Institution provides good information on creating your own classroom exhibit of rocks and minerals: www.smithsonianeducation.org/educators/lesson_plans/minerals/lesson1_ main.html. From this page, it is possible to go to other useful links about minerals. The U.S. Geological Survey also provides helpful information about minerals and their use as resources. This link is titled “Do We Take Minerals for Granted?” and can be accessed at http://minerals.usgs.gov/granted.html. Additional interesting mineral information can be found at this web page as well. Rocks Topic: minerals/rocks Go to: www.scilinks.org Code: PSCG222 222 A rock normally is an aggregate of mineral grains, although a few rocks are formed of fossils, fragments of fossils, or fragments of rocks. Different types of rocks form in different ways. Igneous rocks result from the solidification or crystallization of molten rock. They may form at Earth’s surface (where molten rock is called lava) or below Earth’s surface (where molten rock is called magma). Two common igneous rocks are granite and basalt. They differ in their mineral composition and mineral grain size. Granite forms several kilometers below Earth’s surface, where the insulating properties of the surrounding rocks cause magma to cool slowly. This allows for the growth of large mineral grains. Basalt forms at or close to Earth’s surface, where rapid heat loss results in relatively small mineral grains. Sedimentary rocks form from rock fragments, minerals, or fossils that are compressed beneath the weight of overlying sediments. They also form by chemical precipitation of minerals dissolved by water. Sedimentary rocks form at low temperatures at or very close to Earth’s surface. Common sedimentary rocks are sandstone, limestone, and shale. Metamorphic rocks are those in which the original mineral composition, grain size, or grain shape has changed as a result of exposure to moderately high temperatures, high pressure, or both. Most metamorphic rocks form below Earth’s surface. Marble, slate, and schist are common metamorphic rocks. Over time, or in response to changing conditions, rocks may change from one type into another. The rock cycle depicted in Figure R4.1 summarizes the steps by which one rock type transforms into another. The counterclockwise arrows indicate changes in rocks and rock materials that may take place. The words next to the arrows refer to processes by which the changes are accomplished. Shortcuts, indicated by interior arrows, may occur in this cycle. For example, igneous rocks can be metamorphosed to form metamorphic rocks by heat and pressure, or sedimentary and metamorphic rocks may undergo weathering to become sediments and/or dissolved mineral material. (The dissolution of rock material during weathering and the possibility of that material being precipitated to form sedimentary rock is not included on Figure R4.1 in order to keep the diagram relatively simple; examples of such chemically precipitated rocks are some limestones that form in oceans and salt [halite] deposits that can form in desert lakes or in an isolated part of an ocean.) National Science Teachers Association Copyright © 2011 NSTA. All rights reserved. For more information, go to www.nsta.org/permissions. Reading 4 Compaction and Cementation SEDIMENTARY ROCK Weathering, Erosion, and Deposition SEDIMENT Figure R4.1 Heat and Pressure Weathering, Erosion, and Deposition METAMORPHIC ROCK Melting Heat and Pressure MAGMA Weathering, Erosion, and Deposition IGNEOUS ROCK Cooling and Crystallization The rock cycle. Rock types plus magma and sediment are in capital letters. Processes that alter rocks or sediments are in capital and lower case. Arrows indicate how one type of rock material may change as geological conditions change. These changes need not occur, or they could take many millions of years to happen. Strictly speaking, before you can accurately identify a rock, you must know what minerals are in it. If a rock has decomposed or weathered, even geologists may have a difficult time recognizing those minerals and determining what rock it is—or what it used to be. It is usually easy to tell when rocks and minerals have been weathered; they may crumble when handled and their surfaces may be dirty or covered with what appear to be brownish stains. Typically, geologists examining a weathered rock will break it with a hammer to expose a fresh or unweathered surface. The geologists will then study the rock’s minerals with a magnifying glass and scratch grains with a knife blade to determine hardness. By identifying minerals, they can give the rock a provisional name. If the rock needs further study, geologists might use a specialized (petrographic) microscope using polarized light or analytical instruments that provide chemical data to identify the minerals and thus definitively name the rock. The identity of a rock or mineral usually provides clues as to the type of environment where it formed, which is often quite different from Earth’s surface. For example, a typical granite that is now exposed at Earth’s surface probably crystallized at a temperature of about 700°C, at high pressures, and at a depth within Earth of at least 4–10 km. That rock, now on the surface of Earth, was exposed when the overlying rocks were eroded and weathered away. This action is part of the uplifting process on Earth’s crust. Limestone provides another example of a rock that indicates what an environment was like in the past: A coral-containing limestone that is found today on land in a relatively cold climate originally formed in a warm, shallow ocean. (That is where we find most corals living today.) Assuming the limestone formed where it is now found, Project Earth Science: Geology, Revised 2nd Edition Copyright © 2011 NSTA. All rights reserved. For more information, go to www.nsta.org/permissions. 223 Reading 4 the environment at that location must have changed significantly since the time the rock formed. (An alternate explanation could be that something, perhaps a glacier, transported the rock a long distance to a new location with a different environment.) The identity of rock and mineral specimens also may provide information on the location of ancient plate boundaries. Unusual rocks and minerals may form at specific types of plate boundaries. For example, peralkaline volcanic rocks, which have an unusual chemical composition, currently are forming at several active divergent plate boundaries where continents are pulling apart (e.g., East Africa Rift). Rocks of similar composition that are approximately 700 million years old occur in the central and southern Appalachian Mountains. These rocks are believed to mark the time and location where an early supercontinent began to split into North America and Africa, with the Atlantic Ocean gradually forming between them. Many of the rock cycle processes occur as a result of plate motion and interaction. At subduction zones, where one plate slides beneath another, seafloor sediment on the underlying plate is carried along with oceanic crust down into the asthenosphere where some of the rock may remelt. Some of this material will recrystallize into new igneous rock. The heat that is released as the molten rock cools and recrystallizes metamorphoses adjacent rocks. High pressures created when two plates push together or “collide” also can metamorphose any sedimentary, igneous, and metamorphic rocks at the plate boundary. Conclusions Specific minerals and rocks form in response to specific environmental conditions. Locating and recognizing these minerals and rocks is how past environmental conditions on Earth are determined. 224 National Science Teachers Association Copyright © 2011 NSTA. All rights reserved. For more information, go to www.nsta.org/permissions.