About Sedimentary Rock
Sedimentary rocks are one of the three basic types of rocks along with igneous and metamorphic rocks. Sedimentary rocks form from broken pieces of other rocks, from minerals, or from the remains of living creatures and plants. Compacted by weight or cemented together, each sedimentary rock tells the story of the world as it once was.
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History
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Sedimentary rocks formed when small pieces of sand, silt and clay are formed when the pieces were compacted by weight or cemented together by materials, such as minerals that dissolve in water. They are cool rather than hot since they form on the earth's suface, usually in underwater areas.
Types
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There are three types of sedimentary rocks. Clastic sedimentary rocks are made of particles that are named according to the grain size. Clay are the smallest, then silt, sand and pebbles. Biologic, or organic sedimentary rocks form when living organisms die and are compressed and cemented together. Organic sedimentary rocks are made of remains such as plants, dinosaur bones, plankton skeletons, coral, clamshells and insects. Chemical sedimentary rocks form when seas dry up and minerals come from the deposits. They're made of mineral crystals.
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Function
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Sedimentary rocks make up 3/4 of the surface of the earth. They can explain what conditions were like when they were formed. They can tells us what animals and plants existed at different times. They let us know what the world was like in the past. Fossils, water current marks, mud cracks and microscopic features give clues about the world in the past.
Geography
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Since the sea used to be hundreds of meters higher than now, sedimentary rock was exposed as the sea becoame shallower. Areas such as the central United States contain a lot of sedimentary rock like sandstone, limestone, shale and coal. Sedimentary rock also becomes visible when land rises around the lithospheric plates on the earth.
Identification
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Clastic sedimentary rocks, made of smaller rocks cemented together, usually have layers and sometimes have fossils. They form along beaches and rivers or underwater in lakes and oceans. Conglomerate is made of pebble-sized or larger pieces, sandstone is made of sand-sized pieces, and shale is made of pieces smaller than sand in visible layers. Chemical sedimentary rocks are usually light gray, sometimes containing crystals. Limestone is white or gray in color and is made of the mineral calcite. You can see the remains of living organisms in organic sedimentary rocks. Fossils often have patterns or holes that are the remains of the creature or plant that has become part of the sedimentary rock.
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