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How are sedimentary rocks made?

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Quick Answer

Sedimentary rocks form from the deposition and compression of inorganic and organic material on land or in water. Over 75 percent of the Earth's crust contains sedimentary rock.

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Sedimentary rocks begin as sand, mud, clay or even organic material; the type of material determines the specific type of sedimentary rock. The material breaks away through weathering as erosion carries and deposits the material in another location. Over time, these materials pack together and pressure builds. With pressure comes low-level heat, approximately 200 degrees Fahrenheit. The combination of pressure and heat cements the material together to form solid rock. Sand forms sandstone while clay forms shale. Limestone originates from the remains of dead sea life.

There are three basic types of sedimentary rock: clastic, organic and chemical. Clastic sedimentary rocks form from eroded material from the Earth's crust. Sandstone and shale belong to this group of rocks. Organic sedimentary rocks originate from the remains of living things, both plant and animal. Some rocks form from the leftover shells of marine organisms. Others, like coal, come from layers of dead plant matter. The settling of minerals in shallow water results in chemical sedimentary rock. Some limestones and cherts form in this way; rock salt and gypsum are also chemical sedimentary rocks.

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