Sedimentary rocks form at or near the earth's surface from the weathered remains of pre-existing rocks or organic debris. The term sedimentary rock applies both to consolidated, or lithified sediments (bound together, or cemented) and unconsolidated sediments (loose, like sand). Although there is some overlap, most sedimentary rocks belong to one of the following groups: clastic, chemical, or organic.
Mechanical weathering breaks up rocks, while chemical weathering dissolves and decomposes rocks. Weathering of igneous, metamorphic, and sedimentary rocks produces rock fragments, or clastic sediments, and mineral-rich water, or mineral solutions. After transport and laying down, or deposition, of sediments by wind, water, or ice, compaction occurs due to the weight of overlying sediments that accumulate later. Finally, minerals from mineral-rich solutions may crystallize, or precipitate, between the grains and act as cement. Cementation of the unconsolidated sediments forms a consolidated rock. Clastic rocks are classified based on their grain size. The most common clastic sedimentary rocks are shale (grains less than 1/256 mm in diameter), siltstone (1/256/16 mm), sandstone (1/16 mm), and conglomerate (greater than 2 mm).
Chemical or crystalline sedimentary rocks form from mineral solutions. Under the right conditions, minerals precipitate out of mineral-rich water to form layers of one or more minerals, or chemical sediments. For example, suppose ocean water is evaporating from an enclosed area, such as a bay, faster than water is flowing in from the open ocean. Salt deposits will form on the bottom of the bay as the concentration of dissolved minerals in the bay water increases. This is similar to putting salt water into a glass and letting the water evaporate; a layer of interlocking salt crystals will precipitate on the bottom of the glass. Due to their interlocking crystals, chemical sediments always form consolidated sedimentary rocks. Chemical rocks are classified based on their mineral composition. Rock salt (composed of the mineral halite, or table salt), rock gypsum (composed of gypsum), and crystalline limestone (composed of calcite) are common chemical sedimentary rocks.
Organic sedimentary rocks form from organically derived sediments. These organic sediments come from either animals or plants and usually consist of body parts. For example, many limestones are composed of abundant marine fossils, so these limestones are of organic rather than chemical origin. Coal is an organic rock composed of the remains of plants deposited in coastal swamps. The sediments in some organic rocks (for example, fossiliferous limestone) undergo cementation; other sediments may only be compacted together (for example, coal). Geologists classify organic rocks by their composition.
The origin (clastic, chemical, or organic) and composition of a sedimentary rock provide geologists with many insights into the environment where it was deposited. Geologists use this information to interpret the geologic history of an area, and to search for economically important rocks and minerals.
See also Depositional environments; Lithification; Mineralogy; Sedimentation; Stratigraphy
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