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SEDIMENTARY ROCKS

Sedimentary rocks are the most common rock type on the earth surface. As you can see on this map, compared to the other rock types, sedimentary rocks are the most widespread. If you've ever held a piece of sandstone, walked into a concrete building decorated with limestone blocks, or sprinkled salt on your food, you've encountered sedimentary rock.

These rocks are the result of the deposition and subsequent cementation of material (sediment), or are formed by chemical processes. Pieces of rock as big as boulders or as small as grains of dust can make up a sedimentary rock. These pieces, called clasts, can be carried by wind, water, or glaciers. As they are deposited and accumulate into layers called strata, water creeps into the spaces between clasts. The water can carry dissolved chemicals that dry into cement, gluing the clasts together to form a rock.

Often, clasts of a certain size will become cemented together into a single layer of rock. The different sizes fall into specific categories, from which a general rock name is derived. Thus, mud, the smallest particle sizes, turns into mudstone and shale. The next largest, silt, forms siltstone. Sand makes up sandstone. The three largest clast types, pebbles, cobbles, and boulders, make up rocks called conglomerates (if the clasts are rounded) or breccia (if they are angular).

Certain sedimentary rocks don't come from clasts, but have chemical sources. Some of those have inorganic origins. Examples include evaporites, salts that remain behind when a body of water evaporates. Some of these salts are quite useful, such as halite, used as table salt, and gypsum, which is used in plaster. Other chemically deposited rocks come from organic sources. These include calcium carbonate structures such as shells and coral reefs that are consolidated into limestone and the calcium- or silicon-based "oozes," sediments made up of dead microorganisms that settle to the ocean floor.

Sedimentary rocks also play an important role in the development of natural resources. Plant and animals can be mixed into sediment and preserved as fossils. When that sediment turns into rock, the organic material can, over millions of years, be transformed into fossil fuels such as oil and coal.