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
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.