sedimentary rock

Sedimentary rock is one of the three main rock types (the others being igneous and metamorphic rock). Sedimentary rock is formed by deposition and consolidation of mineral and organic material and from precipitation of minerals from solution. The processes that form sedimentary rock occur at the surface of the Earth and within bodies of water. Rock formed from sediments covers 75-80% of the Earth's land area, and includes common types such as limestone, chalk, dolostone, sandstone, conglomerate, some types of breccia, and shale.^[1]

Sedimentary rocks are classified by the source of their sediments, and are produced by one or more of:

• clastic rock formed from fragments broken off from parent rock, by
  • weathering in situ or
  • erosion by water, ice or wind

followed by transportation of sediments, to the place of deposition;

• biogenic activity; or
• precipitation from solution.

The sediments are then compacted and converted to rock by the process of lithification.

Sedimentary rocks are formed because of the overburden pressure as particles of sediment are deposited out of air, ice, wind, gravity, or water flows carrying the particles in suspension. As sediment deposition builds up, the overburden (or 'lithostatic') pressure squeezes the sediment into layered solids in a process known as lithification ('rock formation') and the original connate fluids are expelled. The term diagenesis is used to describe all the chemical, physical, and biological changes, including cementation, undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface weathering.

Sedimentary rocks are laid down in layers called beds or strata. That new rock layers are above older rock layers is stated in the principle of superposition. There are usually some gaps in the sequence called unconformities. These represent periods in which no new sediments were being laid down, or when earlier sedimentary layers were raised above sea level and eroded away.

Sedimentary rocks contain important information about the history of Earth. They contain fossils, the preserved remains of ancient plants and animals. Coal is considered a type of sedimentary rock. The composition of sediments provides us with clues as to the original rock. Differences between successive layers indicate changes to the environment which have occurred over time. Sedimentary rocks can contain fossils because, unlike most igneous and metamorphic rocks, they form at temperatures and pressures that do not destroy fossil remains.

The sedimentary rock cover of the continents of the Earth's crust is extensive, but the total contribution of sedimentary rocks is estimated to be only 5% of the total. As such, the sedimentary sequences we see represent only a thin veneer over a crust consisting mainly of igneous and metamorphic rocks.