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Sedimentary Rock Formation
Sedimentary rock formation
happens from sediments.
It happens wherever sediments are accumulated in the bottom of water bodies, or deposited by winds in deserts or moving ice sheets in polar regions.
First, the sediment material is formed from rock by erosion, or from the bones or shells of dead organisms.
The eroded material is always carried by wind, water or moving ice.
All three carriers do the same - they sort the eroded material by grain size. Heaviest particles are dropped first, while the lightest ones travel furthest in wind or water stream, or under moving ice, until they are finally deposited.
Over millions of years, the deposits accumulate. As new layers are laid on the top, the older ones get pushed deeper down, where rising pressures and temperatures help to turn the sediments into rock in a process called lithification. In the process, water is squeezed out of the rocks, so they dry and harden.
Powdery sediments turn into rock by compaction only. Sandy sediments, which contain the hard particles of quartz, need to be cemented togehter by cements such as calcite, clay minerals, iron compounds or silicate minerals.
Because of this, and because the sediments are first sorted by grain size, sedimentary rocks generally tend to have quite homogenous grain size. The fine grained ones particularly tend to be uniform in grain size, while larger ones such as conglomerates and breccias have large particles, but also fine grained cement, because they needed to be cemented together.
If fossils happen to form and be buried under layers, they will be embedded in sedimentary rock. This is the only type of rock that can have fossils. Igneous rocks never have, while metamorphics theoretically can but very seldom do in practice.
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