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Sedimentary Rocks
Sedimentary rocks form from
sediments.
While 90% of the Earth's crust is made of igneous rock, on the very surface sedimentaries cover about 80% of the Earth's surface.
They are formed near the Earth's surface, where sediments accumulate on the sea/ocean bottom, in riverbeds, in deserts and on beaches.
Newer layers push the older ones down, which lithify under the pressures from the layers above.
Clastic sedimentaries are formed from rock matter eroded by physical weathering.
Chemical sedimentaries are formed from rock material eroded by chemical weathering.
Biochemical and organic sedimentaries are formed from parts of dead plants and animals.
Another way of grouping them is by grain size.
Because the sediments are transported before they are deposited, they are sorted by grain size. That is why the new rocks tend to have quite uniform grain size.
By Mark via Flickr.com
Sedimentaries are very valuable for scientists when working out the geological time scale.
The presence of fossils in some sedimentaries is a big help there, not only with relative ages, but also because it makes it possible to work out in what conditions a rock formed.
Sedimentaries are the only type of rocks that contain fossils, - igneous rocks never do, and metamorphics can do very rarely.
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