Sedimentary rocks are formed from the weathered products of pre-existing rocks or pieces of once-living organisms that accumulate on the Earth’s surface.
The starting material for sedimentary rock formation is sediment which is simply loose fragments of rock or material created by chemical, mechanical or biological processes.
The stages in the formation of a sedimentary rock are:
- Weathering – existing rocks are broken down by chemical and mechanical processes to produce both rock fragments and chemicals dissolved in water.
- Erosion and Transportation– the weathered rock fragments are removed from where they were formed and transported to a new location. Moving water, wind, ice, gravity and biological processes are the main ways that weathered rock is transported.
- Deposition – the weathered rock material eventually stops moving and builds up in a suitable location. This could occur where fast flowing water slows down or where biological processes extract chemicals from the water.
- Burial and Compaction – In the right circumstances, thick piles of sediment can accumulate and the lower sediments in the pile are compacted by the weight of the overlying material.
- Lithification (cementation) – is the process that turns sediment into rock. Once the sediments have been compacted, new minerals grow to cement the particles together to form a solid rock.
There are two basic types of sedimentary rocks:
- Clastic Sedimentary Rocks – composed of fragments (clasts) of pre-existing rocks that have been compacted and cemented together.
- Biogenic and Chemical Sedimentary Rocks – chemicals dissolved in water that are deposited as solid materials by physical, chemical or biological processes.
Learn more about the 5 stages of sedimentary rock formation here (3m 35s):
Learn more about sedimentary rock classifcation here (8m 39s):
Courtesy of Mike Sammartano
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