Sediments are usually formed from matter which falls to the bottom of oceans and lakes. The matter includes tiny pieces of other rocks, and dead animals, plants and microorganisms. The three most common sedimentary rocks are limestone, sandstone, and shale.
Sedimentary rocks cover 75–80% of the Earth’s land area, but they make up only 5% of the Earth’s crust. The relative abundances of the different types of sedimentary rocks are:
- All others—–5%
Squeezed and compressed over time, the sediments become ‘consolidated’ (made solid) into layers of rock.
Dissolved minerals drip into the cracks. When the water evaporates, the minerals become part of the rock. It is the sticking together of sediment that forms a rock.
Consolidated rocks may be changed some more, by water seeping through, or by heat and extreme pressure.
Types of sedimentary rock
Sediments are formed by three processes:
- Solids swept down from the land
- Bits & pieces laid down in water, e.g. shells
- Chemicals in water
Sedimentary rocks may be found anywhere on Earth.
Cross-bedding in a fluviatile sandstone, Middle Old Red Sandstone (Devonian) on Bressay, Shetland Islands