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Sedimentary rocks

Note: All pictures are meant as guides only, as often the difference between one sedimentary rock type and another is only in the size of the grains.

On the surface of the Earth, wind, water and ice weather and erode away rocks, the particles of rock, minerals and the remains of plants and animals. These transported particles eventually accumulate as sediments in low lying areas such as beaches, lakes, rivers, seas or even the bottom of the ocean.

Here the particles settle, compact under pressure and harden into sedimentary rocks as they are buried under more and more sediment which accumulates on top of them. This process may occur over thousands or even millions of years and produce thicknesses of hundreds of metres of sedimentary rocks.

Of all the different types of rock, sedimentary rocks are probably the easiest to identify. They usually form in layers, are soft and easy to scratch and tend to crumble more easily than other rock types. Fossils - the remains of plants and animals - are only found in sedimentary rocks.

Classification
Sedimentary rocks are classified according to how they physically form. This is reflected in easily identifiable properties such as composition and texture.

Clastic rock is the most common sedimentary rock formed from particles of mineral and rock. Particles range in size from large boulders through sand to fine silt and clay. Conglomerate and breccia form from compacted gravel and boulders; sandstone is derived from compacted sand; mudstone and shale form when fine silt and clay are compacted.
Organic rock forms from the remains of living things. The most common examples are coal (decayed remains of land plants) and limestone (calcium carbonate remains of ocean organisms such as corals and shells).
Chemical rock forms from minerals as they come out of solution and crystallise. Examples include rock salt (halite) and chert.
Pyroclastic rock forms when ash and boulders are blasted out of a volcano in an explosive eruption and settle as sediments. Ash compacts to form tuff whilst boulders form volcanic breccia. The Easter Island statues where carved out of tuff.
Residual rock forms when existing rock is weathered where it sits removing some mineral components whilst others are left behind. Some important minerals are formed in this way, for example bauxite is formed in tropical climates from when weathering removes other components, leaving iron oxide behind.

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	FAQ:
			How can sand on a beach turn into a rock? Some sedimentary rocks have beautiful bands of colour. How do these bands form? Why is there so much variety in the colour of sedimentary rocks?


	Related Topics:
			Weathering Erosion Deposition The structure of the Earth Earthquakes Volcanoes Tsunamis Rocks Igneous rocks Metamorphic rocks Building with rocks


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			Question 1 Question 2 Question 3 Question 4


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