Rocks are classified by their minerals and chemical make-up. The processes that formed them are also noted. Rocks may be igneous, sedimentary and metamorphic. Rock types may change in a so-called 'rock cycle'.
- For the main article, see Igneous rock
Igneous rocks are formed when molten magma cools, either above or below the surface. They are divided into two main categories: plutonic rock and volcanic rock. Plutonic or intrusive rocks are made when magma cools and crystallizes slowly within the Earth's crust (example granite). Volcanic or extrusive rocks result from magma reaching the surface either as lava or ejecta (examples pumice and basalt).
- For the main article, see Sedimentary rock
Diagram showing the placement, age and thickness of the rock units exposed in the Grand Canyon
Sedimentary rocks are the most common rocks on Earth. They form at or near the Earth's surface. Sedimentary rock is formed in layers which were laid down one by one on top of another. Some of the layers are thin, some are thick. Layers are made by deposition of sediment, organic matter, and chemical precipitates.
Deposition is followed by squeezing of sediment under its own weight, and cementation. This process is called 'consolidation': it turns the sediment into a more or less hard substance.
The approximate amounts of different kinds of sedimentary rock are:
- Shale (including mudstone, and siltstone): 60%
- Sandstones 20%.
- Carbonate rocks (limestone and dolomite): 15%.
- All others: 5%.
Only sedimentary rocks have fossils.
- For the main article, see Metamorphic rock
Metamorphic rocks are formed by rocks coming under great pressure and high temperatures. These temperatures and pressures are found under mountains and volcanoes, especially when continental plates move together. These conditions change the make-up of the original minerals.