A rock is a naturally occurring solid. It is made of minerals (which are crystalline), or other mineral-like substances. The minerals in the rocks vary, making different kinds of rock. The Earth's outer solid layer, the lithosphere, is made of rock. That means the Earth's crust is made of rock.
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
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.
Impact on human life
Rocks have had an impact on human life. They have been used by humans for over 2 million years. The mining of rocks for their metal has been one of the most important things in human advancement.
- Blatt, Harvey and Tracy, Robert J. 1996, Petrology. 2nd ed, Freeman. Template:Catalog lookup linkScript error: No such module "check isxn".
- "Sediment and sedimentary rocks." .