|
Igneous Igneous rocks are formed from molten magma and are divided into two main categories: plutonic rock and volcanic. Plutonic or intrusive rocks result when magma cools and crystallises slowly within the Earth's crust, while volcanic or extrusive rocks result from magma reaching the surface either as lava (when the molten rock is in the mantle, it is called magma, when it is above the crust, it is called lava) or fragmental ejecta. Intrusive igneous rocks usually take a few thousand years to cool whereas extrusive igneous rocks take only a few days or weeks to cool and solidify. Sedimentary Sedimentary rocks are formed by deposition of either clastic sediments, organic matter, or chemical precipitates (evaporites), followed by compaction of the particulate matter and cementation during diagenesis. Sedimentary rocks form at or near the Earth's surface when the pressure of the surrounding crust forces the rock together. Sedimentary rock is also common under the ocean as the pressure of the sea can also force different rocks together. Metamorphic Metamorphic rocks are formed by subjecting any rock type (including previously-formed metamorphic rock) to different temperature and pressure conditions than those in which the original rock was formed. These temperatures and pressures are always higher than those at the Earth's surface and must be sufficiently high so as to change the original minerals into other mineral types or else into other forms of the same minerals (e.g. by recrystallisation). The transformation of one rock type to another is described by the geological model called the rock cycle. Impact on Society Rocks have had a huge impact on the cultural and technological advancement of the human race. The mining of rocks for their meta ore properties has been one of the most important factors of human advancement. Humanity's advancement has been decided by the kind of metals available to them, which are obtained from rocks. examples of this is the iron age, bronze age and copper age. For many thousands rocks have also been used to construct housing buildings and infrastructure. See also | ||||||||||
|
| |||||||||||
 |
|
| |