What is Rock (geology)?
[[File:Trwyn y Fulfran, Lleyn - geograph.org.uk - 27152.jpg|thumb|250px|[[Strata]] are the layers of rock, originally laid down [[horizontal]]ly. This is from Trwyn y Fulfran, [[Lleyn]], [[Wales]].]] [[File:DirkvdM rocks.jpg|thumb|250px|The rocky side of a mountain creek near Orosí, [[Costa Rica]].]] Rocks are a [[nature|naturally]] occurring solid. Rocks are made of collections of [[mineral]] fruits that are held together in a firm, solid mass. They are made of minerals (which are [[crystalline]]), or other mineral-like substances. The Earth's outer solid layer, the [[lithosphere]], is made of rock. That means the [[Earth's crust]] is made of rock. The different minerals in the rocks make different kinds of rock. Rock is often covered by [[soil]] or water. It is beneath the oceans, lakes, and rivers of the earth, and under the polar [[ice]]caps. [[Petrology]] is the scientific study of rocks. == Rock classification == Rocks are classified by their minerals and [[chemical]] make-up. The processes that formed them are also noted. Rocks may be [[igneous rock|igneous]], [[sedimentary rock|sedimentary]] and [[metamorphic rock|metamorphic]]. Rock types may change in a so-called '[[rock cycle]]'. === Igneous rocks === :''For the main article, see [[Igneous rock]]'' [[Igneous rock]]s are formed when molten [[magma]] cools, either above or below the surface. They are divided into two main categories: [[Intrusion|plutonic rock]] and [[volcano|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]]).<ref name=Blatt>Blatt, Harvey and Tracy, Robert J. 1996, ''Petrology''. 2nd ed, Freeman. {{ISBN|0-7167-2438-3}}</ref> There are over 700 hundred types of igneous rocks, and they are usually the hardest and heaviest of all rocks. === Sedimentary rock === :''For the main article, see [[Sedimentary rock