Igneous rocks form by the cooling and solidification of molten rock, called magma.
Igneous rocks have a wide range of compositions from acidic to basic depending on the tectonic setting in which they form. (See figure: Classification of Igneous Rocks, under the Reference Material tab.)
Igneous rocks are produced in a number of tectonic settings:
- Mid-ocean ridges
- Subduction zones
- Above hot-spots (mantle plumes); continental and oceanic
Magma is formed at depth within the earth and moves upwards, towards the surface, because it has a lower density than the surrounding solid rock. It will either erupt onto the surface of the Earth, or solidify en route, depending on the relative densities of the magma and the surrounding rock, and on the thermal properties of the magma.
Magmas which solidify at the surface (in air, ice or water) cool rapidly, producing many small crystals; fine-grained extrusive rocks are produced. If they cool very rapidly a glass (amorphous, non-crystalline solid) may be formed.
Magmas solidifying at depth cool more slowly, growing larger crystals; coarse-grained intrusive rocks are formed.