Molten rock inside the crust of a planet that is capable of intruding into adjacent rocks or extruding towards the surface. Igneous rocks are derived from magma through solidification and associated processes or by the eruption of magma on the surface.

Depending on the conditions under which magma cools, the resulting rocks may have coarse or fine granules.

Igneous rocks are subdivided into two large groups: plutonic or intrusive rocks, formed from a slow and deep cooling of magma; and volcanic or extrusive rocks formed by rapid and surface cooling, or near it, of magma.

Magma and gases break the weakest areas of the Earth's outer crust or lithosphere to reach the surface. These weaknesses are found mostly along the boundaries between tectonic plates, which is where most of the volcanism is concentrated. When magma and gases reach the surface through chimneys or fissures in the crust, they form geological structures called volcanoes.

◄ PreviousNext ►
Magellan (Clouds of)Stellar magnetism