Also called fire rocks, igneous rocks are rocks formed by the cooling and solidification of molten rock either below or above the earth's surface. The root of the term "igneous" actually means fire- a derivative of Latin and Greek. Igneous rock is one of three broad classifications of rock types; sedimentary rock and metamorphic rock comprise the two other major rock-types.
When magma (molten rock below the earth's surface) becomes trapped in underground pockets, it cools to form intrusive igneous rock. Lava (erupted magma) breaches the earth's surface as a result of volcanic activity. Extrusive igneous rock forms when lava cools above the earth's surface- a process that is much faster than intrusive igneous rock formation. This type of rock is also called volcanic rock- being a direct result of volcanic activity. Plutonic rock is a type of igneous rock formed by deep magma under the earth's surface.
Igneous rock can have a smooth, glassy appearance or a crystallized appearance with a wide range of textures as well. The appearance is based on conditions surrounding rock formation, chemistry and mineral content. Extrusive igneous rock tend to be much smoother with a uniform layer because of the faster cooling process that precludes crystallization. As a result, extrusive igneous rock has an "aphanitic" appearance- a term derived from Greek meaning invisible or inconspicuous.
Intrusive igneous rocks cool more slowly because of the higher temperature and pressure underground. Intrusive igneous rock has a "phaneritic" appearance since the crystals are visible to the naked eye. The differences in surrounding conditions for igneous rock formation significantly influence the properties of the rock. The mineral content, colour and texture of igneous rocks contribute to their classification as well.
Popular examples of igneous rock include coarse granite, fine-grained basalt and the glassy obsidian rock. Igneous rocks are varied because they form at different layers under the earth's surface. Even above the earth's surface, differences emerge based on the viscosity of lava and its distribution over the earth's surface. Igneous rock that was formed hundreds of thousands of years ago can appear above the earth's surface because of centuries of weathering that exposes the subterranean rock. This is the main reason for the formation of volcanic arcs with intermediate igneous rock.
Igneous rocks are a common form of rock because its formation is a direct result of plate tectonic activity, which occurs in several different parts of earth. The range of these rocks is impressive, owing to the combination of minerals, conditions and processes that affect this rock's formation.
For a more detailed view of igneous rock formation and classification, check the sources for this article below.