Image:North america rock volcanic.jpg|Volcanic rock in North America. Image:North america rock plutonic.jpg|Plutonic rock in North America.

Igneous rocks (etymology from Latin ignis, fire) are rocks formed by solidification of cooled magma (molten rock). They may form with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. This magma can be derived from partial melts of pre-existing rocks in either the Earth's mantle or crust. Typically, the melting is caused by one or more of the following processes — an increase in temperature, a decrease in pressure, or a change in composition. Over 700 types of igneous rocks have been described, most of them formed beneath the surface of the Earth's crust. These have diverse properties, depending on their composition and how they were formed.

Geologic significance

Igneous rocks make up approximately ninety-five percent of the upper part of the Earth's crust, but their great abundance is hidden on the Earth's surface by a relatively thin but widespread layer of sedimentary and metamorphic rocks.

Igneous rocks are geologically important because:

• their minerals and global chemistry give information about the composition of the mantle, from which some igneous rocks are extracted, and the temperature and pressure conditions that allowed this extraction, and/or of other pre-existing rock that melted;
• their absolute ages can be obtained from various forms of radiometric dating and thus can be compared to adjacent geological strata, allowing a time sequence of events;
• their features are usually characteristic of a specific tectonic environment, allowing tectonic reconstitutions (see plate tectonics);
• in some special circumstances they host important mineral deposits (ores): for example, tungsten, tin, and uranium are commonly associated with granites, whereas ores of chromium and platinum are commonly associated with gabbros.

Morphology and setting

In terms of modes of occurrence, igneous rocks can be either intrusive (plutonic), extrusive (volcanic) or hypabbysal.

Intrusive igneous rocks

The central cores of major mountain ranges consist of intrusive igneous rocks, usually granite. When exposed by erosion, these cores (called batholiths) may occupy huge areas of the Earth's surface.

Coarse grained intrusive igneous rocks which form at depth within the earth are termed as abyssal; intrusive igneous rocks which form near the surface are termed hypabyssal.

Extrusive igneous rocks