Igneous rock

Igneous rock (from the Greek word for fire) are rocks that form from when hot, molten rock (magma) crystallizes and solidifies. The melt originates deep within the Earth near active plate boundaries or hot spots, then rises toward the surface.

Igneous rocks are categorized by mineral composition as well as by the rock's crystal structure. Generally, rocks which have cooled quickly have finer, less developed crystals composing the rock, while rocks which have cooled slowly have had a chance to form larger crystals. The rate of cooling is often determined by whether the magma or lava cooled on the surface (extrusive) or under the ground (intrusive/plutonic).

Types of Igneous Rocks
Igneous rocks are divided into two groups, intrusive or extrusive, depending upon where the molten rock solidifies.

Extrusive
Extrusive, or volcanic, igneous rock is produced when magma exits and cools outside of, or very near the Earth's surface. These are the rocks that form at erupting volcanoes and oozing fissures. The magma, called lava when molten rock erupts on the surface, cools and solidifies almost instantly when it is exposed to the relatively cool temperature of the atmosphere.

Quick cooling means that mineral crystals don't have much time to grow, so these rocks have a very fine-grained or even glassy texture. Hot gas bubbles are often trapped in the quenched lava, forming a bubbly, vesicular texture. Pumice, obsidian, and basalt are all extrusive igneous rocks.

Extrusive igneous rocks include:
 * basalt
 * andesite
 * rhyolite

Intrusive
Intrusive, or plutonic igneous rock forms when magma is trapped deep inside the Earth. Great globs of molten rock rise toward the surface. Some of the magma may feed volcanoes on the Earth's surface, but most remains trapped below, where it cools very slowly over many thousands or millions of years until it solidifies. Slow cooling means the individual mineral grains have a very long time to grow, so they grow to a relatively large size. Intrusive rocks have a coarse grained texture. The image at right shows granite, an intrusive igneous rock.

Intrusive rocks include:
 * gabbro
 * diorite
 * granite

Additional information

 * Igneous rock by Wikipedia