Igneous rocks, also called volcanic rocks, are formed from melted rock that has cooled and solidified. When rocks are buried deep within the Earth, they melt because of the high pressure and temperature; the molten rock (called magma) can then flow upward or even be erupted from a volcano onto the Earth's surface. When magma cools slowly, usually at depths of thousands of feet, crystals grow from the molten liquid, and a coarse-grained rock forms. When magma cools rapidly, usually at or near the Earth's surface, the crystals are extremely small, and a fine-grained rock results. A wide variety of rocks are formed by different cooling rates and different chemical compositions of the original magma. Obsidian (volcanic glass), granite, basalt, and andesite porphyry are four of the many types of igneous rock.
Common
igneous (volcanic rocks) are basalt, andesite, and rhyolite. When
magmas crystallize deep underground they look different from volcanic
rocks because they cool more slowly and, therefore, have larger crystals.
Igneous rocks cooled beneath the Earth's surface are called intrusive
rocks. The intrusive equivalents of basalt, andesite, and rhyolite are
gabbro, diorite, and granite, respectively.
Information provided by: http://interactive2.usgs.gov