EXTRUSIVE and INTRUSIVE VOLCANIC ROCKS
Extrusive Igneous Rocks (Volcanic Rocks):
Volcanic rocks (also called extrusive igneous rocks) include all the products resulting from eruptions of lava. Igneous rocks form from melted rock, called magma. On the surface of the Earth, igneous rocks usually occur in a cooled state, except where molten rock is erupting from an active volcano. Magma that flows out onto the Earth's surface is called lava. Rocks that crystallize quickly from cooling lava at or near the Earth's surface are called volcanic rocks. Common volcanic rocks are basalt, andesite, and rhyolite.
Intrusive Igneous Rocks (Plutonic Rocks):
Plutonic rocks (also called intrusive igneous rocks ) are those that have solidified below ground. 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.
See Also: Igneous Rocks
-- Excerpts from:
USGS/NPS Geology of the Parks Website, September 2001; Barker, 1997, Collecting Rocks: USGS General Interest Publication; and the Volcano Hazards Program Website Photoglossary, July 2001