Home » Rock Directory

Igneous Rock

A rock which originated as molten magma from beneath the earth's surface and subsequently came to the surface as an extrusion, or remained below ground as an intrusion.

Igneous rocks are formed from molten magma or lava. The word, igneous means "fire". All igneous rock starts deep in the earth as hot, molten magma. If the magma cools and hardens inside the earth it is called "intrusive" rock. These rocks cool slowly and have large crystals. When the magma comes out of the earth's crust through a volcano, it is called "extrusive". It cools off quickly, and the crystals that form are very small. Molten, or hot, liquid rock is called MAGMA when it is still inside the earth, but once it comes out through a volcano it is called LAVA.

The nature of the rock depends in part on the rate at which it cooled; as intrusions of magma slowly solidify, enough time elapses for large crystals to form whereas extrusions cool quickly, leaving little time for crystal growth. Thus, a coarse-grained, intrusive igneous rock has a fine-grained, extrusive counterpart; granite is coarse rhyolite and gabbro is coarse basalt. Igneous rocks are also classified as acid or basic, according to whether their silica content is high (e.g. granite), or low (e.g. basalt).

Any of various crystalline or glassy, noncrystalline rocks formed by the cooling and solidification of molten earth material (magma). Igneous rocks comprise one of the three principal classes of rocks, the others being metamorphic and sedimentary rocks. Though they vary widely in composition, most igneous rocks consist of quartz, feldspars, pyroxenes, amphiboles, micas, olivines, nepheline, leucite, and apatite. They may be classified as intrusive or extrusive rocks.

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 called fire rocks and are formed either underground or above ground. Underground, they are formed when the melted rock, called magma, deep within the earth becomes trapped in small pockets. As these pockets of magma cool slowly underground, the magma becomes igneous rocks. Igneous rocks are also formed when volcanoes erupt, causing the magma to rise above the earth's surface. When magma appears above the earth, it is called lava. Igneous rocks are formed as the lava cools above ground.

a) Have an IGNEOUS ROCKS QUIZ.

b) To contrast magma and lava see: Igneous Rocks

c) A summary of Igneous Rock.

[ Close ]