Igneous Rocks

Igneous rocks are formed by the cooling and hardening of molten material called magma. The word igneous comes from the Latin word igneus, meaning fire. There are two types of igneous rocks: intrusive and extrusive. Intrusive igneous rock forms within Earth's crust; the molten material rises, filling voids in the crust, and eventually hardens. Intrusive rocks are also called plutonic rocks, named after the Greek god Pluto, god of the underworld. Extrusive igneous rocks form when the magma, called lava once it reaches the surface, pours out onto the earth's surface. Extrusive rocks are also known as volcanic rocks.

Igneous rocks are classified according to their texture and mineral or chemical content. The texture of the rock is determined by the rate of cooling. The slower the rock cools, the larger the crystals form. Because the magma chamber is well insulated by the surrounding country rock, intrusive rocks cool very slowly and can form large, well developed crystals. Rapid cooling results in smaller, often microscopic, grains. Some extrusive rocks solidify in the air, before they hit the ground. Sometimes the rock mass starts to cool slowly, forming large crystals, and then finishes cooling rapidly, resulting in rocks that have larger crystals surrounded by a fine-grained matrix. This is known as a porphyritic texture. Other extrusive rocks cool before the chemical constituents of the melt are able to arrange themselves into any crystalline form. These are said to have glassy texture and include the rocks obsidian and pumice.

The chemistry of the magma determines the minerals that will crystallize and their relative abundance in the rocks that form. Light-colored igneous rocks are likely to contain high proportions of light colored minerals, such as quartz and feldspars and are called felsic. Dark rocks will contain iron and magnesium-rich minerals like pyroxene and olivine and are known as mafic rocks. Those rocks with a color falling between the two are said to have an intermediate composition.

Once the basic composition and texture of the rock are determined, they are combined to establish the name of the rock. For example, a coarse-grained, felsic rock is called granite and a fine-grained felsic rock is called rhyolite. These two rocks are composed of the same minerals, but the slow cooling history of the granite has allowed its crystals to grow larger. These are some of the most familiar igneous rocks because continental portions of the crust are built largely of rock that is similar in composition to these felsic rocks. Coarse-grained and fine-grained mafic rocks are called gabbro and basalt, respectively. Each of these is easily recognized by their dark color. In general, oceanic crustal plates are primarily mafic in chemistry. Diorite and andesite are the respective names for coarse- and fine-grained rocks of intermediate composition. While geologists sometimes use more detailed classification systems, this basic method is used for preliminary differentiation of crystalline igneous rocks.

Certain igneous rocks are named on the basis of particular features. Fragmental rocks like tuff and volcanic Rope-like, twisted lava like this is called pahoehoe. JLM Visuals. Reproduced by permission.

breccia are named on the basis of the size of particles of volcanic material ejected during an eruption. Tuff is composed of fine particles of volcanic ash, while breccia includes larger pieces. Obsidian, pumice, and often scoria have a non-crystalline, glassy texture that can be distinguished on the basis of the quantity of trapped gas. Obsidian contains no such gas and pumice has so many gas bubbles that it will sometimes actually float on water.

Earth's crustal plates are continually shifting, being torn open by faults, and altered by earthquakes and volcanoes. As old plates are drawn downward into the mantle, old rock material is recycled through melting. New igneous material is continually added to the crust along plate margins and other locations through igneous intrusions and volcanic activity. Igneous rocks represent both the ancient history of the formation of the earth and modern episodes of regeneration. Associated igneous processes are evidence of the continuing activity of Earth's interior and the form and composition of each of the igneous rocks give clues as to the conditions and processes under which they formed.