Igneous Rock

I

Introduction

Igneous Rock, rock formed when molten or partially molten material, called magma, cools and solidifies. Igneous rocks are one of the three main types of rocks; the other types are sedimentary rocks and metamorphic rocks. Of the three types of rocks, only igneous rocks are formed from melted material. The two most common types of igneous rocks are granite and basalt. Granite is light colored and is composed of large crystals of the minerals quartz, feldspar, and mica. Basalt is dark and contains minute crystals of the minerals olivine, pyroxene, and feldspar.

II

Types of Igneous Rocks

Geologists classify igneous rocks according to the depth at which they formed in the earth’s crust. Using this principle, they divide igneous rocks into two broad categories: those that formed beneath the earth’s surface, and those that formed at the surface. Igneous rocks may also be classified according to the minerals they contain.

A

Classification by Depth of Formation

Rocks formed within the earth are called intrusive or plutonic rocks because the magma from which they form often intrudes into the neighboring rock. Rocks formed at the surface of the earth are called extrusive rocks. In extrusive rocks, the magma has extruded, or erupted, through a volcano or fissure.

Geologists can tell the difference between intrusive and extrusive rocks by the size of their crystals: crystals in intrusive rocks are larger than those in extrusive rocks. The crystals in intrusive rocks are larger because the magma that forms them is insulated by the surrounding rock and therefore cools slowly. This slow cooling gives the crystals time to grow larger. Extrusive rocks cool rapidly, so the crystals are very small. In some cases, the magma cools so rapidly that crystals have no time to form, and the magma hardens in an amorphous glass, such as obsidian.

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One special type of rock, called porphyry, is partly intrusive and partly extrusive. Porphyry has large crystals embedded in a mass of much smaller crystals. The large crystals formed underground and only melt at extremely high temperatures. They were carried in lava when it erupted. The mass of much smaller crystals formed around the large crystals when the lava cooled quickly above ground.

B

Classification by Composition

Geologists also classify igneous rocks based on the minerals the rocks contain. If the mineral grains in the rocks are large enough, geologists can identify specific minerals by eye and easily classify the rocks by their mineral composition. However, extrusive rocks are generally too fine-grained to identify their minerals by eye. Geologists must classify these rocks by determining their chemical composition in the laboratory.

Most magmas are composed primarily of the same elements that make up the crust and the mantle of the earth: oxygen (O), silicon (Si), aluminum (Al), iron (Fe), magnesium (Mg), calcium (Ca), sodium (Na), and potassium (K). These elements make up the rock-forming minerals quartz, feldspar, mica, amphibole, pyroxene, and olivine. Rocks and minerals rich in silicon are called silica-rich or felsic (rich in feldspar and silica). Rocks and minerals low in silicon are rich in magnesium and iron. They are called mafic (rich in magnesium and ferrum, the Latin term for iron). Rocks very low in silicon are called ultramafic. Rocks with a composition between felsic and mafic are called intermediate.