What is Igneous Rock?
Igneous rock forms when molten minerals cool from a liquid into a solid. Above ground, the molten rock takes the form of lava spewing from volcanic fissures while below ground, magma cools in small caves of air. These processes form volcanic or plutonic rock, respectively. Igneous comes from the root word "ignis," which means, "of fire," which differentiates this form of rock from sedimentary or metamorphic rock.
The asthenosphere, part of the earth's mantle, is made of liquefied rock known as magma that churns at temperatures between 1100-2400° F (590-1300° C). Sometimes the magma reaches hollow parts in cooler rocks through the movement of tectonic plates. It slowly cools, hardening and crystallizing into intrusive rock also called plutonic igneous. In more dramatic geologic shifts, volcanoes eject lava to the open air where it cools much more quickly into extrusive, volcanic igneous.
Different types of rocks can be identified by their composition, the kind of minerals they join, and their density. Examples of extrusive igneous rock are basalt, pumice, andesite, scoria, and obsidian. Igneous rock, since it cools so quickly, traps air bubbles and doesn't have much time to create larger lumps of mineral. In fact, many volcanic rocks are primarily silica, a kind of glassy sand. As children, we are astonished to discover that a rock called pumice can be so light it floats on water.
Some intrusive rocks are granite, diorite, rhyolite, and gabbro, with finer grain and lower density. Plutonic types tend to have coarse grains because their slow formation allows the minerals to crystallize into larger hunks. You can often see their constituent minerals by using a magnifying glass to look closely, like at a granite monument. Porphyry starts by cooling slowly and then reaches the earth's surface where it cools quickly, so it is a mixture of intrusive and extrusive.
The three compositional types of igneous rock are mafic, felsic, and intermediate. Mafic rocks are made of the minerals pyroxene, olivine, and feldspar. Like flaked obsidian, they have dark colors like green and black. Combinations of feldspar and quartz create felsic rocks in much lighter colors, like white or pink, that sparkle in the light. Intermediate types lie somewhere in the middle, with medium shades of grey and green made of amphibole, feldspar, and biotite.
Written by S. Mithra
