Earth scientists divide rocks into three broad categories based on how a rock forms. These three categories are called igneous, sedimentary, and metamorphic rocks. Although sedimentary rocks are the most common type at the earth’s surface, igneous rocks are by far the most common in the earth as a whole, and make up the oldest rocks on the planet.
ORIGIN OF IGNEOUS ROCKS
The name igneous comes from the Latin word “ignis,” which means “fire.” These rocks earned the name because they include hardened remnants of glowing, white-hot lava early scientists watched erupt from fiery volcanoes. Such dramatic events illustrate a trait that all igneous rocks share: they harden, or crystallize, from super-hot liquid. When that melted rock reaches the earth’s surface, it’s called lava. The great majority never reaches the surface, however; and molten rock that remains deep underground is called “magma.” Both forms have a temperature of many hundreds of degrees.
CLASSIFYING IGNEOUS ROCKS
Igneous rocks are a general type, and rocks that crystallize from a melt (molten rock) are known by hundreds of different names. Naming of an igneous rock is based on two characteristics. The first is what geologists call texture, which in the case of igneous rocks means crystal size. Rocks with mineral crystals big enough to see with the naked eye are called “intrusive.” Formation of relatively large crystals means that they cooled slowly, while the molten rock was deep underground where it remained insulated from cool conditions at the surface. These rocks crystallize from magma in giant chambers miles underground, often below volcanic chains. The most familiar of these rocks is granite, a pinkish stone used on the outside of buildings and in counter tops.
The other main division of igneous rocks flood from volcanoes and other locations where lava reaches the surface. These rocks cool much faster, so fast that their crystals are too small to see without a magnifying glass. The name geologists have given to this igneous rock subtype is “extrusive,” because they were extruded or squeezed out onto the surface. The most familiar extrusive igneous rock is dark gray to black basalt.
Geologists also identify other less-common igneous rock textures. One is volcanic glass, which cools so rapidly that it lacks individual mineral grains. The most common glass is called obsidian. Sometimes lava pours into open water, which explodes into steam on contact. The resulting glassy rock is almost entirely frozen bubbles, so this rock light enough to float. Geologists name this rock pumice.
Pegmatite is another special rock, because it is made up of large crystals – sometimes the size of a railroad boxcar. It’s also special because many minerals in pegmatite contain rare or unusual elements. Thin veins of pegmatite can extend for long distances, and often contain minerals with valuable ores of gold and other metals.
COMPOSITION OF IGNEOUS ROCKS
Composition, or mineralogy, is the second quality scientists use to classify igneous rocks. The simplest division is into “mafic” and “felsic” igneous rocks. Mafic rocks are rich in minerals that have large amounts of magnesium and iron; such as olivine, biotite, and pyroxene. These minerals make the rocks dark in color; mostly dark gray, brown, or black. Felsic rocks contain minerals with more aluminum and silicon; such as muscovite, potassium feldspar, and quartz. Felsic rocks are light-colored; usually pink or light gray. Felsic and mafic rocks are divided even further depending on the minerals that are present. Precisely classifying an igneous rock requires careful study of its minerals.
When both composition and texture of an igneous rock are known, a geologist can assign it a name. For instance, a dark, fine-grained rock is likely basalt – a mafic extrusive igneous rock. A light, coarse-grained igneous rock may be granite – a felsic intrusive rock. To a geologist, then, those names say a lot about the conditions under which an igneous rock formed.
MELTED ROCK? HOW DOES THAT HAPPEN?
The principle of plate tectonics tells us that the earth’s surface is in constant motion – slow motion, but over millions of years even slow motion covers long distances. The theory explains that the surface is made of many large plates jostling against each other. Where two plates move toward each other, one – usually an oceanic plate – slips under the other. This motion drives the cool edge of the plate deep into the heat underneath the other plate, and everything on the plate goes along for the ride. It’s much like a giant conveyor belt.
When the cool rocks reach the high temperatures (and pressure) at depth, they begin to melt. This melted rock is not as dense as the rocks around it, and so a bubble of magma begins to rise. Since oceanic plates are mostly mafic (dark) rocks, the magma is also mafic. Basaltic extrusive rocks are common along these plate boundaries, in places like Japan or Indonesia.
When two continents collide, the result is a huge crumpled pile. The mass of this pile presses the underside of the continent downward to where it begins to melt as well, and form its own bubble of magma. Since continental crust is granitic, this magma is felsic (light).
TO SUMMARIZE
Igneous rocks form from a hot liquid, called magma. They are subdivided based on the size of the crystals present and from the minerals that make up the crystals. From just these two characteristics, a geologist can determine whether the rock came from a continent or an ocean basin, and whether it formed near the surface or at depth. Pretty cool for something so hot!
For more information:
Oregon State University: http://volcano.oregonstate.edu/education/vwlessons/lessons/Igrocks/Igrocks1.html
Colorado School of Mines: http://csmres.jmu.edu/geollab/Fichter/IgnRx/Introigrx.html
U. S. Geological Survey: http://geomaps.wr.usgs.gov/parks/rxmin/rock.html#igneous
about.com: http://geology.about.com/cs/basics_roxmin/a/aa011804a.htm
