Igneous rocks get their name from the Latin word for fire, “ignis.” This makes sense, because early naturalists could watch them forming as fiery streams of lava flowing from volcanoes turned into solid rock. The word igneous is now used to describe all rocks that form when molten rock, or magma, solidifies; whether this happens on the ground near a volcano or deep beneath the Earth’s surface. Forming from melted rock is the key difference between igneous rocks and the other two major types of rocks: Metamorphic rocks are formed by heat and pressure that doesn’t melt the rock, and sedimentary rocks form at low temperatures on the surface.
Although this type of rock is formed when magma freezes into solid crystals, there are still many different kinds of igneous rocks. Geologists who study these rocks use several different features to divide them into smaller groups, groups that provide hints about how they formed and what geologic activities have taken place in the area.
Geologists and rock hounds divide igneous rocks into two general classes based on the size of the mineral crystals that formed when the rock cooled. A rule of thumb is that if the crystals are visible to the naked eye, the rock cooled slowly, usually deep in the ground. If the crystals are too small to see without a magnifier or microscope, the rock cooled quickly, from “lava,” or magma that reaches the surface. The fine-grained varieties are called extrusive igneous rocks and the coarser ones are called intrusive. The most familiar igneous rocks are examples of the two types: Granite is intrusive and basalt is extrusive.
Even division on crystal size can have exceptions: The rocks obsidian and pumice solidify so quickly that they seem to have no crystals, much like ordinary glass. At the other extreme, the mineral crystals in pegmatite can be the size of telephone poles, cars or even houses. A porphyry is sort of in between: It’s an igneous rock with large crystals embedded in a matrix of tiny crystals.
Crystal size, or intrusive vs. extrusive, only provides a clue about where the rock cooled; so geologists must look more closely at the rock to get more information. Both fine- and coarse-grained rocks can be divided based on the minerals that are present. Minerals that are rich in silicon and aluminum are most common in one broad class of igneous rocks. These are the “felsic” igneous rocks, most of which are light in color, and include granite (coarse-grained) and rhyolite (fine-grained). Minerals that are rich in iron and magnesium occur in darker, “mafic” igneous rocks. This group includes gabbro (coarse-grained) and basalt (fine-grained).
This division is, once again, overly simple. That’s because the magma from which rocks solidify can have any elements present in any amounts; and so can the rocks created. There are many different names for rocks besides the four named above, all of which are based on the crystal size and the minerals present. The most common minerals found in igneous rocks are quartz and the two feldspars, plagioclase and orthoclase. Most of the common rock types are defined based on the percentage of those three minerals present, so mineral identification is very important to rock identification. Scientists often examine igneous rock samples under a microscope to make mineral identification easier.
Why are earth scientists concerned about the minerals present in igneous rocks? Because the minerals tell them what was melted to form the magma. For instance, gabbro contains no quartz but lots of the minerals rich in iron and magnesium. This composition suggests that the rocks from which magma formed did not include quartz-rich rocks, which are usually found on or near the continents. Gabbro forms most often at oceanic spreading centers and where oceanic crust has been melted at a plate boundary. Granite contains abundant quartz, which suggests that it forms from melted continental material. An intermediate extrusive rock – andesite, named for the Andes Mountains – is believed to have formed from melted oceanic and continental crust that mixed together.
With a well-equipped laboratory, a geologist can break down an igneous rock far enough to measure the amount of the major elements that went into making the minerals present; elements like silicon, potassium, calcium and sodium. The scientists compare the chemistry and minerals of an unknown sample to those of samples from known geological and tectonic environments. This allows scientists to make educated guesses about what processes were going on deep within the Earth when the new sample was created. That sort of information is vital to solving the puzzle geologists call a geologic history, one of the key ingredients in the search for mineral ores and even petroleum.