Igneous rocks are the first great class.
Origin of Igneous Rocks
Igneous rocks begin as hot, fluid material, and the word "igneous" comes from the Latin for fire. This material may have been lava erupted at the Earth's surface, or magma (unerupted lava) at shallow depths, or magma in deep bodies (plutons). Rock formed of lava is called extrusive, rock from shallow magma is called intrusive and rock from deep magma is called plutonic.
Igneous rocks form in three main places: where lithospheric plates pull apart at mid-ocean ridges, where plates come together at subduction zones and where continental crust is pushed together, making it thicker and allowing it to heat to melting. (To learn more about how igneous rocks form, see About Volcanism.)
People commonly think of lava and magma as a liquid, like molten metal, but geologists find that magma is usually a mush — a liquid carrying a load of mineral crystals. Magma crystallizes into a collection of minerals, and some crystallize sooner than others. Not just that, but when they crystallize, they leave the remaining liquid with a changed chemical composition. Thus a body of magma, as it cools, evolves, and as it moves through the crust, interacting with other rocks, it evolves further. This makes igneous petrology a very complex field, and this article is only the barest outline.
Igneous Rock Textures
Tell the three types of igneous rocks apart by their texture, starting with the size of the mineral grains. Extrusive rocks cool quickly (over periods of seconds to months) and have invisible or very small grains, or an aphanitic texture. Intrusive rocks cool more slowly (over thousands of years) and have small to medium-sized grains. Plutonic rocks cool over millions of years, deep underground, and can have grains as large as pebbles — even a meter across. Both intrusive and plutonic rocks have phaneritic texture.
Because they solidified from a fluid state, igneous rocks tend to have an equigranular texture, a uniform fabric without layers, and the mineral grains are packed together tightly. Think of the texture of a piece of bread as a similar example.
In many igneous rocks, large mineral crystals "float" in a fine-grained groundmass. The large grains are called phenocrysts, and a rock with phenocrysts is called a porphyry; that is, it has a porphyritic texture. Phenocrysts are minerals that solidified earlier than the rest of the rock, and they are important clues to the rock's history.
Some extrusive rocks have distinctive textures. Obsidian, formed when lava hardens quickly, has a glassy texture. Pumice and scoria are volcanic froth, puffed up by millions of gas bubbles giving them a vesicular texture. Tuff is a rock made entirely of volcanic ash, fallen from the air or avalanched down a volcano's sides. It has a pyroclastic texture. And pillow lava is a lumpy formation created by extruding lava underwater.
Igneous Rock Types: Basalt, Granite and More
Igneous rocks are classified by the minerals they contain. The main minerals in igneous rocks are hard, primary ones: feldspar, quartz, amphiboles and pyroxenes (together called "dark minerals" by geologists), and olivine along with the softer mineral mica.
The two best-known igneous rock types are basalt and granite, which differ in composition. Basalt is the dark, fine-grained stuff of many lava flows and magma intrusions. Its dark minerals are rich in magnesium (Mg) and iron (Fe), hence basalt is called a mafic rock. So basalt is mafic and either extrusive or intrusive. Granite is the light, coarse-grained rock formed at depth and exposed after deep erosion. It is rich in feldspar and quartz (silica) and hence is called a felsic rock. So granite is felsic and plutonic.
These two categories cover the great majority of igneous rocks. Ordinary people, even ordinary geologists, use the names freely. (Stone dealers call any plutonic rock at all "granite.") But igneous petrologists use many more names. They generally talk about basaltic and granitic or granitoid rocks among themselves and out in the field, because it takes lab work to determine an exact rock type according to the official classifications. True granite and true basalt are narrow subsets of these categories. (Get deeper into classification)
But a few of the less common igneous rock types can be recognized by non-specialists. For instance a dark-colored plutonic mafic rock, the deep version of basalt, is called gabbro. A light-colored intrusive or extrusive felsic rock, the shallow version of granite, is called felsite or rhyolite. And there is a suite of ultramafic rocks with even more dark minerals and even less silica than basalt. Peridotite is the foremost of those.
Where Igneous Rocks Are Found
The deep sea floor (the oceanic crust) is made of basaltic rocks, with ultramafic rocks underneath. Basalts are also erupted above the Earth's great subduction zones, either in volcanic island arcs or along the edges of continents. However, continental magmas tend to be less basaltic and more granitic. (more on arc volcanism)
The continents are the exclusive home of granitic rocks. Nearly everywhere on the continents, no matter what rocks are on the surface, you can drill down and reach granitoid eventually. In general, granitic rocks are less dense than basaltic rocks, and thus the continents actually float higher than the oceanic crust on top of the ultramafic rocks of the Earth's mantle. The behavior and histories of granitic rock bodies are among geology's deepest and most intricate mysteries.
Get more help identifying igneous rocks in the Rock Identification Tables.
NEXT: About Sedimentary Rocks