There are three great categories of rocks: igneous, sedimentary, and metamorphic. Most of the time, they're simple to tell apart. They are all connected in the endless rock cycle, moving from one form to another and changing shape, texture, and even chemical composition along the way. Igneous rocks form from the cooling of magma or lava and compose much of the Earth's continental crust and nearly all of the oceanic crust.
Identifying Igneous Rocks
The key concept about all igneous rocks is that they were once hot enough to melt. The following traits are all related to that.
- Because their mineral grains grew together tightly as the melt cooled, they are relatively strong rocks.
- They're made of primary minerals that are mostly black, white, or gray. Any other colors they may have are pale in shade.
- Their textures generally look like something that was baked in an oven. The even texture of coarse-grained granite is familiar from building stones or kitchen counters. Fine-grained lava may look like black bread (including gas bubbles) or dark peanut brittle (including larger crystals).
Igneous rocks (derived from the Latin word for fire, ignis) can have very different mineral backgrounds, but they all share one thing in common: they formed by the cooling and crystallization of a melt. This material may have been lava erupted at the Earth's surface, or magma (unerupted lava) at depths of up to a few kilometers, known as magma in deeper bodies.
Those three different settings create three main types of igneous rocks. Rock formed of lava is called extrusive, rock from shallow magma is called intrusive, and rock from deep magma is called plutonic. The deeper the magma, the slower it cools, and it forms larger mineral crystals.
Where They Form
Igneous rocks form at four main places on Earth:
- At divergent boundaries, like mid-ocean ridges, plates drift apart and form gaps that are filled by magma.
- Subduction zones occur whenever a dense oceanic plate is subducted underneath another oceanic or continental plate. Water from the descending oceanic crust lowers the melting point of the above mantle, forming magma that rises to the surface and forms volcanoes.
- At continental-continental convergent boundaries, large landmasses collide, thickening and heating the crust to melting.
- Hot spots, like Hawaii, form as the crust moves over a thermal plume rising from deep in the Earth. Hot spots form extrusive igneous rocks.
People commonly think of lava and magma as a liquid, like molten metal, but geologists find that magma is usually a mush - a partially-melted fluid loaded with mineral crystals. As it cools, magma crystallizes into a series of minerals, some of which crystallize sooner than others. As the minerals crystallize, they leave the remaining magma with a changed chemical composition. Thus, a body of magma evolves as it cools and also as it moves through the crust, interacting with other rocks.
Once magma erupts as lava, it freezes quickly and preserves a record of its history underground that geologists can decipher. Igneous petrology is a very complex field, and this article is only a bare outline.
The three types of igneous rocks differ in their textures, starting with the size of their mineral grains.
- Extrusive rocks cool quickly (over periods of seconds to months) and have invisible or microscopic grains or an aphanitic texture.
- Intrusive rocks cool more slowly (over thousands of years) and have visible grains of small to medium-size, or phaneritic texture.
- Plutonic rocks cool over millions of years and can have grains as large as pebbles - even meters across.
Because they solidified from a fluid state, igneous rocks tend to have a uniform fabric without layers, and the mineral grains are packed together tightly. Think of the texture of something you would bake in the oven.
In many igneous rocks, large mineral crystals "float" in a fine-grained groundmass. The large grains are called phenocrysts, and 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 that give 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.
- Pillow lava is a lumpy formation created by extruding lava underwater.
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), as well as olivine, along with the softer mineral mica. The two best-known igneous rock types are basalt and granite, which have distinctly different compositions and textures.
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. It can be either extrusive or intrusive.
Granite is the light, coarse-grained rock formed at a depth that is exposed after deep erosion. It is rich in feldspar and quartz (silica) and hence is called a "felsic" rock. Therefore, granite is felsic and plutonic.
Basalt and granite account for the great majority of igneous rocks. Ordinary people, even ordinary geologists, use the names freely. Stone dealers call any plutonic rock "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 laboratory work to determine an exact rock type according to the official classifications. True granite and true basalt are narrow subsets of these categories.
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 seafloor (the oceanic crust) is made almost entirely of basaltic rocks, with peridotite underneath in the mantle. 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.
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 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.