Major Groups of Igneous Rocks

“Hot rocks” aren’t just valuable jewels stolen from stores and safes. Scientists think that “hot rocks” are what made the Earth, and they’re still making the planet today.

Humans like to think of the planet on which we live as “solid rock.” In reality, however, the Earth is a big ball of rock so hot that it turns liquid. The liquid phase of rock is known as magma, and rocks that crystallize directly from liquid are known as igneous rocks.

Magma occurs in the Earth’s crust in what are known as “magma chambers,” or places where the temperature is high enough that rocks melt. Nobody knows for sure what these chambers or pockets look like. All that’s known for sure is that they exist in places where the pressure is low enough for the magma to remain in a liquid state. Only rocks that form directly from a liquid state are igneous rocks.

To determine whether a rock belongs to the igneous type, geologists evaluate two factors of the rock’s components: texture and composition. Within these two broad categories are almost innumerable varieties of rocks with differing amounts of texture and composition.

Texture refers to the crystals, or individual mineral grains, in the ultimate rock. The size of the grain depends upon how quickly the rock cooled. There are two basic types of texture: intrusive and extrusive.

“Intrusive” igneous rocks are those that cooled at a level in the earth’s crust that was insulated by the sediment or rock around them. In other words, the magma flowed into a pocket surrounded by other types of earth materials and cooled slowly, allowing large grains of crystal to form. Although grain size in intrusive igneous rocks varies widely, most large-grained rocks are considered intrusive.

“Extrusive” rocks are those with fine mineral grains. This kind of rock usually results from a volcanic explosion, and often its grains are too small to see without magnification.

Coarse-grained rocks that have mineral crystals large enough to see with the naked eye are called “phaneritic.” Rocks with grains too small to see without a magnifying glass are known as “aphanitic.” Obsidian, the glassy rock that sometimes forms from a volcanic eruption, is an extrusive aphanitic rock.

Composition is the other factor by which geologists classify igneous rocks. The location of the magma determines composition, or what minerals will be present when the rock is formed. In general, rocks that form in the crust or mantle of the Earth will have more oxygen and silicon – not surprising, since these two elements form 75 percent of the Earth’s crust. These rocks are known as “mafic.” Two examples: If the mafic magma erupts at the Earth’s surface, the result is basalt; if the magma stays underground in a chamber, the result is gabbro. (“Gabbro” was named for a town in Tuscany where it was first classified, and is used to name just about any dark, coarse-grained igneous rock). The classifications “basalt” and “gabbro” refer solely to the rocks’ differing textures; both kinds are made up of essentially the same amounts of minerals.

Rocks that form deeper in the Earth tend to contain less than 65 percent oxygen and silicon, and much more aluminum, and sodium and potassium. They also generally have very low or non-existent quantities of the other three elements that make up the planet’s composition, namely iron, calcium and magnesium. These are known as “felsic” rocks.

Geologists now know that felsic and intermediate magmas result from compression and subduction. In other words, rocks are pushed back into the Earth where the temperature melts them again.This allows the lighter elements to become more concentrated in the rock.

The following chart, based on information available from the University of Oregon, shows the various classifications of igneous rocks based on composition (horizontal) and texture (vertical).

Felsic                     Intermediate          Mafic              Ultramafic

Aphanitic             Rhyolite                  Andesite               Basalt             Nature can’t produce this type
(fine grained)

Intermediate        Dacite                     Diabase

(coarse-grained)   Granite                   Diorite                  Gabbro             Peridotite

Glassy                  Obsidian

Frothy                  Pumice                   Scoria