Types Of Igneous Rocks

Types Of Igneous Rocks

Geologists classify igneous rocks according to the depth at which they formed in the earth’s crust. Using this principle, they divide igneous rocks into two broad categories: those that formed beneath the earth’s surface, and those that formed at the surface. Igneous rocks may also be classified according to the minerals they contain.

A Classification by Depth of Formation

Rocks formed within the earth are called intrusive or plutonic rocks because the magma from which they form often intrudes into the neighboring rock. Rocks formed at the surface of the earth are called extrusive rocks. In extrusive rocks, the magma has extruded, or erupted, through a volcano or fissure.

Geologists can tell the difference between intrusive and extrusive rocks by the size of their crystals: crystals in intrusive rocks are larger than those in extrusive rocks. The crystals in intrusive rocks are larger because the magma that forms them is insulated by the surrounding rock and therefore cools slowly. This slow cooling gives the crystals time to grow larger. Extrusive rocks cool rapidly, so the crystals are very small. In some cases, the magma cools so rapidly that crystals have no time to form, and the magma hardens in an amorphous glass, such as obsidian.

One special type of rock, called porphyry, is partly intrusive and partly extrusive. Porphyry has large crystals embedded in a mass of much smaller crystals. The large crystals formed underground and only melt at extremely high temperatures. They were carried in lava when it erupted. The mass of much smaller crystals formed around the large crystals when the lava cooled quickly above ground.

B Classification by Composition

Geologists also classify igneous rocks based on the minerals the rocks contain. If the mineral grains in the rocks are large enough, geologists can identify specific minerals by eye and easily classify the rocks by their mineral composition. However, extrusive rocks are generally too fine-grained to identify their minerals by eye. Geologists must classify these rocks by determining their chemical composition in the laboratory.

Most magmas are composed primarily of the same elements that make up the crust and the mantle of the earth: oxygen (O), silicon (Si), aluminum (Al), iron (Fe), magnesium (Mg), calcium (Ca), sodium (Na), and potassium (K). These elements make up the rock-forming minerals quartz, feldspar, mica, amphibole, pyroxene, and olivine. Rocks and minerals rich in silicon are called silica-rich or felsic (rich in feldspar and silica). Rocks and minerals low in silicon are rich in magnesium and iron. They are called mafic (rich in magnesium and ferrum, the Latin term for iron). Rocks very low in silicon are called ultramafic. Rocks with a composition between felsic and mafic are called intermediate.

B1 Felsic Rocks

The most felsic, or silicon-rich, mineral is quartz. It is pure silicon dioxide and contains no aluminum, iron, magnesium, calcium, sodium, or potassium. The other important felsic mineral is feldspar. In feldspar, a quarter or a half of the silicon has been replaced by aluminum. Feldspar also contains potassium, sodium, or calcium but no magnesium or iron.

Felsic intrusive rocks are classified as either granite or granodiorite, depending on how much potassium they contain. Both are light-colored rocks that have large crystals of quartz and feldspar. Extrusive rocks that have the same chemical composition as granite are called rhyolite and those with the same chemical composition as granodiorite are called dacite. Both rhyolite and dacite are fine-grained light-colored rocks.

B2 Intermediate Rocks

Rocks intermediate in composition between felsic and mafic rocks are termed syenite, monzonite, or monzodiorite if they are intrusive and trachyte, latite, and andesite if they are extrusive. Syenite and trachyte are rich in potassium while monzodiorite and andesite contain little potassium.

B3 Mafic Rocks

The mafic rock-forming minerals are olivine, pyroxene, and amphibole. All three contain silicon and a lot of either magnesium or iron or both. All three of these minerals are often dark colored.

Mafic intrusive rocks are termed diorite or gabbro. Both are dark rocks with large, dark, mafic crystals as well as crystals of light-colored feldspar. Neither contains quartz. Diorite contains amphibole and pyroxene, while gabbro contains pyroxene and olivine. The feldspar in diorite tends to be sodium-rich, while the feldspar in gabbro is calcium-rich. Extrusive rocks that have the same chemical composition as diorite or gabbro are called basalt. Basalt is a fine-grained dark rock.

Ultramafic rocks are composed almost exclusively of mafic minerals. Dunite is composed of more than 90 percent olivine; peridotites have between 90 and 40 percent olivine with pyroxene and amphibole as the other two principal minerals. Pyroxenite is composed primarily of pyroxene, and hornblendite is composed primarily of hornblende, which is a type of amphibole.