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Metamorphic Rock & Igneous Rock Types

All of the rocks found on our planet can be divided into three types, based upon how they were formed: igneous, metamorphic and sedimentary. Igneous rocks are volcanic rocks formed from cooling magma or lava. They are divided into intrusive, having cooled under the Earth's surface, and extrusive, having cooled above the Earth's surface. Metamorphic rocks are created by combinations of immense pressure and heat. They are divided into foliated rocks and non-foliated rocks.

Foliated Metamorphic Rocks
- Foliated metamorphic rocks exhibit a texture that is layered, lineated or banded. The minerals within these rocks have a preferred orientation, having been forced into alignment by extreme pressure. This type of metamorphic process is called Barrovian. Examples of foliated metamorphic rocks are slates, phyllites, schists, gneisses and micas.
Non-Foliated Metamorphic Rocks
- Non-foliated metamorphic rocks are formed by several different processes, including contact, regional and hydrothermal. They exhibit a granular nature, with equidistant grains. The minerals in non-foliated rocks display no preferred orientation. Hornfels is an example of a non-foliated metamorphic rock formed by contact metamorphism only. Quartzite and marble are examples of non-foliated metamorphic rocks formed by contact and regional processes. Soapstone and serpentinite are examples of non-foliated metamorphic rocks formed by a hydrothermal process.
Intrusive Igneous Rocks
- Intrusive igneous rocks are formed by magma that cools very slowly, below the Earth's surface. This cooling process can take thousands or millions of years. They exhibit a coarse grain. All igneous rocks can be classified by their chemical composition. Felsic rocks are light colored and high in silica. Mafic rocks are dark in color and low in silica. Intermediate rocks fall, as the name implies, between felsic and mafic compositions. There are also ultra-mafic rocks that are extremely high in ferromagnesian minerals. Gabbro is an example of an intrusive, mafic rock. Diorite is an example of an intrusive, intermediate rock. Granite is an example of an intrusive, felsic rock. Granite is one of the most abundant types of igneous rock on the planet. Peridotite is an example of an intrusive, ultra-mafic rock.
Extrusive Igneous Rocks
- Extrusive igneous rocks are formed by lava that cools quickly, above the Earth's surface. This cooling process takes place in days or weeks. These rocks are further classified as having cooled either fast or very fast. Igneous rocks that have cooled quickly exhibit a fine grain, while igneous rocks that have cooled very quickly are glassy in nature. Basalt is an example of a fast-cooling, mafic rock. Basalt, like granite, is another of the most abundant types of igneous rock. Andesite is an example of a fast-cooling, intermediate rock. Rhyolite is an example of a fast-cooling, felsic rock. Komatite is an example of a fast-cooling, ultra-mafic rock. Scoria is an example of an igneous rock that cooled very quickly. Scoria can be mafic or intermediate in nature. Pumice and obsidian are examples of felsic-class rocks that cooled very quickly.

Rocks & Minerals