Most Common Types of Igneous Rocks

Types

• There are two kinds of igneous rocks. They are named for the process by which they formed. Intrusive igneous rocks refer to rocks that form in chambers beneath the earth's surface. Magma flows into these underground chambers where it begins to cool slowly, forming rocks with large crystals. Conversely, extrusive igneous rocks form at the earth's surface. They form due to rapidly cooling lava, creating rocks with small crystals.

Granite

• Granite is a medium to coarse-grained igneous rock that is formed intrusively. Granite is commonly seen in ornamental stonework, monuments, architecture and construction. Light in color, granite is composed of quartz and feldspar minerals. It is high in silica, potassium and sodium, but low in iron, calcium and magnesium.

Basalt

• Basalt is one of the most common types of igneous rocks in the world. The majority of the ocean floor is composed of basalt. This smooth, black igneous rock forms due to erupting magma beneath the ocean floor that cools as it comes in contact with the ocean water. Basalt is a finely grained rock that contains small crystals. It is composed of plagioclase and pyroxene and commonly used for aggregate.

Gabbro

• Crushed gabbro is commonly used as concrete aggregate, railroad ballast and road metal. This coarse-grained igneous rock is formed intrusively and composed of layers of minerals such as feldspar and augite. Occasionally it will contain olivine, a green crystalline mineral. It is generally dark in color. Gabbro can be cut and polished to form what is known as black granite.

Pumice

• While most common types of igneous rocks are very hard, pumice is the exception to the rule, as it is very brittle. Pumice is an extrusive igneous rock that is formed due to cooling lava. Gas and air bubbles formed during the cooling process create tiny holes and crevasses within the rock. Pumice is a very common igneous rock and is used as an exfoliate. It is used to make emery boards and hand soap.