What Are the Properties of Igneous Rocks?

Granite rock is a coarse intrusive igneous rock that is formed when magma from continental crust cools over millions of years.
••• Hemera Technologies/AbleStock.com/Getty Images

Igneous rock, also known as volcanic rock, is formed by the cooling of magma or lava. This type of rock is classified by cooling time and the type of magma it is formed from. The properties of these rocks vary greatly, including their chemical composition, grain structure, texture and color.

Igneous Rock

Igneous rock is produced by the melting of the Earth’s crust into magma. There are two primary types of igneous rock: intrusive and extrusive. Intrusive igneous rock is produced by the slow cooling of magma below the surface. Extrusive igneous rock is produced by the rapid cooling of lava above the surface. In addition to cooling times, igneous rock is further categorized by the type of magma it is formed from, whether felsic, intermediate, mafic or ultra mafic.

Cooling Times

The slow cooling of intrusive igneous rocks enables the growth of large mineral crystals within the rock. These crystals give intrusive igneous rock its coarse nature. Examples of intrusive igneous rock include granite, diorite, gabbro and peridotite. The rapid cooling of extrusive igneous rocks does not allow for the formation of crystallization, producing fine-grained, vesicular and glassy rock. Examples of fine-grained extrusive rock include rhyolite, andesite and basalt. The fastest cooling lava produces scoria, pumice and glasslike obsidian.

Felsic Igneous Rock

Felsic igneous rock is formed by magma that is dominated by silicon and aluminum. This magma is produced by continental crust, characterized by highly viscous magma or lava that is low in temperature and high in gas content. Additional mineral content includes potassium feldspar, sodium-plagioclase feldspar, quartz and biotite. When cooled, this rock is light in color. Granite is an example of slow-cooling felsic igneous rock. Rhyolite is an example of a fast-cooling felsic igneous rock. Pumice and obsidian are examples of very fast-cooling felsic igneous rock.

Intermediate Igneous Rock

Intermediate igneous rock is formed by magma that has a composition between felsic and mafic. It is typically formed by subduction zones involving oceanic plates. The composition of intermediate rocks includes feldspar, amphibole, pyroxene, biotite and quartz. Diorite is an example of a slow-cooling intermediate igneous rock. Andesite is an example of a fast-cooling intermediate igneous rock. Scoria is an example of a very fast-cooling intermediate igneous rock.

Mafic Igneous Rock

Mafic igneous rock is formed by magma that is dominated by ferromagnesian minerals. This magma is typically found in oceanic divergent zones, characterized by fluid magma that is high in temperature and low in gas content. In addition to magnesium and iron silicates, mafic igneous rock may include other minerals, such as:

  • calcium-plagioclase feldspar
  • pyroxene
  • olivine
  • amphibole

Gabbro is an example of slow-cooling mafic igneous rock. Basalt is an example of fast-cooling mafic igneous rock. Scoria can also be formed by very fast-cooling mafic lava.

Ultra Mafic Igneous Rock

Ultra mafic igneous rock is almost entirely ferromagnesian in nature, with the addition of olivine. Peridotite is an example of a slow-cooling ultra mafic igneous rock. There are no forms of fast-cooling ultra mafic rock, and peridotite is rarely found on the Earth’s surface.

Related Articles

Types of Intrusive Igneous Rock With Large Crystals
What Minerals Make Up Pumice?
What Is the Composition of a Lava Rock?
Three Types of Rocks That Form When Lava Cools
A List of Extrusive Igneous Rocks
List of Igneous Rocks That Do Not Contain Quartz
How Is Tourmaline Formed?
How to Identify Basalt
What Is Deformation in Earth Science?
Characteristics of Intrusive Rocks
Stones Found in Arizona
Volcanic Rock Types by Mauna Loa
What Type of Plate Boundary Is the Aleutian Trench?
What Type of Volcano Is Not Associated With a Plate...
Important Uses of Sphalerite
What Is the Difference Between the Crust & the Lithosphere?
Differences Between Extrusive and Intrusive Rocks
What Is Shearing in Geology?
Difference Between Granite & Limestone
List of Minerals Found Under the Sea Bed