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Earth & Space Science: Session 3

A Closer Look: Igneous Rocks

What are igneous rocks?

Igneous rocks are the most common rocks on Earth. All of Earth's ocean floor, its entire mantle, and much of the continental crust consists of igneous rock. Igneous rock forms as molten (liquid) rock cools and solidifies. Most of this molten rock originates under the Earth’s surface in a zone within the upper mantle where it is extremely hot but the pressure is not great enough to keep the rock solid. There are two major types of igneous rock: extrusive and intrusive.

How are different types of igneous rocks formed?

extrusive rock

Extrusive, or volcanic igneous rock forms when magma erupts, or extrudes, onto the surface of the Earth as lava. This occurs along active plate margins, such as mid-oceanic spreading ridges and subduction zones, as well as at intraplate settings, like hot spots. Lava cools and hardens quickly at the surface of the Earth, as finely grained rock with many tiny crystals. In some cases, the lava cools so quickly that the molten material does not have enough time to arrange itself into crystalline structures. This kind of igneous rock is called volcanic glass. Another product of volcanic eruption is pyroclastic debris, which are particles formed from the gas and lava that explode into the atmosphere. Pyroclastic debris includes fine particles of volcanic glass called ash, and larger pieces of rock called, depending on their size, cinders and bombs. The most common extrusive igneous rock is basalt, which is the rock that comprises oceanic crust. Basalt accounts for more than 90% of all volcanic rock on the planet.

Intrusive, or plutonic, igneous rock forms when when magma beneath the Earth’s surface rises upward and pushes its way, or intrudes, into pre-existing crustal rocks. Features associated with this ascension of magma include sills (nearly horizontal intrusion of magma that is injected between layers of rock); dikes (nearly vertical injection of magma that cuts across layers of rock); laccoliths (when a sill domes upward, looking like a blister); and batholiths (immense, deep, dome-shaped intrusions of igneous rock). Magma cools and solidifies more slowly underground than at the Earth's surface, which produces igneous rocks with coarse crystals that can easily be seen with the naked eye. One of the most well known intrusive igneous rocks is granite, which comprises much of the continental crust.

In addition to being categorized as extrusive or intrusive, igneous rocks are also classified in other ways:

  • Mafic igneous rocks are dense, and rich in iron- and magnesium-bearing minerals and are usually dark in color.
  • Felsic igneous rocks are rich in less dense minerals, such as quartz, and are often light in color.
  • Granular igneous rocks consist of crystals that are large enough to be easily seen, such as granite.
  • Aphanitic igneous rocks are made of tiny crystals that cannot be seen with the naked eye, such as basalt.
  • Glassy igneous rocks are composed mostly of volcanic glass. Obsidian is one example.
  • Porphyritic igneous rocks have larger crystals embedded in a finer grained matrix.
  • Pyroclastic igneous rocks are volcanic rocks that form from explosive eruptions that shatter magma and can be either cemented together or unconsolidated fragments or slivers. Pumice and ash are examples.
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