CK12 Earth Science

Composition

Igneous rocks are classified according to how and where they formed (in other words, if
they’re intrusive or extrusive) and their mineral composition (describing the minerals they
contain). The mineral compositions of igneous rocks are usually described as being felsic,
intermediate, mafic, or ultramafic (as examples, seeFigure4.16andFigure4.17). Felsic
rocksaremadeoflight-colored, low-densitymineralssuchasquartzandfeldspar. Maficrocks
are made of dark-colored, higher-density minerals such as olivine and pyroxene. Intermediate
rocks have compositions between felsic and mafic. Ultramafic rocks contain more than 90%
mafic minerals and have very few light, felsic minerals in them. Table 4.1shows some
common igneous rocks classified by mode of occurrence and mineral composition.

Table 4.1: Common Igneous Rocks

Mode of Occur-

rence

Mineral Com-

position

Felsic Intermediate Mafic Ultramafic

Extrusive Rhyolite Andesite Basalt Komatiite

Intrusive Granite Diorite Gabbro Peridotite

The rocks listed in the table above are the most common igneous rocks, but there are
actually more than 700 different types of igneous rocks. Granite is perhaps the most useful
one to humans. We use granite in many building materials and in art. As discussed in the
introduction to this lesson, pumice is commonly used for abrasives. Peridotite is sometimes
mined for peridot, a type of gemstone used in jewelry. Diorite is extremely hard and is
commonly used for art. It was used extensively by ancient civilizations for vases and other
decorative art work (Figure4.18).

Lesson Summary

• Igneous rocks form either when they cool very slowly deep within the Earth or when
  magma cools rapidly at the Earth’s surface.


• Composition of the magma will determine the minerals that will crystallize forming
  different types of igneous rocks.

Review Questions

1. What is the difference between an intrusive and an extrusive igneous rock?


2. Why do extrusive igneous rocks usually have smaller crystals than intrusive igneous
   rocks?


3. How are igneous rocks classified?