
| Types of Igneous Rocks |

Igneous rock is in the making as lava pours down a well-developed lava channel near a series of erupting vents on the northeast rift zone of Mauna Loa Volcano.
Igneous rocks

The term igneous comes from the Latin ignis, meaning "fire". Igneous is used to describe rocks that crystallize out of hot molten material in the Earth called magma. When magma pushes up through Earth's crust to the surface, it is called lava. Both magma and lava cool and harden to form igneous rocks.

Intrusive vs. Extrusive

Igneous rocks can be classified into two main categories: intrusive and extrusive. A trick to help kids remember intrusive and extrusive is to think of intrusive -- inside and extrusive -- exit.

Intrusive rocks come from magma. They cool slowly deep in Earth's crust. When magma cools underground, the crust acts like a blanket, insulating it, keeping it warm longer. Because the magma cools slowly, crystals of different minerals have time to grow. The molecules in the magma have time to arrange themselves into crystal formations before the magma hardens. Intrusive rocks have large crystals that can be seen with the naked eye. A common example of an intrusive igneous rock is granite.

Pahoehoe Lava Field, Hawaii.
Extrusive igneous rocks come from lava. Lava, at the surface, is exposed to air and water which causes the molten rock to cool rapidly. Solidifying rocks at the surface cool too quickly for large crystals to form. Molecules in the lava do not have time to arrange themselves to form large crystals. Extrusive rocks have crystals that are too small to see without magnification. A common example of an extrusive igneous rock is basalt. Some extrusive rocks, such as obsidian and pumice, cool so rapidly that they completely lack crystal structure and are considered a volcanic glass. Pumice is just like obsidian except it is tiny shards of glass.


Within the two main categories of intrusive and extrusive, rock can be classified even further using texture and chemical composition. The word "texture" has nothing to do with how the rock feels. Texture, in geology, is used to describe how the rock looks. The most noticeable textural feature of igneous rocks is grain size. Grain size refers to the size of the individual mineral crystals. As mentioned above, intrusive igneous rocks, such as granite have large, individual crystals visible to the naked eye. The textural term used to describe a rock with large crystals is coarse-grained. In contrast, fine-grained rocks, such as basalt, are igneous rocks that have crystals too fine to see with the naked eye. Under magnification they are still very small but easily identifiable with a few optic tests. Quickly-cooled lavas can contain trapped bubbles of gas, which are called vesicles. The resulting texture is described as vesicular.

Chemical Composition

Chemical composition of igneous rock can often be estimated just from looking at the rock. Geologists look at the proportions of light-colored and dark-colored minerals in an igneous rock to estimate the chemical makeup of rock. Light-colored or felsic, minerals have more silica in them. Silica is one of the most abundant elements on Earth and is the chief component of quartz. Felsic minerals are most often colorless, white, gray or pink but can be any number of colors. The dark, or mafic, minerals are richer in iron and magnesium. Mafic minerals are chiefly black, brown, dark gray and sometimes green.

The mineral proportions of the rock are what allows geologists to classify rocks chemically. Depending on the proportion of light minerals to dark minerals, igneous rocks can be broken into four main types: felsic, intermediate, mafic and ultramafic. The following list gives more information about igneous chemical categories.

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