Igneous rocks are one of the three main types of rocks and make up about 95% of the upper crust of the Earth. There are over 700 different types of igneous rocks that have been discovered and most of them are formed under the Earth’s crust. They are studied by geologists to discover important information about how they were formed. There has been some controversy about how they are classified in the past. Today they are classified by how they occurred, texture (how the rock looks), chemical and mineral composition as well as the geological shape.
The types of igneous rocks
There are two basic types of igneous rocks; intrusive and extrusive igneous rocks. These both form differently and are the most basic way to classify these diverse rocks.
How igneous rocks form
Intrusive igneous rocks, also called plutonic rocks, are formed from magma (this is what lava is called when it is underground). This magma is very hot and consists mainly of molten rock. The magma slowly cools down and solidifies deep in the Earth’s surface, which creates intrusive igneous rocks. The slow solidification process means that these rocks have a coarse grain.
Extrusive igneous rocks, also called volcanic rocks, form on or above the surface of the earth. The magma rises to the surface and breaks through (typically from a volcano), when it is called lava. When this molten rock is exposed to air it cools and solidifies quickly and creates extrusive igneous rocks. The faster cooling process means that these rocks have a fine grain.