Igneous rocks are formed when molten material, or magma, solidifies. The magma moves upward from pockets a few kilometers below the Earth’s surface, through fractures in older solid rock. There the magma cools, forming rocks of mineral crystals.
Most igneous rock consists of silicate minerals— chemical compounds that contain silicon and oxygen atoms. These rocks also contain mostly metallic elements. The mineral grains in igneous rocks are very tightly interlocked, and so the rock is normally very strong.
Quartz, which is made of silicon dioxide (SiO2 ), is the most common mineral of all rock classes. It is quite hard and resists chemical breakdown. Silicate minerals in igneous rocks are classed as felsic, which are light- colored and less dense, and mafic, which are dark- colored and more dense.
Felsic rock contains mostly felsic minerals, and mafic rock contains mostly mafic minerals. Ultramafic rock is dominated by two heavy mafic minerals and is the densest of the three rock types. Three common igneous rocks are granite (felsic, intrusive); andesite (felsic, extrusive); and basalt (mafic, intrusive).
Magma that solidifies below the Earth’s surface and remains surrounded by older, preexisting rock is called intrusive igneous rock. Because intrusive rocks cool slowly, they develop mineral crystals visible to the eye. If the magma reaches the surface and emerges as lava, it forms extrusive igneous rock.
Extrusive igneous rocks cool very rapidly on the land surface or ocean bottom and thus show crystals of only microscopic size. You can see formation of extrusive igneous rock today where volcanic processes are active.
A body of intrusive igneous rock is called a pluton. Granite typically accumulates in enormous plutons, called batholiths. As the hot fluid magma rises, it melts and incorporates the older rock lying above it. A single batholith extends down several kilometers and may occupy an area of several thousand square kilometers.
One is a sill, a plate-like layer formed when magma forces its way between two preexisting rock layers. In the example shown, the sill has lifted the overlying rock layers to make room.
A second kind of pluton is the dike, a wall-like body formed when a vertical rock fracture is forced open by magma. These vertical fractures conduct magma to the land surface in the process of extrusion. The dike rock is fine- textured because of its rapid cooling. Magma entering small, irregular, branching fractures in the surrounding rock solidifies in a branching network of thin veins.