Intrusive rocks crystallize from magmas that have been intruded into the earth's crust at depths far below the surface. These intrusions are then usually exposed millions or billions of years later through the processes of uplift, mountain‐building, and erosion. Other intrusive rocks are discovered through deep‐drilling programs. Country rock is the surrounding rock that the magma invades. A contact then separates the cooled intrusive rock from the country rock. Contacts are rarely straight lines, are quite irregular, and mark the change in rock type. The edge of the intrusive rock is usually very fine grained because it is here where the most rapid cooling took place. This edge of the intrusion is called the chill zone. The grain size in the intrusion increases away from the chill zone toward the center, where it remained the hottest for the longest time. The intrusive rock often contains xenoliths—fragments of the country rock that were torn away during the emplacement of the magma and that are generally most abundant near the contact with the country rock.
Plutonic rocks. Intrusive rocks that were formed deep in the earth's crust are called plutonic rocks and are generally coarse grained (mineral grains greater than 1 millimeter in diameter), large, and often associated with mountain‐building.
Mafic, felsic, and intermediate intrusive rocks. Intrusive rocks are classified the same way extrusive rocks are—according to the relative amounts of feldspars, quartz, and ferromagnesian minerals. Gabbro is a mafic rock and has the same chemistry and mineralogy as basalt; diorite is an intermediate rock equivalent to andesite; and granite is a felsic rock equivalent to rhyolite. For example, a magma that would form rhyolite if it vented at the surface would crystallize into a granite in a subterranean chamber kilometers below the surface. Granite is the most common intrusive rock on the continents; gabbro is the most common intrusive rock in oceanic crust.
Ultramafic intrusive rocks. Ultramafic intrusions are almost completely composed of ferromagnesian minerals, mostly olivine and pyroxene. They contain less than 45 percent silica and are thought to be derived from the mantle. A typical ultramafic intrusion is called a peridotite.