Igneous rocks and minerals form from magmas, molten rock that originates beneath Earth’s surface. Magma often collects in large magma chambers, but magmas are also mobile; they can flow through fissures and sometimes reach the surface. Mid-ocean ridges and subduction zones account for most magmas at or near Earth’s surface. Continental rifts and hot spots, places where anomalous heat rises from depth, account for the rest.
Magmas are complex liquids that vary greatly in composition and properties. They have temperatures as great as 1,400 °C (2,500 °F) and often originate in regions 50 to 200 km (30 to 125 mi.) deep in the Earth. They may be partially crystalline, containing crystals of high-temperature minerals such as leucite, olivine, or clinopyroxene. Because magma has a lower density than the solid upper mantle and crust of the Earth, buoyancy moves it upward. The race between upward movement and cooling ultimately determines whether magma becomes an intrusive or extrusive igneous rock.
Magma solidifies as an intrusive rock if it crystallizes before it reaches the surface. Intrusive rocks often form plutons (a general term given to any intrusive igneous rock body), so geologists sometimes use the terms intrusive and plutonic interchangeably. If magma reaches the surface while molten, or partially molten, we term it lava. The cooling lava forms an extrusive rock, also called a volcanic rock (although it does not necessarily form a volcano).
Magmas that reach Earth’s surface cool quite quickly. In contrast, those that cool at some depth in the Earth do so more slowly, and cooling rate directly affects grain size of an igneous rock. The common plutonic rock granite contains crystals of quartz and potassium feldspar that are easily seen with the naked eye. A common volcanic rock, rhyolite, may contain the same minerals, but we need a microscope to see the crystals (Figure a). The difference is due to cooling rate. Intrusive rocks cool and crystallize slowly; other very hot rocks surround and insulate them as crystallization occurs. Crystals in granites have a long time to form and grow. In contrast, volcanic rocks, such as rhyolite, crystallize rapidly because extrusion exposes the lava to water or to the cool atmosphere at the surface of the Earth.
 Figure (a)
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| Three igneous rocks: coarse-grained granite (right), medium-grained granite (right), and rhyolite (left). Granite and other intrusive rocks are generally coarser grained than extrusive rocks such as rhyolite because they had a longer time to crystallize. |
Sometimes extrusive igneous rocks cool so quickly that no crystals form. This is especially likely to occur if lava meets water. The result is a rock composed of glass called obsidian. In other cases, different minerals may grow to distinctly different sizes. The result is a porphyry in which coarse crystals called phenocrysts are floating in a sea of fine grained crystals called ground mass.
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Quickly cooled plutonic rocks may be very fine grained and difficult to tell from volcanic rocks. Some petrologists, therefore, prefer to classify and name igneous rocks based on their grain size rather than their genesis (origin). They divide rocks into those containing very fine grains (aphanitic), rocks containing very coarse grains (phaneritic), and rocks containing combinations of large and small crystals (porphyritic).