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Igneous Rocks

Rock Cycle: Igneous

Igneous rock solidifies and crystallizes from a molten state. Igneous rock forms from either magma (molten rock beneath Earth’s surface) or lava (the name for magma after it emerges at Earth’s surface). When a rock becomes reintroduced to the lower mantle, it will melt into magma, the source material for igneous rocks. Igneous rock makes up 90% of Earth’s crust, but is frequently covered by sedimentary rock, soil, or oceans.

There are two main types of igneous rock: Intrusive and Extrusive.

 

Intrusive igneous rock forms by magma intruding into and cooling underneath Earth's crust. Extrusive igneous rock forms when magma reaches Earth's surface and cools rapidly upon introduction to air or water. When magma reaches Earth's surface, it is called lava.
Intrusive igneous rock forms by magma intruding into and cooling underneath Earth's crust. Extrusive igneous rock forms when magma reaches Earth's surface and cools rapidly upon introduction to air or water. When magma reaches Earth's surface, it is called lava.

 

Intrusive

Intrusive igneous rock is formed when magma intrudes into Earth’s crust, then cools and hardens below Earth’s surface, usually over a very long period of time. Although these rocks form beneath Earth’s surface, igneous rock formations can emerge above Earth’s surface through exogenic processes like weathering and erosion.

 

Sometimes, a pool of magma will harden to create a pluton, a generic term for an igneous intrusion, or any body of intrusive rock beneath Earth’s surface. Sometimes, multiple plutons are linked together, resulting in a batholith with a surface greater than 100 square km (40 square miles). This image depicts the Ladakh Batholith, which is part of the Ladakh Range in the western Himalayan mountains.
Sometimes, a pool of magma will harden to create a pluton, a generic term for an igneous intrusion, or any body of intrusive rock beneath Earth’s surface. Sometimes, multiple plutons are linked together, resulting in a batholith with a surface greater than 100 square km (40 square miles). This image depicts the Ladakh Batholith, which is part of the Ladakh Range in the western Himalayan mountains.

 

Plutons can also include the magma conduits of ancient volcanoes that have cooled and hardened, as in the Devil's Rock formation in Wyoming state, U.S.
Plutons can also include the magma conduits of ancient volcanoes that have cooled and hardened, as in the Devil's Rock formation in Wyoming state, U.S.

 

Extrusive

Extrusive igneous rock forms when magma extrudes onto Earth’s surface as lava, cools and hardens above Earth’s surface. Typically, extrusive igneous rock results from volcanic eruptions and flows. Extrusive igneous rock forms much more quickly than intrusive rock, because of the sudden cooling resulting from exposure to air or water.

 

Basalt is the most common extrusive igneous rock. It makes up the bulk of the ocean floor and accounts for 71% of Earth’s surface.
Basalt is Earth's most common extrusive igneous rock. It makes up the bulk of the ocean floor and accounts for 71% of Earth’s surface.

 

Grain Size

The location and rate of cooling determines the crystalline structure of different igneous rocks. Coarser, larger-grained materials indicate the slower cooling of magma beneath Earth’s surface (e.g. granite).

On the other hand, finer, smaller-grained materials indicate the faster cooling of lava (e.g. basalt, which is the most common extrusive igneous rock). If cooling is so rapid that crystals cannot form, the result is a glassy, rock-like obsidian.

Intrusive igneous rocks like diorite (left) have large, course-grained structures, while extrusive igneous rocks like basalt (right) have smaller, fine-grained structures.
Intrusive igneous rocks like diorite (left) have large, course-grained structures, while extrusive igneous rocks like basalt (right) have smaller, fine-grained structures.