Rocks, like all materials on Earth, are part of a cycle in which they form, break down, and are eventually recycled. The rock cycle may be said to begin when molten magma rises up from the Earth’s mantle and cools at or near the Earth’s surface. When magma cools, the minerals crystallize, forming igneous rocks. Igneous rocks that form on the surface of the Earth are known as extrusive (meaning “to push out from”) or volcanic (after the Roman God of fire, Vulcan) rocks. Those that crystallize underground, often deep within the crust, are known as intrusive or plutonic (after the Greek God of the underworld, Pluto) rocks. Extrusive igneous rocks are said to form from lava rather than magma — lava is essentially the rock material that is left when gases escape from magma as it reaches the surface.
Extrusive igneous rocks are found in abundance near their source: volcanoes or fissures that extrude lava. Volcanoes are found throughout the world, but are particularly common along tectonic plate boundaries. The Cascade Range in the northwestern U.S., for example, is dotted by a number of active volcanoes like Mount St. Helens, which last errupted in 1980. Composite volcanoes like Mt. St. Helens are explosive; they build up pressure until the overlying rock can no longer contain the underlying magma. Other volcanoes, like those found in Hawaii, release lava slowly over long periods of time and therefore do not build up enough pressure to explode. Extrusive igneous rocks tend to sprawl over a wide area, creating layers upon layers of new rock as lava is continuously or periodically released from the volcanoe or fissure.
Intrusive igneous rocks are said to “form at depth;” they never reach the surface. Much of the magma that finds its way into the Earth’s crust actually cools at depth, often forming large rock structures known as plutons. Some plutons can cover hundreds of kilometers and add significantly to the thickness of the crust at certain locations. Due to crustal uplifting and the weathering of rocks at the surface, plutons may be exposed after being confined underground for millions of years. Mt. Rushmore in South Dakota was actually carved into an enormous exposed pluton, known as the Black Hills. Other recognizable plutons in the U.S. include Stone Mountain, Georgia, Mount Washington, and much of Yosemite National Park.
Once igneous rock is exposed to the atmosphere it begins to undergo the process of weathering, in which wind, water, ice, and chemicals begin to physically and chemically break down the rock. Physical weathering breaks the rock into smaller and smaller pieces, while chemical weathering actually alters the chemical nature of the rock by adding or removing elements. As the rock breaks down into smaller pieces, it may be transported to other locations by running water, strong wind, the movement of glacial ice, or simply downhill under the influence of gravity (a process known as mass wasting). Eventually the broken-down rock, or the minerals that once composed the rock, are deposited in some low lying location like a river floodplain, desert basin or ocean basin. At this point the weathered rock is known as sediment.
Sediment is deposited in a series of layers, and over thousands or millions of years, the layers may be buried hundreds of meters beneath the Earth’s surface. Under the pressure of the overlying layers, lose sedimentary particles eventually begin to fuse together in a process known lithification, which means “conversion to rock.” Sometimes the lithification process is a chemical, rather than physical, process. Minerals once suspended in ground water can settle (or precipitate) out, forming new layers of rock. Whether through a physical or chemical process, or a combination of the two, lithified sediment becomes sedimentary rock. Most of the Earth’s surface is actually covered by sedimentary rock. Most of the rock layers seen at the Grand Canyon are sedimentary rocks layers formed from sedimentary deposits laid down millions of years ago.
Both igneous and sedimentary rock are often subjected to enormous pressures when buried deep within the Earth. At a depth of several kilometers, the heat and pressure may be enough to deform or partially melt the rock. While rocks tend to be brittle when cool, they can bend and flow easily when subjected to enough heat and pressure. Chemicals can also alter the mineral structure of the rock, producing a new material. If a rock is permanently altered either by physical or chemical means, it is said to be metamorphic. This process, known as metamorphism (meaning to “change form”), can happen to any type of rock, including existing metamorphic rock. Large quantities of metamorphic rock is produced during the process of mountain building, when extreme compressional forces provide the heat and pressure necessary to bend, squeeze, and permanently alter rock on a massive scale.
Rock may become buried deep enough to melt completely, in which point it once again becomes magma. This occurs on a huge scale when large pieces of the Earth’s crust, known as tectonic plates, descend into the mantle. The rock may eventually melt and return to the surface as magma or lava, producing a new “generation” of igneous rock. While this may complete the rocks cycle, it’s important to note that it is by no means a simple, one-way process. Sedimentary rocks, for example, can be broken down into sediments, eventually forming new sedimentary rocks. Likewise, sedimentary, metamorphic and igneous rocks can all undergo metamorphosis, be broken down into sediments, or undergo melting, provided that the right conditions are in place.