5.1: The Rock Cycle

The Rock Cycle

The rock components of the crust are slowly but constantly being changed from one form to another and the processes involved are summarized in the rock cycle. The rock cycle is driven by two forces: (1) Earth’s internal heat engine, which moves material around in the core and the mantle and leads to slow but significant changes within the crust, and (2) the Sun which powers the hydrological cycle, moving water, wind and air along Earth’s surface.

The rock cycle is still active on Earth because our core is hot enough to keep the mantle moving, our atmosphere is thick enough to support circulation, and we have liquid water. On some other planets or their satellites, such as the Moon, the rock cycle is virtually dead because the core is no longer hot enough to drive mantle convection and there is no atmosphere or liquid water.

Figure \(\PageIndex{1}\): The Rock Cycle. (Creative Commons Attribution 4.0 International License; Steven Earle)

In describing the rock cycle, we can start anywhere we like, although it’s convenient to start with magma because from magma, new minerals and igneous rock are created. Magma is rock that has been heated to the point of being entirely molten. Magma can either cool slowly within the crust (over centuries to millions of years)—forming intrusive igneous rock, or erupt onto the surface, where it is called lava, and cool quickly (within seconds to years)—forming extrusive igneous rock (volcanic rock). Intrusive igneous rock typically crystallizes at depths of hundreds of meters to tens of kilometers below the surface.

Figure \(\PageIndex{2}\): Granite, an example of an igneous rock. Zoom in close to notice the interlocking fabric of minerals crystals of different size and shape. See if you can identify the common minerals: quartz, sodium-rich plagioclase, hornblende and biotite. (CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.)

Plate tectonic processes of mountain building can allow for all types of rock to be uplifted and exposed at the surface. These include rock that is buried deep within the continents and even rock that is part of the crust of the deep ocean basins. Once rock of any type is exposed at the surface, it is attacked by the weathering agents of water, wind, ice and gravity. Rock is weathered, both physically (breaking into smaller pieces) and chemically (by alteration of the minerals). These weathering products—mostly small rock and mineral fragments—are eroded, transported, and then deposited as sediments. Transportation and deposition occur through the action of glaciers, streams, waves, wind, and other agents, and sediments are deposited in rivers, lakes, deserts, and the ocean. Sediments will eventually become buried by more sediments in some type of an accumulating basin. At depths of hundreds of meters or more, they become compressed and cemented into sedimentary rock.

Figure \(\PageIndex{3}\): Quartz sandstone, an example of a sedimentary rock. Zoom in close and notice the rounded quartz grains that are cemented together by white silica cement. The reddish color is the result of chemical weathering of minor iron minerals. (CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.)

Any rock type that becomes buried deep within the crust is subjected to increasing heat and confining pressure. Tectonic forces of colliding plates can add additional stress to the buried rock. These changing conditions affect the stability of the pre-existing minerals and the fabric, or texture, of the original rock. This results in the formation of metamorphic rock.

Figure \(\PageIndex{4}\): Mica schist, an example of a metamorphic rock. Tiny muscovite mica minerals give this sample a “glittery” appearance. This metamorphic rock began its life as a clay rich sedimentary rock, like shale. Increased temperature and pressure conditions caused by mountain building deep within the crust caused the metamorphism. The black crystals are the iron-oxide mineral, magnetite. (CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.)

Did I Get It? - Quiz

Exercise \(\PageIndex{1}\)

Which two forces drive the rock cycle?

a. Earth's internal heat and the biological cycle

b. The Sun and gravity

c. Earth's internal heat and the Sun

d. The hydrological cycle and volcanoes

Answer

c. Earth's internal heat and the Sun

Exercise \(\PageIndex{2}\)

When describing the processes of the rock cycle, you must begin with igneous rocks.

a. No, the rock cycle processes are continuous and you can begin anywhere.

b. Yes, the rock cycle begins with the formation of igneous rocks.

Answer

a. No, the rock cycle processes are continuous and you can begin anywhere.

Exercise \(\PageIndex{3}\)

Agents of the weathering process include

a. Water, temperature, wind and gravity

b. Ice, water, pressure and wind.

c. Gravity, water, wind and ice.

Answer

c. Gravity, water, wind and ice.

Exercise \(\PageIndex{4}\)

Metamorphic conditions of change include

a. chemical and physical weathering

b. erosion and transportation

c. pressure and temperature

Answer

c. pressure and temperature