HiSET Science: Cycling of Earth's Materials

Rock Cycle
The rock cycle is the process whereby the materials that make up the Earth transition through the three types of rock: igneous, sedimentary, and metamorphic. Rocks, like all matter, cannot be created or destroyed; rather, they undergo a series of changes and adopt different forms through the functions of the rock cycle. Plate tectonics and the water cycle are the driving forces behind the rock cycle; they force rocks and minerals out of equilibrium and force them to adjust to different external conditions.

Viewed in a generalized, cyclical fashion, the rock cycle operates as follows: rocks beneath Earth's surface melt into magma. This magma either erupts through volcanoes or remains inside the Earth. Regardless, the magma cools, forming igneous rocks. On the surface, these rocks experience weathering and erosion, which break them down and distribute the fragments across the surface. These fragments form layers and eventually become sedimentary rocks. Sedimentary rocks are then either transformed to metamorphic rocks (which will become magma inside the Earth) or melted down into magma.

Rock Formation
Igneous Rocks: Igneous rocks can be formed from sedimentary rocks, metamorphic rocks, or other igneous rocks. Rocks that are pushed under the Earth's surface (usually due to plate subduction) are exposed to high mantle temperatures, which cause the rocks to melt into magma. The magma then rises to the surface through volcanic processes. The lower atmospheric temperature causes the magma to cool, forming grainy, extrusive igneous rocks. The creation of extrusive, or volcanic, rocks is quite rapid. The cooling process can occur so rapidly that crystals do not form; in this case, the result is a glass, such as obsidian. It is also possible for magma to cool down inside the Earth's interior; this type of igneous rock is called intrusive. Intrusive, or plutonic, rocks cool more slowly, resulting in a coarse-grained texture.
Sedimentary Rocks: Sedimentary rocks are formed when rocks at the Earth's surface experience weathering and erosion, which break them down and distribute the fragments across the surface. Fragmented material (small pieces of rock, organic debris, and the chemical products of mineral sublimation) is deposited and accumulates in layers, with top layers burying the materials beneath. The pressure exerted by the topmost layers causes the lower layers to compact, creating solid sedimentary rock in a process called lithification.
Metamorphic Rocks: Metamorphic rocks are igneous or sedimentary rocks that have "morphed" into another kind of rock. In metamorphism, high temperatures and levels of pressure change preexisting rocks physically and/or chemically, which produces different species of rocks. In the rock cycle, this process generally occurs in materials that have been thrust back into the Earth's mantle by plate subduction. Regional metamorphism refers to a large band of metamorphic activity; this often occurs near areas of high orogenic (mountain-building) activity. Contact metamorphism refers to metamorphism that occurs when 'country rock' (that is, rock native to an area) comes into contact with high-heat igneous intrusions (magma).

Plate Tectonics Rock Cycle
The plate tectonics rock cycle expands the concept of the traditional rock cycle to include more specific information about the tectonic processes that propel the rock cycle, as well as an evolutionary component. Earth's materials do not cycle endlessly through the different rock forms; rather, these transitive processes cause, for example, increasing diversification of the rock types found in the crust. Also, the cycling of rock increases the masses of continents by increasing the volume of granite. Thus, the tectonic rock cycle is a model of an evolutionary rock cycle. In this model, new oceanic lithosphere is created at divergent plate boundaries. This new crust spreads outward until it reaches a subduction zone, where it is pushed back into the mantle, becomes magma, and is thrust out into the atmosphere.
It experiences erosion and becomes sedimentary rock. At convergent continental plate boundaries, this crust is involved in mountain building and the associated metamorphic pressures. It is eroded again, and returns to the lithosphere.

Role of Water
Water plays an important role in the rock cycle through its roles in erosion and weathering: it wears down rocks; it contributes to the dissolution of rocks and minerals as acidic soil water; and it carries ions and rock fragments (sediments) to basins where they will be compressed into sedimentary rock. Water also plays a role in the metamorphic processes that occur underwater in newly-formed igneous rock at mid-ocean ridges. The presence of water (and other volatiles) is a vital component in the melting of rocky crust into magma above subduction zones.


Metamorphism
Metamorphism is the process whereby existing sedimentary, igneous, or metamorphic rocks (protoliths) are transformed due to a change in their original physiochemical environment, where they were mineralogically stable. This generally happens alongside sedimentation, orogenesis, or the movement of tectonic plates. Between the Earth's surface and a depth of 20 kilometers, there exists a wide range of temperatures, pressure levels, and chemical activity. Metamorphism is generally an isochemical process, which means that it does not alter the initial chemical composition of a rock. The changes a rock undergoes in metamorphism are usually physical. Neither a metamorphosing rock nor its component minerals are melted during this process—they remain almost exclusively in a solid state.

Metamorphism, like the formation of plutonic rock bodies, can be studied only after metamorphic rocks have been exposed by weathering and erosion of the crustal rocks above.

Factors
Heat is a primary factor in metamorphism. When extreme heat is applied to existing rocks, their component minerals are able to recrystallize (which entails a reorganization of the grains or molecules of a mineral, resulting in increased density, as well as the possible expulsion of volatiles such as water and carbon dioxide). High levels of thermal energy may also cause rocks to contort and deform. Pressure is another factor affecting the metamorphism of rocks. Increased pressure can initiate recrystallization through compression. Pressure forces can also lead to spot-melting at individual grain boundaries. Lithostatic, or confining, pressure is created by the load of rocks above a metamorphosing rock.

Pore-fluid pressure results from the release of volatiles due to thermal energy. Directed pressure is enforced in a certain direction due to orogenesis:
This type of pressure is responsible for foliation, or layering, which entails parallel alignment of mineral particles in a rock, characteristic of metamorphism. Chemical activity affects metamorphism due to the presence of volatiles in pore fluids.

Biogeochemical Cycle
The term biogeochemical cycle refers to one of several chemical processes in which chemical elements are (re)cycled among biotic
(living) and abiotic (nonliving) constituents of an ecosystem. The theory of relativity necessitates the presence of such cycles in nature by virtue of its supposition that energy and matter are not created or destroyed in a closed system such as Earth's ecosystem. Generally, a biogeochemical cycle operates as follows: inorganic compounds, such as carbon, are converted from water, air, and soil to organic molecules by organisms called autotrophs.

Heterotrophs (organisms that cannot independently produce their own food) consume the autotrophs; some of the newly formed organic molecules are transferred. Finally, the organic molecules are broken down and processed once again into inorganic compounds by secondary and tertiary consumers and replaced within water, air, and soil. Carbon, nitrogen, and phosphorus provide examples of nutrients that are recycled in the Earth's ecosystem.