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Rock cycle facts for kids

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Magma fountain sprays liquid rock from deep underground

The rock cycle is the process by which rocks of one kind change into rocks of another kind.

There are three main kinds of rocks: igneous rock, metamorphic rock, and sedimentary rock. Each of these rocks can change into the other kinds by physical processes: cooling, melting, heat, weathering/erosion, compacting (squeezing tightly together), cementing, and pressure.

When heated deep under ground, rocks become magma (liquid rock). Above ground, it is called lava. Sediment, the particles from rock erosion and weathering, is the basis for sedimentary rock of the future.

Igneous rock is hardened magma, which can happen above or below ground. It can melt into magma, erode into sediment, or be pressed tightly together to become metamorphic.

Metamorphic rock is igneous or sedimentary rock that has been heated and squeezed. It can erode into sediment or melt into magma. It is formed under extreme pressure and temperature deep inside mountain chains.

Sedimentary rock is compacted sediments which can come from any of the other rocks, plus remains of living things. It can erode back into sediment, or be pressurized into metamorphic rock and can be melted to magma , which forms igneous rocks .

These processes can occur in different orders, and the cycle goes on forever. Earth has several processes for changing rocks. Wind and water can create sediment from rocks, and movement of one tectonic plate against another creates enormous heat and pressure which affects rocks greatly. Subduction converts all kinds into magma, which eventually rejoins the cycle as igneous rocks.

Forces that drive the rock cycle

Plate tectonics

In 1967, J. Tuzo Wilson published an article in Nature describing the repeated opening and closing of ocean basins, in particular focusing on the current Atlantic Ocean area. This concept, a part of the plate tectonics revolution, became known as the Wilson cycle. The Wilson cycle has had profound effects on the modern interpretation of the rock cycle as plate tectonics became recognized as the driving force for the rock cycle.

Spreading ridges

At the mid-ocean divergent boundaries new magma is produced by mantle upwelling and a shallow melting zone. This juvenile basaltic magma is an early phase of the igneous portion of the cycle. As the tectonic plates on either side of the ridge move apart the new rock is carried away from the ridge, the interaction of heated circulating seawater through fractures starts the retrograde metamorphism of the new rock.

Subduction zones

The Juan de Fuca plate sinks below the North America plate at the Cascadia subduction zone.

The new basaltic oceanic crust eventually meets a subduction zone as it moves away from the spreading ridge. As this crust is pulled back into the mantle, the increasing pressure and temperature conditions cause a restructuring of the mineralogy of the rock, this metamorphism alters the rock to form eclogite. As the slab of basaltic crust and some included sediments are dragged deeper, water and other more volatile materials are driven off and rise into the overlying wedge of rock above the subduction zone, which is at a lower pressure. The lower pressure, high temperature, and now volatile rich material in this wedge melts and the resulting buoyant magma rises through the overlying rock to produce island arc or continental margin volcanism. This volcanism includes more silicic lavas the further from the edge of the island arc or continental margin, indicating a deeper source and a more differentiated magma.

At times some of the metamorphosed downgoing slab may be thrust up or obducted onto the continental margin. These blocks of mantle peridotite and the metamorphic eclogites are exposed as ophiolite complexes.

The newly erupted volcanic material is subject to rapid erosion depending on the climate conditions. These sediments accumulate within the basins on either side of an island arc. As the sediments become more deeply buried lithification begins and sedimentary rock results.

Continental collision

On the closing phase of the classic Wilson cycle, two continental or smaller terranes meet at a convergent zone. As the two masses of continental crust meet, neither can be subducted as they are both low density silicic rock. As the two masses meet, tremendous compressional forces distort and modify the rocks involved. The result is regional metamorphism within the interior of the ensuing orogeny or mountain building event. As the two masses are compressed, folded and faulted into a mountain range by the continental collision the whole suite of pre-existing igneous, volcanic, sedimentary and earlier metamorphic rock units are subjected to this new metamorphic event.

Accelerated erosion

The high mountain ranges produced by continental collisions are immediately subjected to the forces of erosion. Erosion wears down the mountains and massive piles of sediment are developed in adjacent ocean margins, shallow seas, and as continental deposits. As these sediment piles are buried deeper they become lithified into sedimentary rock. The metamorphic, igneous, and sedimentary rocks of the mountains become the new piles of sediments in the adjoining basins and eventually become sedimentary rock.

An evolving process

Diagram of the rock cycle. Legend:
1 = magma;
2 = crystallization (freezing of rock);
3 = igneous rocks;
4 = erosion;
5 = sedimentation;
6 = sediments & sedimentary rocks;
7 = tectonic burial and metamorphism;
8 = metamorphic rocks;
9 = melting.

The plate tectonics rock cycle is an evolutionary process. Magma generation, both in the spreading ridge environment and within the wedge above a subduction zone, favors the eruption of the more silicic and volatile rich fraction of the crustal or upper mantle material. This lower density material tends to stay within the crust and not be subducted back into the mantle. The magmatic aspects of plate tectonics tends to gradual segregation within or between the mantle and crust. As magma forms, the initial melt is composed of the more silicic phases that have a lower melting point. This leads to partial melting and further segregation of the lithosphere. In addition the silicic continental crust is relatively buoyant and is not normally subducted back into the mantle. So over time the continental masses grow larger and larger.

The role of water

The presence of abundant water on Earth is of great importance for the rock cycle. Most obvious perhaps are the water driven processes of weathering and erosion. Water in the form of precipitation and acidic soil water and groundwater is quite effective at dissolving minerals and rocks, especially those igneous and metamorphic rocks and marine sedimentary rocks that are unstable under near surface and atmospheric conditions. The water carries away the ions dissolved in solution and the broken-down fragments that are the products of weathering. Running water carries vast amounts of sediment in rivers back to the ocean and inland basins. The accumulated and buried sediments are converted back into rock.

A less obvious role of water is in the metamorphism processes that occur in fresh seafloor volcanic rocks as seawater, sometimes heated, flows through the fractures and crevices in the rock. All of these processes, illustrated by serpentinization, are an important part of the destruction of volcanic rock.

The role of water and other volatiles in the melting of existing crustal rock in the wedge above a subduction zone is a most important part of the cycle. Along with water, the presence of carbon dioxide and other carbon compounds from abundant marine limestone within the sediments atop the down going slab is another source of melt inducing volatiles. This involves the carbon cycle as a part of the overall rock cycle.

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