Rock cycle
For example, an igneous rock such as basalt may break down and dissolve when exposed to the atmosphere, or melt as it is subducted under a continent.
If the conditions no longer exist for the magma to stay in its liquid state, it cools and solidifies into an igneous rock.
Silicification, the replacement of the minerals by crystalline or crypto-crystalline silica, is most common in felsic rocks, such as rhyolite, but is also found in serpentine, etc.
Regional metamorphism refers to the effects on large masses of rocks over a wide area, typically associated with mountain building events within orogenic belts.
These rocks commonly exhibit distinct bands of differing mineralogy and colors, called foliation.
The new basaltic oceanic crust eventually meets a subduction zone as it moves away from the spreading ridge.
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.
On the closing phase of the classic Wilson cycle, two continental or smaller terranes meet at a convergent zone.
The high mountain ranges produced by continental collisions are immediately subjected to the forces of erosion.
[8] Erosion wears down the mountains and massive piles of sediment are developed in adjacent ocean margins, shallow seas, and as continental deposits.
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.
Running water carries vast amounts of sediment in rivers back to the ocean and inland basins.
[12] 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.
