User:Kent G. Budge/sandbox

Petrology
In a subduction zone, loss of water from the subducted slab induces partial melting of the overriding mantle and generates low-density, calc-alkaline magma that buoyantly rises to intrude and be extruded through the lithosphere of the overriding plate. Most of the water carried downwards by the slab is contained in hydrous (water-bearing) minerals, such as mica, amphibole, or serpentinite minerals. Water is lost from the subducted plate when the temperature and pressure become sufficient to break down these minerals and release their water content. The water rises into the wedge of mantle overlying the slab and lowers the melting point of mantle rock to the point where magma is generated.

However, while there is wide agreement on the general mechanism, research continues on the explanation for focused volcanism along a narrow arc some distance from the trench. The distance from the trench to the volcanic arc is greater for slabs subducting at a shallower angle, and this suggests that magma generation takes place when the slab reached a critical depth for the breakdown of an abundant hydrous mineral. This would produce an ascending "hydrous curtain" that accounts for focused volcanism along the volcanic arc. However, some models suggest that water is continuously released from the slab from shallow depths down to 70 to 300 km, and much of the water released at shallow depths produces serpentinization of the overlying mantle wedge. According to one model, only about 18 to 37 percent of the water content is released at sufficient depth to produce arc magmatism. The volcanic arc is then interpreted as the depth at which the degree of melting becomes great enough to allow the magma to separate from its source rock.

It is now known that the subducting slab is located anywhere from 60 to 173 km below the volcanic arc, rather than a uniform depth of around 120 km, which requires more elaborate models of arc magmatism. For example, water released from the slab at moderate depths might react with amphibole minerals in the lower part of the mantle wedge to produce water-rich chlorite. This chlorite-rich mantle rock is then dragged downwards by the subducting slab, and eventually breaks down to become the source of arc magmatism. The location of the arc depends on the angle and rate of subduction, which determine where hydrous minerals break down and where the released water lowers the melting point of the overlying mantle wedge enough for melting.

The location of the volcanic arc may be determined by the presence of a cool shallow corner at the tip of the mantle wedge, where the mantle rock is cooled by both the overlying plate and the slab. Not only does the cool shallow corner suppress melting, but its high stiffness hinders the ascent of any magma that is formed. Arc volcanism takes place where the slab descends out from under the cool shallow corner, allowing magma to be generated and rise through warmer, less stiff mantle rock.

Magma may be generated over a broad range of depth of the slab but become focused into a narrow volcanic arc by a permeability barrier at the base of the overriding plate. Numerical simulations suggest that crystallization of rising magma creates this barrier, causing the remaining magma to pool in a narrow band at the apex of the barrier. This narrow band corresponds to the overlying volcanic arc.

Central Pangean mountains
Equals Hercynian megasuture?

Sources frequently used, in year order




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