EXERCISE 5.11 Origin of Intermediate Magmas in Subduction Zones Section: Date:_-

Question

EXERCISE 5.11 Origin of Intermediate Magmas in Subduction Zones Section: Date:_- Name Course Melting above the subducted slab produces mafic magma, as described above, yet intermediate rocks (andesite, diorite) between oceanic and continental lithosphere, explain how this intermediate magma forms (a) A relatively small amount of intermediate magma occurs in in many subduction zones. Considering the magmatic processes discussed in section 5.4, and the difference oceanic volcanic arcs. Remembering that melting above the subducted slab produces mafic magma, explain the origin of the intermediate (b) Much more intermediate magma erupts in continental arcs. Why?

Explanation / Answer

(a) Below 150 km under the Earth, heat directly cause the volatiles present within the subducting zone to get released into ultramafic asthenosphere of overriding tectonic plate. Partial melting of this asthenosphere normally produces basaltic magma in huge amounts, which will migrate upward through lithosphere just to get erupted at the surface.
So depending u[on the amount of thecrystallization that is occuring during this transit, this basaltic (mafic) magma may extrude as an intermediate one.

(b) In continental arcs igneous rocks formation is more complex than that of the oceanic crust. Oceanic arc crust have more mafic magma whereas the continental arc crust have intermediate magma in majority. So that is the reaon why intermediate magma erupts there. Also the partial melt of the subduction oceanic part produces the prmary magma which is further contaminated by the continental crust materials and intermediate magma is formed.

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