Question

How is magma produced at a subduction zone?

Answer 1

Hint: The molten material formed during the pressure and temperature variation in the earth deep inside which comes out as lava is rich in minerals. Most of the iron rich minerals and the hard rocks are a result of cooling down of this material.

Complete answer:
Magma is generated within the asthenospheric wedge above a subducted plate which may result from both anhydrous decompression melting and also as a by fluid-fluxed melting. While the previous process produces tholeiitic basalt magma, the latter process produces a spread of magmas—from alkali basalt to high-Mg andesite. The fluids which are formed during the process are mostly from chlorite breakdown reaction that takes place in this process. Serpentine breakdown could also be involved in cooler slabs, but amphibole breakdown as a source of fluid remains debatable.

The fluid released could also be a supercritical fluid or a fluid + melt composite that rises up to the warmer part of the asthenospheric wedge, where it triggers more voluminous magma generation. Hydrous melting of the different types of silicic sedimentary rocks and the basaltic rocks located within the subducted oceanic crust below the ocean floor can also form small volumes of granitic melts which eventually form granite, the hardest rock.

Note: The magma comes out of the earth as it is lighter than the mantle and hence it comes out of the subduction zones of the Earth. In case it does not find an opening, it consolidates itself and ultimately results in a volcanic eruption.