Explain which type of melting (flux, decompression, heat transfer) is responsible for mafic magmas in each of the following plate-tectonic settings and why the resulting magma is mafic.

Solution :

The type of melting that is responsible for the mafic magmas for the following plate tectonic settings :

a). Mid - ocean ridges

At the mid oceanic ridges, the two plates diverges away and the new oceanic crust is formed by the effusion of the magma. The earth's interior is exposed as the two plates are moved away.

b). Continental rift :

A new oceanic crust is formed by the effusion of the magma and the two plates diverges away. Thus it causes the formation of the melt in the continental drift. As a result, the crustal thickness decreases.

c). Oceanic and continental volcanic arc :

The oceanic arc and the volcanic arc are the locations of the convergence. The denser plate of the arcs subducts under the less denser plates or the  lighter plates and it moves downwards into the mantle, which causes the increase in temperature of the surroundings.