Final answer:
Magma primarily forms in subduction zones due to the addition of volatiles released from subducting hydrated minerals, which lowers the mantle's melting temperature.
Step-by-step explanation:
Magma forms primarily as a consequence of the addition of volatiles in subduction zones. This happens because oceanic crust-topped tectonic plates subduct below another plate, bringing hydrated minerals into the mantle. As these minerals heat up and destabilize at depth, they release water, reducing the melting temperature of the overlying mantle and producing magma, which can lead to volcanic activity.
By contrast, mid-ocean ridges involve magma generation primarily through decompression melting as plates pull apart, whereas hotspot volcanism is caused by mantle plumes from the core-mantle boundary heating the lithosphere above. Transform faults, on the other hand, typically feature shallow earthquakes and are not associated with the generation of magma.