Final answer:
Magma primarily forms at subduction margins due to the addition of volatiles such as water, which lowers the mantle's melting temperature as the oceanic crust subducts and releases these substances.
Step-by-step explanation:
Magma forms primarily as a consequence of the addition of volatiles at subduction margins. This process is known as flux melting and occurs when one tectonic plate with oceanic crust subducts under another plate. Water and other volatiles released from the subducting plate lower the melting temperature of the mantle above the plate, leading to magma formation. This is in contrast to magma generation at mid-ocean ridges due to decompression melting, hot-spot volcanoes as a result of mantle plumes, and the general lack of volcanism at transform faults. Subduction margin volcanoes play a significant role in giving us access to material from deeper within the Earth and are often located closer to coastlines due to the nature of oceanic-continental convergence.