Final answer:
Volcanic activity is usually not found close to the trench in subduction zones because the required melting of the asthenosphere occurs above the subducting slab, at a certain depth further away from the trench. The location of volcanoes relative to the coastline depends on the subduction angle, while hotspot volcanic activity, like that of the Hawaiian Islands, can occur far from plate boundaries.
Step-by-step explanation:
Volcanic activity does not typically occur close to the trench in a subduction zone because the processes required for magma formation happen at a specific depth. Partial melting of the asthenosphere occurs above the subducting slab, which usually takes place not immediately adjacent to the trench, but rather at a distance further away from the trench towards the overriding plate. This happens as the subducted plate sinks and reaches a level hot enough for the surrounding rocks to undergo flux melting due to the release of volatiles like water, which lowers the melting temperature of the mantle material above the slab.
In contrast, the location of volcanoes relative to the coastline depends on the angle of subduction. Where subduction angles are steeper, volcanoes tend to be closer to the trench and consequently the coastline. However, if the angle is shallower, the location of volcanism might be further inland. This is because the point of melting, where magma forms, moves further from the trench as the subducting plate descends at a shallower angle.
Mantle plumes and hotspots represent another type of volcanic activity which can form volcanoes far from plate boundaries. The Hawaiian Islands are an example of hotspot volcanism, where magma rises to form volcanoes over a stationary hotspot as the Pacific Plate moves over it.