Final answer:
Volcanic activity is not typically found close to trenches because partial melting that leads to volcanism occurs at a depth beneath the earth's surface due to water released from subducting plates, therefore volcanoes are found further away from the coastline. Hot spots and mid-ocean ridges can cause volcanism directly at plate boundaries or within the interior of plates.
Step-by-step explanation:
Volcanic activity tends to not occur directly adjacent to subduction zone trenches. This is because the process of partial melting of the mantle above the subducting slab, which leads to volcanism, requires certain conditions that are not present immediately at the trench.
As an oceanic plate is forced under another tectonic plate (subduction), it descends into the hotter depths of the Earth. The subducting plate contains minerals with water in their crystalline structures, which begin to destabilize and release water.
This water lowers the melting temperature of the mantle above the subducting plate. However, this process occurs at a certain depth beneath the Earth's surface and not immediately at the trench, which is why volcanoes are typically located at some distance from the coastline where the trench is more proximal.
By contrast, volcanic activity can occur directly at plate boundaries in the case of mantle hot spots or at divergent boundaries such as mid-ocean ridges. Hot spots, like the one beneath Hawaii, are stationary areas where plumes of hot mantle material rise towards the Earth's surface, regardless of their location relative to plate boundaries.
These features can produce volcanism far from trenches and even within the interior of plates. Similarly, mid-ocean ridges experience melting because of decompression as tectonic plates pull apart, allowing magma to rise and form new oceanic crust.