Final answer:
The subduction zone is the tectonic setting where the asthenosphere is farthest from the Earth's surface. Volcanoes are generally closer to the coastline in subduction zones due to the proximity of the melting asthenosphere to the ocean trench.
Step-by-step explanation:
Tectonic Setting and the Asthenosphere
The tectonic setting that places the asthenosphere farthest from the surface is a subduction zone. A subduction zone occurs when two tectonic plates converge and one, typically the denser oceanic plate, is forced underneath the other, often a lighter continental plate. As the subducted plate descends, it heats up and begins to melt, forming magma that can contribute to volcanic activity. This process takes the asthenosphere, which is beneath the lithosphere, deeper into the Earth's interior, moving it further from the surface than in other tectonic settings such as rift zones.
Regarding volcanoes, they are generally found closer to the coastline in subduction zones where the oceanic plate is thrust beneath the continental plate. The reason for this is that the subduction process and the subsequent partial melting of the asthenosphere occur relatively near the oceanic trench that marks the boundary between the converging plates. This proximity to the trench means that the resulting volcanic arcs are often close to the coastline of the overriding plate.