Final answer:
The top of the asthenosphere is closest to the Earth's surface along a mid-ocean ridge, where the tectonic plates are pulling apart and new crust is formed by upwelling magma.
Step-by-step explanation:
The top of the asthenosphere is closest to the surface along a mid-ocean ridge. At these locations, tectonic plates pull apart from each other, and this process is facilitated by upwelling currents in the mantle. The asthenosphere partially melts due to decompression, and the resultant basaltic lava rises to fill the gap created by the separating plates. The upwelling of magma from the asthenosphere creates new oceanic crust, pushing the older crust away from the ridge and towards subduction zones. This process does not just create new crust but also brings the asthenosphere closer to the Earth's surface compared to other areas like subduction zones or transform faults.