Final answer:
The zone where lithospheric plates sink into the upper mantle, causing earthquakes, is known as the Benioff zone. This zone is associated with subduction zones and contributes to the occurrence of earthquakes and volcanic activity due to the interaction between tectonic plates.
Step-by-step explanation:
The zone where plates of the lithosphere sink into the upper mantle, causing earthquakes along the upper boundary of the sinking plate, is known as the Benioff zone. This occurrence is closely related to subduction zones, where one tectonic plate moves under another and is forced to sink due to gravity into the mantle. Subduction zones are a type of convergent boundary where the denser, often oceanic plate, descends beneath a less dense plate, leading to a range of earthquake depths from shallow to deep. Earthquakes in the Benioff zone can vary in angle and depth, with some occurring in a flat-slab subduction scenario, which causes shallower quakes and can affect the uplifting of mountains and volcanic activity.
In the context of plate tectonics, the descending slab can cause partial melting of the asthenosphere above it, contributing to volcanic activity that is frequently found in proximity to coastlines where subduction occurs. This process illustrates the dynamic nature of Earth's lithosphere and its role in shaping the geological features of our planet.