Final answer:
In the circled area indicating a subduction zone, old oceanic crust is subducted beneath a plate, leading to earthquakes, volcanic activity, and mountain building as part of plate tectonics.
Step-by-step explanation:
The circled area in Figure 8.9, likely depicting a subduction zone, is a region where the process of subduction occurs. This is part of the cycle of plate tectonics, where new crust is formed at rift zones and old crust is destroyed at subduction zones. During subduction, an oceanic plate is forced beneath another plate into the upper mantle, where it melts and creates a balance in crustal material. This process is often accompanied by various occurrences such as the formation of volcanoes, the occurrence of earthquakes, and mountain building due to the pressures and temperatures involved.
Subduction zones are marked by features like ocean trenches and are responsible for the creation of geological phenomena such as volcanic arcs and deep-sea trenches. Furthermore, the material from the subducted plate, melting several hundred kilometers below the surface, helps form volcanoes that provide a sample of material from deeper within the planet.