Final answer:
The process at point b due to the convergence of plates of two different densities is subduction, where the denser plate descends into the mantle, leading to melting and possible volcanic activity.
Step-by-step explanation:
At point b in the diagram where two tectonic plates of different densities are converging, the process occurring is known as subduction.
This process involves the denser, often oceanic, plate descending beneath the less dense, often continental, plate into the Earth's mantle. As the oceanic plate subducts, it encounters increasing pressure and temperature, which can lead to the melting of materials and result in volcanic activities or the formation of mountain ranges. Typically, an ocean trench marks a subduction zone, and it's where the older crust is recycled into the mantle, balancing the crust generated at divergent boundaries like mid-ocean ridges.