As the leading edge of plate A continues to move downward, typically in a subduction zone, several key geological processes are expected to occur:

• Increasing Pressure and Temperature: As the plate descends deeper into the Earth's mantle, it encounters progressively higher pressures and temperatures. This causes metamorphism, altering the mineral composition and physical properties of the subducting plate

• Melting and Magma Formation: The intense heat and pressure can cause partial melting of the subducting plate and surrounding mantle material. This melting generates magma, which can rise to the surface and lead to volcanic activity, often forming volcanic arcs above the subduction zone

• Earthquake Generation: The movement and interaction of the descending plate with the overriding plate generate significant stress and strain, frequently resulting in earthquakes along the subduction interface

• Formation of Oceanic Trenches: The bending and sinking of the plate create deep, narrow oceanic trenches at the subduction boundary

• Deformation and Downward Bending (Downreit Effects): The weight and motion of the subducting plate cause flexural bending and deformation of the surrounding rock layers, contributing to geological features such as folds, faults, and basins near the subduction zone

• Release of Volatiles: As the plate descends, water and other volatiles are released from minerals in the plate, which can lower the melting point of the mantle and further promote magma generation

Overall, the leading edge of plate A will continue to subduct into the mantle, driving volcanic activity, seismic events, and the formation of characteristic geological structures like trenches and mountain ranges over geological time scales