Sharp jumps in seismic velocity are called discontinuities . Mohorovicic Discontinuity separates crust and mantle. Origin of layering and discontinuities Several models. Mantle is solid that flows. Convection in mantle responsible for moving plates.
Earthquakes occur at plate boundaries.
Volcanoes occur at plate boundaries and also at hot spots. Presence of oxygen is unique in the solar system. Due to photosynthesis. Carbon dioxide increasing.