Density of sea water is comparatively higher in polar region than at equator due to lower temperature, consequently thrust due to liquid pressure is more over the poles, which make the poles slightly flat and the equator bulged out.
Vertical thrust due to pressure depends on depth, hence where there is more depth experiences higher thrust, leading to subduction and erosion.
Temperature and density differences of sea water cause formation of convection currents. These currents; hot as well as cold, usually create turbulent sea movement. Such movements are the major agents of forces that promotes sea-floor spreading and suduction.