When two oceanic plates share a convergent type of plate boundary, the denser oceanic plate will subduct below the less dense oceanic plate. This will result in the formation of the subduction zone, where the rocks are being pulled down to the mantle. This subduction zone is typically marked by the presence of a narrow depression commonly known as an oceanic trench, that lies just above the zone.
The rocks of the subducting plate undergo partial melting and mix up with the magma that rises upwards towards the surface due to the force exerted by the convection currents. This later gives rise to the formation of volcanoes or a chain of volcanoes which are commonly known as an island arc.
Today we can make observations that would lead us to the same conclusion as newton. If you throw a baseball in the air, it always comes back down. This is because of gravity. duh.
