The answer is gravitational:)
At the subduction zone a very deep trench is formed in the ocean floor. Oceanic and oceanic plate convergence result in the formation of volcano chains. The crust that is pulled under or subducted melts to form magma. This magma rises to the top of the overriding oceanic plates and erupts on the ocean floor.
So the answer would be Volcano
I'm not sure about the magnitude, but the direction of the normal force is upward.
