Seafloor spreading and continental drift are believed to be caused by

Convection current

Explanation:

The seafloor spreading refers to the splitting of the oceanic crust in two opposite direction, resulting in the expansion of the ocean basin. And the continental drift refers to the large scale movement of the continents from one location to another, over the vast ocean water bodies.

Both these processes are believed to be caused by the convection current, that generates in the mantle portion of the earth. In the mantle, the magma being less dense, rises from the interior of the earth, and as it rises upwards, it forms convection cells, that forces the plates to move in different directions sharing a type of plate boundary. This entire process is known as the plate tectonic movement.