The primary force that causes seafloor to spread and continents drift is the Earth's internal convection currents within the Earth's mantle. Convection currents are the movement of heated material away from a heat source and cold material towards it, forming a circular motion. The Earth's heat source is the residual heat from its formation and radioactive decay. Within the Earth, this heat causes the solid rock in the mantle to soften and become more fluid-like. This allows for it to slowly move or "convect." As material within the mantle rises towards the surface, cooler, denser material sinks toward the core, creating a cycle. The movement of the mantle material drags along the tectonic plates on the Earth's surface, causing them to drift and interact with each other. This motion, combined with other geological processes such as subduction and rifting, is what causes the continents to move and shapes the Earth's surface.