Figure 9-15: The Mechanism of Plate Tectonics Convection currents transport rock to a rift where it oozes toward the surface as magma, hardens in the cold ocean, and spreads away from the rift to create new seafloor. Each side of the rift exhibits seafloor spreading and is a separate tectonic plate. When higher-density seafloor meets lower-density continental crust at a subduction zone, the seafloor plunges downward. Slab pull, due to gravity on this slab, is the dominant force moving the plate across Earth. Collision of plates at a subduction zone also leads to mountain-building, earthquakes, and volcanoes.