1. Ridge Push: This force arises from the elevated position of mid-ocean ridges, where new oceanic crust is formed. Gravity pulls the newly formed, relatively warm and buoyant crust downhill, pushing the plates away from the ridge.
2. Slab Pull: This force originates from the sinking of dense oceanic plates (also called slabs) into the mantle at subduction zones. As the slab descends, it pulls the rest of the plate along behind it.
3. Mantle Convection: This force is driven by heat from the Earth's core. Hotter, less dense material rises, while cooler, denser material sinks. This creates a cycle of convection currents within the mantle that can exert a dragging force on the overlying lithospheric plates.
These three forces are not necessarily equal in strength or influence. The relative importance of each force varies depending on the specific location and tectonic setting.
