Seafloor spreading, also called ocean floor spreading, is the process where new oceanic crust is formed at mid-ocean ridges, causing the seafloor to expand. As tectonic plates move away from each other at these ridges, magma rises up from the Earth's mantle to fill the gap, creating new crust. This process is driven by convection currents in the upper mantle and is an example of divergent plate tectonics, where ocean plates separate to make room for the formation of new crust between them.