Seafloor spreading is the process where new oceanic crust is formed at the mid-ocean ridges, because volcanic activity occurs and moves the plates gradully. As magma rises and solidifies and the newer crust is formed the newer crust pushes the older crust, which is why the seafloor distance increases.
theory of lithospheric evolution that holds that the ocean floors are spreading outward from vast underwater ridges. First proposed in the early 1960s by the American geologist Harry H. Hess, its major tenets gave great support to the theory of
the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. A subduction zone is an area on Earth where two tectonic plates move towards one another and subduction occurs. Rates of subduction are typically measured in centimeters per year, with the average rate of convergence being approximately 2 to 8 centimeters per year (about the rate a fingernail grows).