Here, a magma chamber forms underneath oceanic crust and causes sea-floor spreading in the creation of new oceanic crust.[3] Some of the force that pushes the two plates apart is due to ridge push force of the magma chamber.[4] Tension, however, accounts for most of the "opposite directions" pull on the plates. As the separating oceanic crust cools over time, it becomes more dense and sinks farther and farther away from the ridge axis. The cooling and sinking ocean crust causes a tensile stress that also helps drive the pulling apart of the plates at the ridge axis.