Hot magma rises from the mantle at mid-ocean ridges pushing the plates apart. Earthquakes occur along the fractures that appear as the plates move apart.:
All of the earth crusts are cycled through, each plate goes down into the plates crust, and gets heated and then comes back up as new crust after many many years of cycling through. So, when the plates sink into each other, the other side of the plates pull apart from each other.
