A new study links seafloor spreading ridges in the ocean to glacial periods. During glacial periods, water in the oceans move on land and are stored in continental glaciers (much like modern day Greenland and Antarctica). This in turn causes less weight be be pushed down on the ocean crust. Rock in the mantle is allowed to be melted and cause volcanic eruptions due to the lower pressure. The mid-ocean ridges are also synced up with Milankovitch Cycles.