Flexi Says: Divergent boundaries form where convection cells in the mantle bring large amounts of hot, molten mantle rock to the surface. This most commonly occurs in oceanic crust, which covers most of Earth's surface. It sometimes occurs within a continent, but over time new seafloor forms and spreads, so the boundary ends up in the middle of an ocean. There is a very low probability of a new rift forming along the edge of an ocean-continent boundary, because these boundaries make up only a small fraction of Earth's surface and such boundaries tend to form over cooler parts of the mantle where cold oceanic crust sinks back down.
