When continents break apart to create new ocean basins, rifts of remarkable diversity form at the continental edges or margins. Seismic studies show that at one extreme, the 25'30-km-thick continental crust is stretched like gum until it finally breaks to allow a new oceanic spreading center to form. This is a cold rift with little accompanying volcanism. At the other extreme is a hot rift, created when the stretching and rifting of some continental margins produce huge volumes of molten rock. The rock pours out of the rift or becomes trapped and solidifies at the base of the crust.