Rift valleys occur when continental plates are pulling apart from one another.
A new ocean begins when hot mantle material begins to move upward beneath a continent. Geoscientest are still not certain on why that happens. The lithosphere of the continent bulges upward and is stretched sideways. Eventually, it breaks along a long crack, called a rift. Magma rises up to feed volcanoes in the rift. As the rift widens, the ocean invades the rift. A new ocean basin has now been formed, and it gets wider as time goes on.
