At divergent boundaries in the oceans, magma from deep in the Earth's mantle rises toward the surface and pushes apart two or more plates. Mountains and volcanoes rise along the seam.

Diagram of an opening rift valley: at stage B the valley is dominated by rivers, and at stage C by shallow marine environments. Here is a nice summary of sedimentation on divergent margins: be sure to ...

Running along the top of this chain of mountains is a deep crack, called a rift valley. It is here that new ocean floor is continuously created.