Divergent Boundaries
What is a divergent boundary? Well, it is the area where plates move away from each other. When the sea floor spreads a divergent boundary is created. Divergent boundaries that are close to continents make rifts. The rifts(A place where the lithosphere and the earth's crust are being pulled apart.) then make rift valleys. (A rift valley looks like a lake, but it kind of looks like it was cracked open.)
http://en.wikipedia.org/wiki/Rift_valley