Although still an area of active research, it appears that due to complex convection within the Earth's mantle material rises to the base of the lithosphere beneath the divergent plate boundary. This supplies the area with vast amounts of heat and a reduction in pressure that melts rock from the asthenosphere (or upper mantle) beneath the rift area forming large flood basalt or lava flows. Each eruption occurs in only a part of the plate boundary at any one time, but when it does occur, it fills in the opening gap as the two opposing plates move away from each other. The average rate of movement is comparable to how fast human fingernails grow, (about 2 cm a year).
Over millions of years, the plates have moved many hundreds of kilometers away from both sides of the divergent plate boundary. Because of this, rock closest to the boundary is younger than rock further away on the same plate.