Evidence we have today suggests that divergent boundaries form above temperature instabilities near the boundary between the core and mantle. Convection currents diverge where they approach the surface. Tension and high heat weakens the floating plate and it begins to break apart. The two sides move away in opposite directions, creating a divergent plate boundary. The weaknesses between the diverging plates fill with molten rock from below.
Sea water cools the molten rock, which quickly solidifies, forming new oceanic lithosphere.
This continuous process builds a chain of volcanoes and rift valleys called a mid-ocean ridge or spreading ridge . Little by little, as each batch of molten rock erupts at the mid-ocean ridge, the newly created oceanic plate moves away from the ridge where it was created. Continue to convergent boundaries , return to plate tectonics home or use the back button on your browser to return to previous page. This site is a joint project of the Western Region Geologic Mapping Team and the National Park Service .