Places where the plates that make up the earths crust move away from eachother or seperate are called divergent boundaries. When the plates spread on land, valleys are created. Hot magma comes to the earths surfice where the plates seperated creating new crust after it hardens and cools. Speading of the sea floor happens along divergent boundaries when the boundaries are in the ocean. Again magma rises, when it cools and hardens it creates ocean ridges. Sometimes the ridges rise as high as one kilometre above the sea floor.