The boundaries that are between plates that are moving apart are called divergent boundaries. As plate separate, the hot molten magma rises to the Earth's surface and cools then hardens, this forms new crust. On the ocean floor this separation of plates and production of the new crust is called sea-floor spreading. You can think of divergent boundaries as manufacturing plants where the Earth's crust gets formed and subduction zones as recycling ports for mother nature. When the hot magma cools and hardens, it forms ridges of new rock. These ridges can rise up-to a kilometer above the sea-floor. The entire length of the Atlantic ocean has a ridge in the middle where the North American and Eurasian plates are separating. On land divergent boundaries create valleys called rifts. In some places like Iceland, the divergent boundary between the North American plate and the Eurasian plate is clearly visible on land.