Compare this map to the map of known plate boundaries . Plates can move over the molten rock of the asthenosphere. Interactions between plates occur along plate boundaries. Plates can move apart, come together, or slide past each other. The key to understanding plate tectonics is understanding the types of boundaries and what happens at each.
Plates move apart.
Rising hot rock pushes crust upwards. Crust stretches, warps, and cracks. Slabs of broken crust collapse to form rift valleys . Rift valleys lengthen and deepen, and eventually meet the ocean. Valley becomes a long, narrow sea.