Vertical movement of the crust due to weight lifting off and coming down. Continental crust is thicker and less dense (2. 7) than oceanic crust (3. 0) Wegener (continental drift): evidence - fossils, fit of continents (offshore, 2000 meters), mountain chains, glacier deposits (wrong mechanism - moon/sun dragging continents across the earth) Tops worn off due to erosion by waves. Warm material floats to the top (less dense) but then sinks to the bottom when the ridge splits apart and becomes colder, which leads to the more dense material. As the magma bubbles up from the seam in the middle of the ocean, it pushes everything outward and layers on top of previously existing layers of older rock. Volcanoes that form along the ridge are originally close to the surface of the ocean where they can be eroded flat on top. Deep ocean trenches are where the seafloor finally descends back into the mantle.