Cutaway artwork of a subduction zone. The tectonic plates of the Earth's crust move on the semi-plastic, molten layers underneath. The thin oceanic plates spread outward from mid-ocean ridges where molten rock oozes from within the Earth. They are destroyed at subduction zones, where the dense rock is pushed under the less-dense (but thicker) continental rocks. At the subduction zone, melted oceanic plate and seawater rise upwards to form volcanoes.