Some plates, such as the Nazca Plate in the Pacific Ocean, are relatively small and consist entirely of oceanic crust. Other plates, for example, the North American plate, are much larger and mostly consist of continental crust. The top of the asthenosphere is usually about 70 km into the Earth. The movement of the plates is ultimately responsible for most of the Earth s volcanoes and earthquakes. Tectonic plates move because of the generation of heat within the Earth from the decay of radioactive elements.
Hot materials rise from the mantle.
In the upper mantle, they cool, become more dense and sink back into the deep mantle. In the deep mantle, they reheat and rise again. The cycle of rising magmas and sinking cooler materials creates convection currents , which may move the tectonic plates. Hot water rises, cools and then sinks. The cycling of rising, cooling, sinking, heating, and rising again forms convection currents that mix the water shortly before it begins to boil.