Volcanoes form when magma from the earth's mantle rises through ruptures or openings in the crust. The movement of tectonic plates, particularly when they separate from each other, causes pressure that can lead to volcanic eruptions. In 1883, Krakatoa erupted with a force equivalent to 200 megatons, causing massive tsunamis that killed over 36,000 people and lowering the earth's temperatures by ejecting ash high into the atmosphere.