Volcanoes can be summarized in 3 sentences: Volcanoes form when magma pushes up through cracks in the Earth's crust, bursting out as lava or exploding into the air. Powerful eruptions can blast rock and ash high into the sky, leaving behind a crater at the summit and coating the surrounding area with billowy flows of molten rock or jagged cinders. The molten rock cools and hardens over time into new landforms that are gradually weathered by wind and rain.