A volcanic eruption happens when magma, or melted rock from deep inside the Earth, bursts out of a volcano. This magma, called lava when it reaches the surface, flows like a river and can destroy everything in its path. Volcanic eruptions can also send ash and gases into the air, which can travel long distances and make it hard to breathe. Scientists measure volcanic activity using the Volcanic Explosivity Index (VEI), which ranks eruptions from 0 (least explosive) to 8 (most explosive). Some eruptions are small and quiet, while others are explosive and very dangerous. Scientists carefully monitor volcanoes to warn people before they erupt.