This document explains how and why volcanoes erupt through a sequence of events. It describes the structure of the Earth's layers and tectonic plates, and how the movement and collision of these plates can cause magma from the Earth's interior to rise up and erupt from a volcano. The magma travels upwards through the main vent and out the crater when one tectonic plate pushes down forcefully on another, squeezing magma between the gaps of two plates. A volcanic eruption sends an ash cloud into the sky, thick lava trickles down the mountain, and eventually a pyroclastic flow forms - all of which can destroy cities and claim lives.