A volcano erupts when magma (molten rock) from beneath the Earth's surface rises and breaks through the crust. Step 1: Magma formation: Heat and pressure deep within the Earth melt rock, forming magma. This magma is less dense than the surrounding solid rock, so it rises. Step 2: Magma chamber: The rising magma collects in a reservoir called a magma chamber below the volcano. Step 3: Pressure build-up: As more magma enters the chamber, pressure increases. Dissolved gases within the magma also expand, further increasing pressure. Step 4: Eruption: When the pressure becomes too great for the surrounding rock to contain, the magma forces its way up through a conduit (pipe) and erupts onto the surface as lava, ash, and gases.