What causes volcanoes to erupt?

Volcanoes erupt when magma, formed by the melting of Earth’s mantle or crust, rises to the surface. Pressure builds as gases dissolved in the magma expand, and if this pressure exceeds the strength of the overlying rock, an eruption occurs. The type of eruption—explosive or effusive—depends on the magma’s composition and viscosity. Explosive eruptions eject ash, gas, and lava, while effusive eruptions produce flowing lava. Eruptions often occur near tectonic plate boundaries or hotspots.