Why do volcanoes erupt lava?

Volcanoes erupt lava when pressure builds up from magma beneath Earth’s crust. This magma forms when rocks in the mantle melt due to high temperatures and pressure changes. As magma rises, gases within it expand, increasing pressure. When the pressure becomes too great, it forces the magma through cracks in the surface, resulting in a lava eruption. The composition of the magma determines the type of eruption and the characteristics of the lava flow.