Volcanoes & Extrusive Igneous Rocks

How do volcanoes erupt?

Volcanoes erupt when magma reaches the Earth’s surface. As it does so, dissolved gases bubble out of the magma in much the same way as “Coke” fizzes when you open the bottle.

Depending on the viscosity and gas content of the magma, eruptions may be effusive or explosive, and may produce various mixtures of lava, gases and pyroclastics.