How and why do volcanoes erupt?
•Hot, molten rock (magma) is buoyant (has a lower density than the surrounding rocks) and will rise up through the crust to erupt on the surface.
•When magma reaches the surface it depends on how easily it flows (viscosity) and the amount of gas (H2O, CO2, S) it has in it as to how it erupts.
•Large amounts of gas and a high viscosity (sticky) magma will form an explosive eruption!
•Small amounts of gas and (or) low viscosity (runny) magma will form an effusive eruption