Chemistry of magma and its influence on eruptive style Volcanoes form because molten rock called magma breaks through Earth's crust and erupts as lava. The difference between magma and lava is that magma is molten rock below the surface and lava is molten rock at Earth's surface. The viscosity of magma and lava determines how a volcano will behave. Volcanos that are highly explosive and therefore dangerous to people are built-up as highly viscous magma that forces its way to the surface, often exploding violently as trapped gasses try to escape. Volcanoes that are less explosive and less of a threat to people are made up of very fluid magmas.
Fluid magma allows gasses to escape and therefore are less explosive.
Magma with a high silica content are sticky and more viscous. Magmas with a lower silica content are less sticky and more runny. Usually lava is between 600 and 1200 degrees Celcius. Basaltic lavas have the highest temperatures up to 1200 C. Rhyolitic and andesitic lavas are cooler when they erupt. We all know by experience that fluids generally become more runny as they are heated up.