Mount Vesuvius is what's called a composite volcano. Here the magma is made from lots of different types of rocks, liquids and gases, forced deep underground by plate tectonics. Under pressure and heat, this volatile mixture melts, and then rises back up to lurk inside a large magma chamber just below ground. Unlike shield volcanoes, these chambers contain a lot of gas ?" a bit like a bottle of Prosecco ?" which can't escape because of the volcano's solid-rock lid. As more magma and gas bubble up from below, the pressure builds and builds, as if the bottle is being shaken. Finally it bursts, releasing a thick cloud of vapour, ash and rocky debris that hovers above the volcano before collapsing into a pyroclastic flow. This is what makes composite volcanoes so