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A composite volcano can erupt in two ways. One way involves molten magma streaming down the outside of the volcano, an eruption which is commonly depicted in movies. The other way is the way Mt Vesuvius erupted in 79 AD. This type of eruption is called a Plinian, named after Pliny the Elder the uncle of the first man to write an account of the devastation of the eruption, Pliny the Younger. Plinian eruptions occur when gassy magma creating pressure within the magma chamber. This pressure increases until an eruption occurs, with plumes of ash that can reach heights of 50km!