Mt. Vesuvius is a convex-concave composite/strato volcano, and its volcanic cone sits in a the summit caldera of Mount Somma. Most sources claim that the collapse of the Somma rim occurred about 17,000 years ago, creating the foundation in which Mt. Vesuvius came to be.
Monte Somma rose through the convergence of the African and Eurasian plate. The African plate was older, thus more dense, so it pushed beneath the Eurasian plate causing a subduction.
When subduction occurs, a trench forms where the more dense plate sinks. In this case, the trench was formed by the process of the African plate sinking under the Eurasian plate and hitting the mantle. The Vesuvius trench is very long, and runs parallel to the volcano.
The caldera in which Vesuvius sits formed after the highly explosive eruption and collapse of Monte Somma. The emptying of the magma chamber through this kind of eruption pulls the cone in, and creates a massive crater, or a caldera.
A mixture of deposits make up the composite Somma-Vesuvius complex, and when it erupts, viscous magma is released from the earth's crust through the large craters or many vents at the top. An eruption takes place when the plates either converge and heat the magma through friction or when the plates separate and magma seeps up.
The eruptions of the Somma-Vesuvius complex produce large ash deposits through the igneous process of erupting from the volcano, and the sedimentary process of being blown for stretches of miles and deposited as a layer. Fragments of rock and lava (tephra) make up the ash deposit.
Mount Vesuvius's eruption in 79 AD was catastrophic, killing thousands and burying the city of Pompeii in ash. Erupting 200 times since, its eruption's pyroclastic flows pose great danger to Naples.
