Vesuvius was formed as a result of the collision of two tectonic plates, the African and the Eurasian. The former was subducted beneath the latter, deeper into the earth. As the water-saturated sediments of the oceanic African plate were pushed to hotter depths inside the planet, the water boiled off and lowered the melting point of the upper mantle enough to partially melt the rocks. Because magma is less dense than the solid rock around it, it was pushed upward. Finding a weak spot at the Earth's surface, it broke through, thus forming the volcano.