A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. A volcano is an opening in the Earth's crust that allows magma, ash and gasses to erupt from below the surface. It is made up of a magma chamber, a vent, a crater and a cone shaped mountain made of layers of ash and lava. Volcanoes are a specialized type of mountain built by the accumulation of erupted lava, ash, and rock around a central vent, often featuring a crater and the potential for active eruptions. Conversely, mountains are generally, larger, older landforms created by tectonic plate collisions, folding, or uplift that lack internal volcanic activity. Most of the estimated 2,000 deaths at Pompeii occurred on the second day of the eruption, when the top of Vesuvius collapsed and an avalanche of raging-hot volcanic material tore through the city. This fast-moving wall of hot rock and ash, known as pyroclastic flow, killed with both heat and sheer force. Volcanic eruptions are partly driven by pressure from dissolved gas, much as escaping gases force the cork out of a bottle of champagne.