The main vent of a volcano is the outlet chamber in the Earth's crust that allows hot magma to reach the surface. While secondary vents may form to alleviate the pressure caused by a magma chamber, the main vent is responsible for giving volcanoes their familiar cone shape.
Magma chambers are large reservoirs beneath the Earth's crust that hold hot magma. Magma is less dense than the materials that make up the mantle, which causes it to pool below the crust and exert pressure. When the pressure is too great, magma is expelled through the main vent of a volcano.