The inside of a volcano has one or more vents through which gas and lava flow during eruptions. Below the vent is the magma, which is the molten rock that flows upward due to the divergence or convergence of tectonic plates in the Earth's crust. The number of vents and shape of the buildup of magma inside the magma chamber depends upon the type of volcano.
Cinder cone volcanoes have single vents. Their bowl-shaped craters form from particles of cooling lava. Lava does not always flow from the tops, but the discharge of particles may cause lava fountains. Stratovolcanoes, or composite volcanoes, may have one or a cluster of vents. These volcanoes consist of cones formed from multiple layers of lava flows. Magma flows upward from a deep magma reservoir and flows out through breaks in the wall of the crater and cracks and fissures on the sides.
Shield volcanoes may have one or a group of vents through which lava flows. They usually have broad slopes spread out over a large area. The fluid lava flows not only through the vents but from rift zones that open on the flanks. Lava domes are formed by the accumulation of highly viscous lava within a vent chamber. These buildups can cause explosive eruptions or collapse to cause deadly pyroclastic currents of swift-flowing lava.