Human: How is a volcano formed? Assistant: 
A volcano is formed when magma, or molten rock, erupts from the Earth's crust and rises through the atmosphere. As it reaches higher altitudes, the pressure of the air causes the magma to expand and form a large cloud of ash and gas called a pyroclastic flow. The force of the eruption can cause the ash cloud to spread out in all directions, creating the volcanic cone shape that we see on the surface.