What is a volcano?. A volcano is a mountain that forms when molten rock erupts or flows as lava from an opening in Earth\'s surface.. . How do volcanoes form?. A volcano forms when molten rock erupts or flows as lava from an opening in earth\'s surface and builds up a volcanic cone.These openings are called vents.Volcanoes release molten rock, ash, and poisonous gases. All these products result from melting in the mantle or in the crust..