A volcano is an opening in the earth's surface by which lava, hot gases, and rock pieces erupt. Volcanoes are formed when melted rock from deep within the earth blasts through the earth's surface. Volcanoes are mostly mountains and are often coned shaped. The mountain is formed by lava that is built up during eruptions. Eruptions of volcanoes can be magnificent sites to see. In some volcanic eruptions giant fiery clouds rise above the mountain and rivers of lava flow down its sides. In other eruptions, hot red ashes shoot out of the top of the mountain cone.