Structure of volcanoes Volcanoes can be described by their shape or type - shield or composite . Both types of volcanoes have common features: magma chamber - this is where the molten rock is stored beneath the ground at great heat and pressure main vent - this is the channel through which magma rises to reach the Earth's surface secondary vent - some magma may escape through the side of the volcano, particularly if the main vent becomes blocked crater - this is found at the top of the volcano and the magma usually erupts from here cone - this is formed from the material ejected during eruptions and grows in size with each eruption When a volcano erupts there can be lava, ash, steam or gaseous emissions. Sometimes eruptions are explosive and lava is thrown out as volcanic bombs. Composite volcanoes Composite volcanoes are made up of alternating layers of lava and ash. They are usually found at destructive margins . Eruptions are infrequent, but mostly violent and can include volcanic bombs. The eruptions from these volcanoes may be a pyroclastic flow . This is a deadly mixture of hot steam, ash, rock and dust. It moves at very high speeds and has high temperatures (400°C). They are steep-sided as the thick and heavy lava doesn't flow very far before it cools. Examples of composite volcanoes are Mt Unzen in Kyushu, Japan, and Mt Etna in Sicily, Italy. Shield volcanoes Shield volcanoes are usually found at constructive margins or over a hotspot . They have gently sloping sides because the lava is thin and runny so it moves far before it solidifies. Eruptions tend to be often or continuous and mostly gentle. An example of a shield volcano is Kilauea, in Hawaii. Hotspots Hotspots are places where the magma rises up through the crust. They are caused by a static source of magma, often away from plate margins. As the plate moves away from the hotspot, a new volcano island will form.