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.

Magma comes from the magma chamber under the volcano. It travels through the main vent to the crater. Secondary vents on the side of the volcano may also form. The magma may erupt as volcanic bombs.

Composite volcanoes

A composite volcano is made up of layers of ash and lava. It is steep-sided and cone shaped.
  • 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

A shield volcano is made up of layers of lava. It has a gentle slope and a low, wide cone.
  • 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.

A stationary magma plume forms in the mantle. Magma rises to the surface through the crust, forming a volcano. As the crust moves, and island string is created.