Extrusive Structures

Central Vent Eruption

Most volcanic mountains are central vent eruptions. There are three types: Composite, Dome and Shield Volcanoes.

Composite Volcano

• These have alternate layers of lava and pyroclastic material that build up to form a steep sided cone-shaped mountain.
• Most of the material falls close to the crater. If this becomes blocked, the entire top of the volcano can blow off, creating a huge depression called a caldera.

Dome Volcano

• These are also steep sided, rounded volcanoes but are formed by slow moving acidic lava that erupts from the volcano. The material does not flow far so the crater can often become blocked, creating very violent eruptions such as Mount St. Helens.

Shield Volcano

• Have a very wide base with shallow, sloping sides.
• It is created by the fast-flowing basic lava that erupts from the volcano.
• They are common at hotspots, such as Hawaii.