Shield and composite volcanoes
Volcanoes can be described in terms of activity and can be:
- still active and erupt frequently;
- dormant (temporarily inactive but not fully extinct);
- extinct (never likely to erupt again).
Volcanoes can also be described by their shape or type - shield or composite.
- Shield volcanoes are usually found at constructive boundaries.
- They are low, with gently sloping sides.
- They are formed by eruptions of thin, runny lava.
- Eruptions tend to be frequent but relatively gentle.
- Example: Mauna Kea in Hawaii, USA or the Galapagos islands
- Composite volcanoes are made up of alternating layers of lava and ash (other volcanoes just consist of lava).
- They are usually found at destructive boundaries.
- The eruptions from these volcanoes may be a pyroclastic flow rather than a lava flow. A pyroclastic flow is a mixture of hot steam, ash, rock and dust.
- A pyroclastic flow can roll down the sides of a volcano at very high speeds and with temperatures of over 400°C.
- Example: Hekla in Iceland, Mt St Helens in the USA and Mt Kilimanjaro in Tanzania.