Main Features of Volcanoes
- A volcano is formed when magma erupts onto the Earth's surface as lava through a vent in the Earth's crust
- The magnitude of a volcanic eruption is measured on the Volcanic Explosivity Index (VEI)
- The scale is open ended but the highest in recorded human history was an 8 (Tambora 1815)
- All volcanoes have features in common, as shown in the diagram below:
- Due to the type of lava erupted the formation of a volcano varies
- Composite (strato-volcanoes)
- Steep-sided
- Sticky (viscous) lava
- More explosive eruptions
- Formed from alternating layers of ash and lava
- Tend to form on convergent (destructive) plate boundaries
- Shield volcanoes
- Gently sloping sides
- Runny/thin lava
- Less explosive - gentle eruptions
- Tend to form on divergent (constructive) plate boundaries or hot spots
- Frequent eruptions
- Volcanoes may be active, dormant or extinct
- Active
- The volcano has recently erupted and is likely to erupt again
- Dormant
- Has erupted in the last 2000 years and may possibly erupt again
- Extinct
- Shows no evidence of eruption in historic times and there is no evidence of a magma reservoir
- Active
Volcanic eruption features
- Eruptions may include a range of features as shown in the table below:
Feature | Characteristics |
Lava | When magma erupts to the surface it is known as lava. The lava can be thin and runny or thick and slow moving. This depends on the composition of the magma |
Ash | Pulverised solid lava which measures less than 2mm in diameter. Ash is ejected into the atmosphere and can travel thousands of kilometres |
Pyroclastic flow | Fast moving, very hot clouds of poisonous gases mixed with ash. Average speeds of about 100km/h but can move at up to 700km/h |
Lahars | When volcanoes erupt snow and ice on the peak melts and combines with the ash. This creates fast moving mudflows or lahars |
Earthquakes | Magma rising to the surface through the vents in the volcano increases pressure on the Earth's crust leading to earth tremors |
Volcanic bombs | Fragments of molten rock which are ejected from the volcano. These are between 60mm and 5m in diameter |
Worked example
Shield volcanoes are often formed on constructive (divergent) plate boundaries.
Describe two characteristics of a shield volcano.
[2]
- Answer: Any two of the following
- Fluid/thin/runny lava [1]
- Low (height to width ratio) [1]
- Gentle slopes [1]
- Gentle eruption [1]
- Frequent eruptions [1]