Why Are Some Volcanoes More Hazardous Than Others?

This reduces the scientists’ credibility in the eyes of government and local people, and can cause problems the next time an eruption seems likely.
7. Prediction, forecasts and reactions
- Active volcanoes all over the world are monitored using a vast array of scientific equipment. Satellites look for infrared radiation that indicates rising magma and ground instruments measure gas emissions, ground deformation and resulting earthquake activity.
- Geologists use evidence from past eruptions to suggest likely eruptive scenarios and hazard maps can be constructed to indicate those areas at greatest risk.
- Scientists can provide reasonably accurate forecasts of impending eruptions and people can be evacuated from areas considered to be at risk.
- However, governments do not always respond to scientific predictions and disasters can result.
-Scientists do not always get it right and sometimes eruptions that seem imminent never actually occur.
6. Frequency of eruptions and perception of risk
- Volcanoes that erupt virtually continuously are promoted by the local tourist industry. The treat to property, although relatively minor given the nature of the eruptions, is very real to those who witness the outpouring of molten rock. - Sometimes there are hotels and cafes on the flanks of mountains which, although they periodically get damaged during eruptions, are considered worth the risk to build and maintain.
- However, for many people living close to the really dangerous destructive margin volcanoes, eruptions are very infrequent and hundreds of years may elapse between eruptions. Memories are short and people are less inclined to worry about an eruption from a volcano that has been inactive for many generations.
- The frequency of eruptions therefore has a profound impact on people’s perception of the volcanic hazard.
5. Proximity to population centres
- Whilst some volcanoes are located in remote parts of the world, many are located in areas of dense population or close to major cities. Those volcanoes (particularly the explosive ones) close to population centres are much more hazardous than those located in remote regions.
People live close to volcanoes:
- There is little choice but to be geographically close to volcanoes on islands where space is limited.
- Volcanoes can be associated with fertile soils.
- They bring other benefits for human habitation such as building materials and hot water.

3. Explosiveness of eruption
- The explosiveness of an eruption can be measured by the Volcanic Explosivity Index (VEI).
- The VEI is open-ended and ranges from 0, for non-explosive eruptions, to 8, for explosive eruptions.
- The higher the VEI, the greater the potential hazard.
- Eruptions with a value of 0 or 1 tend to have very localised effects and are associated with the more gently fiery eruptions involving mostly lava.
- Eruptions of 2 and above have much greater effects, with huge quantities of pyroclastics being erupted with the potential of affecting both the immediate area, and through global atmospheric circulation systems, parts of the world much further away.

4. The materials ejected
Ash clouds – explosive eruptions blast solid and molten rock fragments (called tephra or pyroclastics) into the air with tremendous force. They pose a considerable threat to aircraft: commercial aircraft inadvertently fly into ash clouds and suffer engine damage as a result. Ash returning to Earth can cause buildings to collapse and can cause death by asphyxiation (severely deficient supply of oxygen).
Pyroclastic flows – these are clouds of incandescent gas, ash and rocks, often reaching temperatures of 800°C and travelling at speeds in excess of 200kph down the volcano’s flanks. They can cause enormous and widespread destruction.
Lahars – these are cement-like mudflows consisting of volcanic ash and water, which can travel down river valleys at speeds of up to 100kph. They often occur in the days following an eruption when people are at their most vulnerable and with the capacity to travel up to 300km they represent a massive threat.
Tsunamis – when a volcano erupts out to sea, huge waves called tsunamis can be generated. They represent a significant threat to coastal communities often kilometres away from the volcano. One of the main issues with tsunamis is that they often impact on communities far away from volcanoes who do not consider themselves to be at risk.

2. Plate Margins
Constructive/divergent – plates are moving away from each other. Magma is produced by the partial melting of the mantle deep below the surface. It is a basic-type magma and therefore has a low viscosity enabling it to flow easily. Lava Plateaux and Basic/Shield volcanoes are volcanoes associated with constructive plate margins. They erupt frequently but not usually violently and are associated with lava flows and ash.
Destructive/convergent – plates are moving towards one another. As one plate dives (subducts) below another, intense pressures and heat cause melting of the rocks and sediment, which can result in the formation of an acidic magma chamber. Very viscous and resistant to flow, there is often a huge build up of pressure, which results in very violent and dangerous eruptions involving ash and pyroclasts.
- Volcanoes that aren’t associated with plate margins result from isolated plumes of rising magma known as ‘hot spots’. The magma is basic in nature and these volcanoes are similar to those associated with constructive margins.

1. Viscosity of magma
- Viscosity/thickness of magma determines the nature and power of an eruption and the resultant shape of the volcano.
Viscosity is determined by:
Temperature - the higher the temperature, the lower the density of the magma and the more easily it will flow.
Dissolved gases - the greater the amount of dissolved gases, the more fluid the magma. Gases remain dissolved in high temperature conditions.
Chemistry – the higher the silica content, the more viscous the magma.
- The more viscous the magma, the greater the potential for explosive eruptions and these represent the greatest potential hazards.

Why are some volcanoes more hazardous than others?