Analyse the factors that cause differences in the hazards caused by volcanoes around the world (40 marks) A hazard is defined as something which has the potential to kill or cause harm. However a hazard is only a risk to people if they are exposed to it. This is when a hazard can cause disasters. For example, a volcano is only a hazard if the population comes in contact with the consequences of its activity. Hazards and their likeliness to become disasters can depend on many different factors, for example how developed a country exposed to the hazard is, the size of the population exposed or the size and scale of the disaster. Volcanoes are a particularly abundant risk to humans as they have many associated hazards, but the risk they pose differs around the world. One factor that heavily influences how hazardous a volcano is, is the plate boundary that it lies on. Subductive plate boundaries tend to produce much more violent eruptions than conservative or constructive margins. Subductive boundaries occur where two plates (one continental and one oceanic) are moving toward each other. The denser plate, the oceanic, is forced underneath the continental.
As it subducts into the asthenosphere, it begins to turn back into magma. This magma is light and less dense than the asthenosphere, so it rises and forces its way through weaknesses in the crust. This is how volcanoes are formed on subductive margins. An example, is the Aleutian Islands which are a chain of volcanoes formed by the subduction of the pacific oceanic plate underneath the Eurasian/north American (?) plate. Volcanoes formed on these boundaries tend to be more hazardous because they erupt more violently. This is because the melted oceanic plate (which supplies the magma) is rich in silica. This produces magma which has a very high viscosity meaning it moves very slowly and cools very rapidly (from about 800 degrees).
The type of volcano it produces tend to be acid dome volcanoes because when the magma reaches the surface it doesn’t have time to move anywhere before it cools so it forms very steep sided convex volcanoes. The hazard increases in these types of volcanoes because the rapidly cooling lava can block the flow of lava at the crater which causes pressure to build up. The pressure is released suddenly and violently in an explosive eruption. These eruptions have been known to project burning clouds of ash and pumice many kilometres into the atmosphere. A common hazard associated with acid dome volcanoes are pyroclastic flows which are clouds of high temperature containing rocks and poisonous gasses. The flows are known to move at hundreds of kilometres an hour. Montserrat in the Caribbean experienced a pyroclastic flow which devastated Plymouth, the capital city of their island. Usually if you are caught in these flows, there is no chance of survival, however in Monserrat there had been a lot of monitoring of the volcano prior to the eruption so the whole south island had been evacuated.
In contrast, volcanoes formed on constructive margins or hotspots tend to be less hazardous because shield volcanoes are formed which erupt more effusively. This is because the lava produced from the spreading plates is basaltic which is low in silica meaning it flows very quickly and cools much slower. This forms a much less hazardous volcano with less violent eruptions because the vent of the volcano doesn’t become blocked so the lava and material erupting out of it is free to move anywhere. The lava is much more freely moving because it contains gas bubbles which give it the freedom to expand and move away from the vent. Volcanoes in Iceland tend to be less effusive because of this reason.