Analyse the factors that cause differences in the hazards posed by volcanoes around the world (40 marks)
A hazard is a situation that poses a level of threat tolife, health, property or environment. The level of hazard posed by different volcanoes can very greatly, from a weak eruption with minimal impact that causes little damage, to a voilent and life threatening explosion. Most of the sixty-plus volcanoes that erupt each year are low risk, however a combination of factors can cause a volcano to be a serious hazard. The factors causing these variations will be explained in this essay.
The first factor that must be considered is the viscosity of the magma. This can determine how powerful an eruption is and what shape the volcano eventually becomes. Viscosity can be affected by three main factors, Firstly, the higher the temperature, the lower the density of the magma, causing it to flow more easily. Secondly, thegreater the amount of dissolved gases in the magma, the less viscous it will be, and lastly the higher the silica content, the more viscous it will be. Thicker, more viscous magma has a greater potential for explosive eruptions and therefore represent the greatest potential hazards. The thickest type of magma is known as Acid Magma. Its relatively low temperature (600C-1000C), high silica content and low proportio of dissolved gases causes its toothpast-like consistency that leads to blockages and powerful eruptions.
This can mean that the eruptions caused by thick magma can be less frequent and more difficult to predict, meaning that when an eruption does occur, it is usually with little or no warning, which can lead to catastrophic consequences asany nearby settlement will be relatively unprepared for the effects of a violent volcanic eruption.Furthermore, acid magma is more likely to produce clouds of smoke and ash due to the explosive nature of the eruption it causes, than thinner, basic lava. Ash clouds, such as the one caused by the volcano Eyjafjallajokull when it erupted in april 2010, which caused sever disruption to European air space, causing thousands of flights to be cancelled due to concerns over the hazards posed to air passengers travelling through the ash cloud.
Anpother factor that can have an impact on the level of hazard posed by a volcano is the type of plate margin on which it occurs. Volcanoes occuring at constructive plate boundaries are usually much less violent than those occuring at destructive plate boundaries. This is because the magma produced by plates moving apart is Basic, and therefoe has a low viscosity, allowing it to flow easily. The lava is produced from a central vent or fissure and erupts regularly but not usually violently. Also,constructive plate boundaries are often found under the sea and create submarine volcanoes, such as along the Mid-Atalntic ridge, so pose few threats to humans. As a result, the hazards posed by volcanoes at constructive plat eboundaries is relatively low. However, the subduction of one plate under another at destrctive plat eboundaries can form an acidic magma chamber, due to the build up of intense heat. Acidic magma is very viscous and resisitant to flow, meaning that there is often a huge build up of pressure, which can result in very violent and dangerous eruptions involving ash and pyroclastic flow.
This can pose a a serious hazard. Pyroclastic flowsa are extremely dense, containing toxic gases at very high temperatures, and can move at speeds over 100km/h. The consequences of such an unpredictable hazard can be extremely seruous andpotentially life threatening, especially if there are channels from the erupting volcano towards a nearby urban settlement. Furthermore, violent eruptions caused by an intense build up of presssure can lead to hazards such as Tephra and even Lava bombs, which aaare fragments of volcanic rock and lava blasted up into the air. Thesecould pose a life threatening risk to an urban population if it is within reach of the tephra ejected by the volcano. The fact that over 80% of the world’s active volcanoes occur at destructive plate boundaries, including the Pacific Ring of Fire, means that the hazards posed by these volcanoes are very serious due to their explosiveness and unpredictability as a result of the acidic lava which is fromed there.
Another factor affecting the hazards posed by a volcano is the materials ejecte when it erupts. Non-explosive eruptions such as those which produce mainly basaltic lava flows, do not usuallyrepresent a particularly serious hazard to people, although they will destroy farmland an buildings. A greater hazard is posed when large quantities of very fluid lava are released, such as when part of acrater collapses, and there is a real danger to people. However, it is whn an eruption is explosive that the greatest risk to people is posed. As the magma is usually very viscous, there is not usually much lava, and so any that is produced cools and solidifies quickly.But features of these eruptions such as ash clouds, pyroclastic flows, lahars and tsunamis, are severe hazards. An ash cloud can reach 20km in jiust 30 minutes, and can be carried hundreds of kilometres further, although tephra produced in the same eruption usually falls within a few kilometres of the vent. The ash cloud can pose a considerable threat to aircraft however, and over the past 15 years, 80 commerical aircraft have inadvertently flown inyo ash clouds and several suffered severeengine damage as a result. The hazard posed to people on the aircraft is that the engine could fail and the aircraft could crash, a potentially life thretening situation.
Furthermore, ash landing on earth can cause buildings to collapse, and people to suffer from asphyxiation, which can cause death. At Pompeii, over 2000 people were killed this way in 79 AD. Lahars are another seruois hazard caused by explosive eruptions. They are cement-like mudflowsconsisting of volcanic ash and water, which can travel down river valleys at speeds of io to 100km/h. They pose such a s eriou shazard as they often occur in th edays after an eruption, at a time when, if peoplehave been affected by the volcano, they are at their most vulnerable. Many of th epople who were killed by the Mount Pinatubo eruption of 1991 were killed by lahars. Another seruous threat posed by volcanic eruptons is the risk of tsunamis. Usually, submarine volcanoes pose limited hazards to people due to the lack of proximity. However an eruption out to sea can cause a Tsunami; an enormous wave. They represent a significant threat to coastal communities and the eruption of Krakatoa in 1883 triggered waves of up to 35 metres in height, which swept along the coast of Java and Sumatra, killing over 36,000 people. Tsunamis can be an incredibly serous hazard as they are an indirect consequence ofvolcanoes, meaning that the people and communities affected are often unprepared for the risks associated iwth volcanoes as they do no consider themselves to be at risk as they are so far away.
Perhaps the most important factor in differentiating the level of hazard posed by a volcano is its proximity to poplation centres. Some volcanoes ar elocated in areas of dense population, or close to major cities. Japan, Indonesia and the Philippines havemany active volcanoes as well as hugh population densities, which means that many people ar eput at risk. Mount Vesuvius in Italy is just a few kilometres away from the city of Naples, which has a population o fjust over 1 million. Clearly the volcanoes, especially explosive obes, close to population centres are much more hazardous than those located in remote regions as they will affect more people if they erupt. However, epople often have no choice but to live in such hazardou slocations, sometimes due to limited space.
The two most important factors that cause differences in the hazards posed by differnt volcanoes around the world are the materials ejected and direct consequences of the volcano, and the proximity of the volcano to a population centre. This is because the more poeple that a volcano can affect, the greater the hazard because it has the potentail to be more damaging by posing a greater threat to human life and helth, as well as property and environment.