Why Are Some Volcanoes More Hazardous Than Others? (40 Marks)

Why are some volcanoes more hazardous than others? (40 marks)

A volcano is a surface landform resulting from the extrusion of magma from underground as lava, ash, rocks, and gasses are erupted in various proportions. Each year, around 60 major volcanoes erupt globally. How hazardous each one is, depends on a variety of human and physical factors that interrelate to determine the level of impact on human activity that each event exerts. I will be looking at how the physical properties of a volcano, interact with human variables to make certain volcanoes more hazardous than others. This essay will incorporate exemplification from countries at different stages of development to discuss human factors and how this links with their physical geography. The scale will be primarily global; both development spread and tectonic activity are best presented in this manner.

The strength of an earthquake, measured on the Richter scale, often determines the level of damage caused by an earthquake. Therefore, it is reasonable to assume that the power of a volcanic event is illustrative of how potentially hazardous an eruption is going to be. The explosiveness can be measured by the Volcanic Explosivity Index (VEI); the higher the VEI, the greater the potential hazard. The type of magma, and ultimately the classification of volcano that is created, is a huge physical factor in determining a volcano`s explosivity. Magma comes in many forms and each chamber is unique. Therefore, a wide spectrum of lava types can occur. The viscosity of the magma is primarily determined by three factors: temperature, dissolved gasses, and the chemical make up. The hotter the magma is and the lower the silica content, the more fluid it will be and therefore the less explosive; if magma has a high level of dissolved gasses, then it is also likely to be less viscous. Thick lava, for example acid lava, has the potential to cause much larger and more explosive eruptions, therefore presenting a larger threat and increased potential for hazardous activity.

The location of a volcano is also a large variable in determining the nature of its eruptions. Volcanoes are generally found in three locations: constructive and destructive plate boundaries, as well as hotspots. The most explosive and potentially the most hazardous volcanoes are found on destructive, convergent plate boundaries. Here, one plate subducts beneath the other generating intense heat and pressure, melting the rock and sediment to form an acidic magma chamber. This viscous magma is resistant to flow and therefore results in violent, dangerous eruptions involving pyroclastics and ash; potentially hazardous both locally and globally. Another factor associated with destructive boundaries is the presence of the phreatic zone: explosive volcanic activity involving steam derived from water that has been trapped as a plate subducts. At constructive/divergent boundaries, the emerging lava is generally basic and therefore has a low viscosity, allowing it to flow easily causing much less violent eruptions. Events occur frequently but not explosively at constructive plate margins. A good example is the Mid Atlantic Ridge where the ocean floor is lined with constant, low risk volcanic activity. The hazard capacity of constructive margins generally increases when activity emerges above sea level, as seen with Iceland. However, the risk still remains low, with the exception of distinct events in which other factors have contributed. The recent Eyjafjallajökull eruption (2010) caused major global disruption, especially to Northern Europe. In this instance, the ice cap situated on top of the main vent was primary in causing the major ash cloud: this is another physical factor. Eruptions can also occur on hotspots, but the magma associated with these volcanoes is generally of low viscosity and basic in nature, producing similar events to those found at divergent boundaries. Volcanoes generally don`t form at collision boundaries. Whilst 75% of all volcanic material is erupted at constructive margins, over 80% of the world`s volcanoes occur at destructive boundaries. It is these that are thought to be the most explosive and thus the most hazardous.

Possibly the most important physical factor that helps to determine how hazardous a volcano can be, is the type of material ejected. Non-explosive eruptions tend to mostly produce lava flows which do not present a huge hazard to human activity. From more explosive, viscous eruptions, you get much more hazardous material, posing a greater threat. Ash clouds can cause serious aircraft damage and asphyxiation from collapse of buildings caused by the weight of the ash. A good example is Mount Vesuvius; whilst it is an out of date case study (AD 79), it does stand as testament to the power of an ash cloud, as 3m of ash falling on Pompeii caused 2,000 deaths. Pyroclastic flows also present great danger. Great clouds of incandescent gas, ash and rocks can often reach 200kph and temperatures of up to 800 degrees Celsius, causing widespread destruction as seen during the likes of the Mt St Helen`s eruption in 1980 and Mt Pinatubo in 1991. Lahars can often occur in the days following an eruption when people are at their most vulnerable; this was also seen in the 1991 Mt Pinatubo eruption. Coastal communities can also come under threat to tsunamis caused by underwater volcanic activity as seen in 1883 with the Krakatoa eruption, which caused waves of up to 35m in height, killing over 36,000 people in Java and Sumatra. The hazard presented by tsunamis can often be greater because the risk perception of coastal communities miles away from a volcanic zone is often much lower.

Human factors also play a huge part in hazard assessment. A volcanic eruption could have a VEI value of 6, but if it is not in close proximity to settlement or human activity, it is not deemed hazardous at all. However, whilst some volcanoes are located in remote regions, such as Alaska, many are located close to areas of dense population, generally because of the benefits of a volcanic area in terms of original factors for settlement. Japan, Indonesia and the Philippines have many active volcanoes close to high population densities putting large numbers of people at risk. The city of Naples, Italy, is just 14 kilometres from Vesuvius which is likely to erupt again in the future, putting the city`s 1 million strong population at risk. Clearly volcanoes such as these pose a much greater hazard than remote ones.

The perception of risk is a huge factor in determining a volcano`s hazard. Many people do not just settle in close proximity to volcanoes because of the benefits associated with increased soil fertility, but also because their perception of the risk is low. If people believe a volcano is being monitored sufficiently or if they deem a volcano to be inactive or low risk, them demographics are unlikely to prepare in the manner they should. Mt Etna is probably the most monitored volcano in the world, due to the high development of places such as Italy. Therefore, people perceive the risk to be low. However, historically the eruptions of Mt Etna are increasing in frequency and explosivity, this will cause greater issues than the famous 2001 eruption, with people becoming more complacent with time. Perhaps it is fair to say that a false sense of safety around a volcano that has been inactive for years presents the greatest hazard. It has been proven that the largest eruptions normally come from volcanoes that have been dormant for centuries. This combined with a low perception of risk ultimately generates a larger hazard than necessary, as seen with Mt Pinatubo. Because the Pilipino volcano had not erupted since 1380, people no longer perceived the risk to be high. Over 700 deaths are attributed to the eruptions of June 1991. Although only 6 of these were believed to be as a direct result of lava flows, a large number of deaths were caused by a lack of preparedness in terms of mitigation. Modifying risk in areas such as the Philippines is now common practice, but was not so consistent in the early 1990s.

Other human factors such as the level of preparedness including prediction and reactions are dependent on levels of development. Hazard salience will come into play in countries of low development, as they prioritise their resources in other areas such as unemployment and famine prevention. This therefore has a huge impact upon the level of preparedness a developing country or region can have towards a volcanic event. The less a country is prepared, the greater the hazard. This is why the death toll in developing countries is (on average) three times greater than that of middle to high income countries for most eruptions. The modification of a volcanic event is impossible with current technology and is anticipated to always be that way. However, what is possible currently, is the modification of the risk by predication and therefore mitigation of the impacts. This has been seen on many occasions in developed countries. Mt St Helens in Washington being a great example where the impacts of the 1980 eruption were modified by adequate prediction through morphology, seismicity and thermal monitoring amongst other techniques, which ultimately allowed the populations of high risk areas to be evacuated avoiding many unnecessary costs. However, whilst it is obvious that governments should be prepared in order to deal with volcanic events effectively, this is not always possible. If a governing body has the decision between protecting 70,000 people from a Cholera outbreak that is imminent, or preparing for a potential hazard then obviously the disease or other scenario would have to take priority. It is in this instance, that the developed world should intervene and make sure that the world's volcanoes are monitored effectively. All explosive, volcanic events have the capacity to shut down entire continents, as seen in 2010 with Eyjafjallajökull with the closure of European airspace, or even the entire world as a result of global climatic shift. It is therefore not solely a local responsibility and must be accounted for on the global political stage. Paramount to all of this however, and something that costs a significant amount less than prediction systems, is awareness. I believe that global and local awareness within the developing and developed world is the key to tackling this issue more effectively and ultimately lowering the hazard potential of volcanic events around the world.

Overall, it is clear that there is an intrinsic link between physical and human factors in the determination of the level of risk that a volcano presents. It is when social, demographic and environmental factors coalesce, that the greatest hazards occur. I believe volcanic hazard to be a vicious cycle; increased fertility attracts greater settlement, posing an increased hazard. After eruption, the cycle starts again. Also, the size of eruption is key in determining the level of hazard perception which, if disproportionally low, can actually generate a much greater hazard, and one that no physical factor could equal. I believe that a lot more needs to be done to promote awareness, as well as further research into prediction systems and mitigation methods. It seems to be trending that human factors are increasingly becoming dominant over physical factors. However, as history has shown, the physicality of the Earth is such that devastating eruptions of colossal proportions are always a possibility, and that no amount of preparedness can equate to the modification of such events. It seems to be a question of scale; as it always is with Geography!