Lastly a high silica content will make the magma more viscous. Magma of a high viscosity can cause very violent eruptions. These pose the greatest threat as there very destructive. This is because it has a tendency to plug the volcano, trapping gasses which build pressure and eventually erupt violently. 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 as any nearby settlement will be relatively unprepared for the effects of a violent volcanic eruption. It also more likely to produce clouds of smoke and ash which can lead to suffocation and is a big killer during eruptions. You then have your thinner, less viscous magma such as AA or pahoehoe. It has a very high temperature, 950 C. It moves quickly due to its low viscosity. However it does result in much less violent eruptions but they are much more frequent. It poses less of threat though as it cools quickly. For example in the Mt Etna eruptions. It was basaltic lava that caused very little damage to buildings. The eruptions here were quite common. However in the eruptions in Montserrat, the eruptions killed 19 people with pyroclastic flows while others suffered inhalation injuries.
The volcano was located on a destructive plate margin so the volcano contained andesitic lava which lead to a very destructive eruption. From this we can see that viscosity is a major factor. At a constructive margin the lava is less viscous and therefore the hazard isnt as severe. While a volcano at a destructive margin eg. Montserrat the lava has a high viscosity and therefore is very destructive imposing a much greater hazard. The location of a volcano is also a large variable in determining the nature of its eruptions. Location affects what type of volcano is formed and therefore the viscosity of the magma as discussed before. This makes location critical in determining the severity of the hazard posed by said volcano. 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. 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. 80% of the worlds volcanoes occur at destructive plate margins, for example where the collision between the Eurasian Plate and the Indian Plate is forming the Himalayas or the subduction of the Nazca Plate beneath the South American Plate to form the Andes. These are the most destructive and hazardous of all.
A human factor that would cause differences in the hazards posed by volcanoes is settlement, which includes where people chose to live in relation to a volcano. For example, in Indonesia many people settle near Mount Merapi because of the rich and fertile soil favoured for farming. As Indonesia is a poor country many people rely on subsistence farming, therefore are willing to risk everything by settling beside a volcano. Furthermore, in the last eruption in 2010 360,000 people were displaced from their homes, meaning not only did they lose their home but also their livelihood; hundreds also lost their lives. Therefore, the hazards posed could have been minimalized if people chose to settle elsewhere. However this is different around the world due to there being different types of volcanoes.
For contrast, In Italy people live near Mt Etna yet they dont rely on subsistence farming. However they are better educated and since Etna erupts so frequently they have developed around the impacts of the volcano becoming used to life near a volcano. Etna of course is also less explosive than Merapi was and therefore any hazard caused is to be greater in Indonesia than in Italy. These human factors increased the severity of the hazard. The perception of risk is a huge factor and may be the most important in determining a volcanos 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, they are unlikely to prepare how 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 may in time impact the people as they become complacent and dont realise a more dangerous eruption may come. However the greatest hazard presented by this is a false sense of safety around a volcano that has been inactive for years. This is especially true for the Montserrat eruptions. It had been inactive for years and people didnt perceive it to be a risk. This is a case where human perception exacerbated the risk posed by Montserrat. Of course capacity to cope plays into this. With Etna, Italy is much more developed and therefore has the economic power to deal with any fall out from an eruption. They were also experienced with dealing with the lava flow. Whether that be with explosions as they did in 1993 or using an earth dam in 1992. With Etna not being as destructive as Montserrat was, there is less impact in Italy with the greatest impact being the closure of Catania airport however this isnt too important as they are used to this and it doesnt have as great of an impact on world travel as there would be if say Heathrow closed. When Montserrat erupted there capacity to cope was much lower as they relied heavily on aid. The aid provided by the UK definitely lowered the severity but it didnt stop the destruction of the south side of the island.
This causes more problems later on. The loss of such an environment damages the eco system. Of course theres economic fallout with having to deal with repairs and loss of business and homes etc. So because there perception of the risk was low that instantly reduced their capacity to cope and then caused the impacts posed by the volcanic hazards to be much more severe. This is just one example of a scenario where the physical factors were out of there control and then the hazard was exacerbated by the human factors. In closing it is clear that physical and human factors are intertwined in causing differences posed by volcanoes. It is because of this that human factors are much more important. We cannot control the physical factors, we cannot change the type of volcano that forms or the type of lava it contains.
In all cases the human factors exacerbate the hazard. In Montserrat they settled on the volcano, making it easy for them to be affected. They had a low perception of risk posed by the volcano. However while the human factors exacerbate the hazard, it is the volcano itself that dictates the hazard. Whether it be that the lava is of low viscosity than the volcano poses less of threat as eruption will be less violent while volcanoes with high viscosity lava will be very destructive and have violent eruptions. There are many physical factors that contribute such as what margin it is on. This leads me to one final conclusion. The difference in hazards I caused by the mixture of human and physical. Where the physical dictates the hazard and then the human affects the impact of the hazard.