A hazard can be define as something which poses a level of threat the life, health, property or environment, a volcano can compromise all these things through the many hazards volcanoes presents. Such as lahars, flash flooding, landslides, pyroclastic flows, ash clouds and many others. There are factors which can influence the severity of the hazard and cause differences in them and can be classified into different categories, such as physical, economic, political and social.

One physical factor which causes differences in a volcanic hazard is the steepness of the volcano. If the volcanoes sides are of a steep gradient then the lava from the volcano would flow must faster than if the sides were a steady gradient. Mount Nyiragongo for example had extremely steep sides, 50 degrees at some points. This, combined with the low viscosity of the basaltic lava, saw lava speeds of up to 60 miles per hour. The hazard of a lava flow in Nyiragongo was much greater than if the volcano had steady sloping sides. The type of lava also influences the hazard, if the lava is rhyolitic then it has a high viscosity due to its high silica content and its low temperature (only between 650-750 Celsius). Therefore it won’t flow very fast, minimising the impact of the hazard, whereas if it is basaltic like Mount Nyiragongo then it will flow extremely fast, intensifying the hazard.

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. 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. An 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 Icelandic 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. 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 therefore the most hazardous.

Another 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, the greater 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) and high silica content causes its thick 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 as any 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 Icelandic volcano when it erupted in April 2010, which caused severe 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.

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 minimalised if people chose to settle elsewhere. However, this isn’t the same all around the world due to different types of volcanoes. For example, Grímsvötn in Iceland is a basaltic volcano which mostly lies underneath Vatnajokull icecaps, therefore there would only be minor hazards due to settlement, as no one would want or could live as close to Grímsvötn as people do Mount Merapi. Iceland is also a richer country so none of its residents would rely on subsistence farming. Overall, this shows that settlement can be a huge factor in worsening hazards posed by volcanoes.

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.

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 combine, that the greatest hazards occur.