There are several reasons that influence volcanic hazards, these include the viscosity of the magma, proximity to population centres, materials that are ejected from the volcano, the magnitude of the eruption and the plate margin that the volcano rests upon.
The viscosity of the magma depends on 3 things, the temperature, dissolved gasses and the chemistry involved in the magma. These factors produce 2 main types of magma called acid and basic. The acid lava is the more viscous (66%+ rich silica) and so this is the more hazardous lava as it has more potential for explosion. The other, less viscous lava is the basic lava which has a 44-52% silica content and has a higher temperature of 1000-2000 degrees compared to the 600-1000 of the acid lava which allows the basic lava to run more smoothly and more fluid like.
Most volcanoes occur at a plate margin because this is the area where the magma is produced. At constructive (divergent) plate margins the magma is produced by the melting of the mantle below the surface and so this produces a basic lava which allows it to flow easily. These types of volcanoes eg, Mid-Atlantic Ridge erupt frequently but do not produce devastating results, they are associated with lava flows and ash. On the other hand volcanoes on destructive (convergent) margins (plates moving towards each other) are much more violent and hazardous. The formation of an ‘acid magma chamber’ is caused by one plate subducting below the other which causes intense pressure and heat causing the melting of rocks and sediment. Because the lava is very viscous and doesn’t flow very easily it allows huge amounts of pressure to build up which can lead to a catastrophic eruption involving pyroclastics.
75% of the worlds volcanic material is erupted at conservative margins however 80% of the worlds most active volcanoes occur at destructive plate boundaries.
The power of a volcano when it erupts plays a major part