Volanoes and Associated Hazards
This revision and study page provides in depth knowledge different types of volcano and eruptions including the variety of primary and secondary hazards associated with them
Volcanoes are not all the same. They have very different structures and sizes and the nature of their eruptions vary in magnitude and explosivity. Volcanoes occur at both destructive (convergent) and constructive (divergent) plate boundaries as well as at hotspots associated with mantle plumes. The physical processes leading to magma formation is quite varied and this explains the differences in volcano structure and eruption.
Destructive plate boundaries are more closely associated with composite or strato-volcanoes that produce more explosive eruptions. Constructive boundaries are more linked with shield volcanoes and lower magnitude eruptions. Hotspots are generally associated with shield volcanoes but in fact can result in all forms of volcano. Cinder volcanoes are most commonly found on the flanks of shield and composite volcanoes. The types of volcano and eruption type can be seen the diagram below.
Different magma produces different eruptions. Magma is produced when rock in the upper mantle melts. Being less dense than the rock around it, it rises upwards. There are three types of magma and they have very different characteristics. Basaltic magma forms at constructive boundaries and hotspots over oceans. Its temperature is hottest and it has lowest silica content. This means that it is more fluid in form and produces faster flowing lava. It also allows for gas to more easily escape and so eruptions tend to be less violent with smaller ash columns. The diagram below shows how pressure release at the ridge leads to partial melting of the mantle and magma formation.
Rhyolitic magma in contrast is cooler and has much greater silica content. This makes it thicker and produces much more viscous lava that is resistant to flow. This magma traps gas and produced highly pressurised eruptions. Rhyolitic magma is much more common with composite volcanoes in destructive plate boundaries. The diagram below shows subduction at two types of destructive plate boundary. The diagram to the left shows how island arcs form when the denser oceanic crust subducts under the lighter oceanic crust at oceanic to oceanic margins. The diagram to the right shows the subduction of dense oceanic crust under the lighter continental crust at oceanic to continental margins. In both cases the subducting crust sinks into the softer asthenosphere. It carries hydrated sediments and rock and so this water lowers the melting point of the rock found in the mantle wedge. This process causes melting of the mantle wedge and partial melting of the crust leading to magma.
Silica is the most important compound in magma influencing the explosively of its eruptions. As magma rises upwards though the crust, silica compounds increase in concentration. This happens for two key reasons. The first reason is because silica poor compounds crystallise, gain density and sink. Silica rich compounds crystallise slower. The second reason is because as the magma rises it assimilates more silica-rich compounds in the surrounding rock and the higher up in the crust the more silica rich rock can be found. Therefore magma rising upwards in continental crust, due to its greater depth will assimilate more silica. Silica rich compounds increase viscosity of magma and trap more gas. For this reason eruptions at destructive boundaries are more explosive.
Composite, Shield and Cinder Volcanoes
The following gallery details the key differences between composite, shield and cinder volcanoes
The hazards of a tectonic event can be classified into two types, primary hazards and secondary hazards. Primary hazards result directly from the erupting volcano and include, lava, ash and gas. When combined these can form pyroclastic flows. Secondary hazards in volcanoes are linked to the structural collapse of the volcano and include rock avalanches such as landslides and lahars. A really good video on primary and secondary hazards from Cardiff University can be watched here.
There are different types of lava but the main two are called Pahoehoe and A'A lava. Pahoehoe lava is smooth and billowy in its appearance. It is low in viscosity and generally travels slowly. A'A lava is much more rough in its appearance, it has higher viscosity and carries lava blocks called clinkers. Lava flow can lead to some secondary hazards such as snow and glacier melt that can cause floods or glacial ouburst floods called jökulhlaup as well as forest fires.
Ash and Gas
Other primary hazards are ash and gas ejections. Powerful eruptions create vast ash columns that can shoot as high as 12 kilometres into the atmosphere. They then produce ash plumes that extend away from the volcano and result in ash fall that can effect places many hundreds of kilometres away from the volcano. Ash can also impact on global climate, if sufficient ash reaches the the top if stratosphere. Ashfall can asphyxiate people and livestock. If it falls in sufficient quantities secondary hazards such as contaminated water and collapsing roofs can endanger life. When this ash fall mixes with rainfall the flanks of the volcano become unstable, resulting in rapid moving mudflows called lahars.
Gases emitted by volcanoes include significant amounts of carbon dioxide, sulfur dioxide, hydrogen sulfide and hydrogen halides . Depending on their concentrations, these gases are all potentially hazardous to people, animals, agriculture, and property. There have been many examples of eruptions and gas releases that have suddenly killed large groups of people. One such event happened in 1986 at Lake Nyos in Cameroon. Here large volumes of volcanic gas made out of carbon dioxide was suddenly released from the depth of the lake, which led to the deaths of thousands of people and cattle.
Pyroclastic flows are a primary hazard of an eruption and occur when superheated gas, pumice and ash fly down the flank of the volcano. Pyroclastic flows can reach speeds of several hundred km/hr. However, they vary enormously and most common speeds are between 20-70km/hr. Temperatures vary from 200 to over 700 °C. Pyroclastic flows form when the upward force of the blast is weak and part of the ash column collapses down the flank of the volcano. Their composition has a surge at the front, heavier blocks and ash at the base and lighter ash and gas above. Pyroclastic flow will knock down nearly anything in its path.
When the pressure within the magma chamber is so great it can lead to lateral blasts, whereby the flank of the volcano becomes unstable and collapses. This leads to block and ash flow called rock avalanches. The scale of these events can be extraordinary. This occurred in 1980 during the Mount St Helens eruption. 2.3 billion cubic meters fell from the mountain destroying more than 600 km2 of territory and millions of trees. Rock avalanches can also cause floods and tsunamis when they hit rivers and large bodies of water.
Lahars occur when large quantities of water mixes with fallen ash. Water can come from torrential downpours caused by ash particles in the atmosphere but also from snow melt or dam failure. When this water mixes with ash, rivers quickly turn into torrents of mud and stones. These lahars can run for some distance away from the volcano, destroying everything in their way. Lahars vary in size and speed. Small lahars less than a few meters wide and several centimeters deep may flow a few meters per second. Large lahars hundreds of meters wide and tens of meters deep can travel at fast speeds. With the potential to flow at speeds up to 100 kilometers per hour and flow distances of more than 300 kilometers. A lahar can cause catastrophic destruction in its path. Lahars caused by the snow melt in the wake of the 1985 Nevado del Ruiz eruption devastated the city of Armero in Colombia, causing the deaths of over 23,000 people.