23 Mar 2015 10 Jan 2018
A natural hazard is defined as a "natural process or phenomenon that may cause loss of life, injury or other health impacts, property damage, loss of livelihoods and services, social and economic disruption, or environmental damage" (UNISDR, 2009). It is clear that volcanoes pose a huge threat to human life and can also have major economic impacts. This short essay aims to present the hazards that arise due to volcanoes and look at what measures are currently (or should be) being taken in order to minimise the risks taken by living in close proximity to one.
http://earthquakescanada.nrcan.gc.ca/nazko/IMG012.jpgThe main and most obvious hazard that occurs due to a volcano (depending on the type) is the eruption. For volcanoes such as Kilauea in Hawaii the effusive eruption is less dangerous due to the lower pressure and lava tends to be erupted rather than other materials. The main hazard from these types of eruptions is the lava itself, which can reach widespread areas in some cases and destroys most things in its path. Volcanoes similar to Mount St. Helens in Washington, USA have extremely dangerous explosive eruptions (also known as Vesuvian eruptions) which involve many different hazards in themselves. "Massive quantities of ash-laden gas are violently discharged to form a cauliflower-shaped cloud high above the volcano" (Tilling, 1985). A report by (Myers & Brantley, 1995) describes the effects of the blast from an eruption such as this: "An explosive eruption blastsmolten and solid rock fragments (tephra)into the air with tremendous force. The largest fragments (bombs) fall back to the ground near the vent, usually within 2 miles. The smallest rock fragments (ash) continue rising into the air, forming a huge, billowingeruption column. ... Eruption columns can be enormous in size and grow rapidly, reaching more than 12 miles above a volcano in less than 30 minutes. Once in the air, the volcanic ash and gas form an eruption cloud. ... Large eruption clouds can travel hundreds of miles downwind from a volcano, resulting inash fallover enormous areas"
Another hazard is known as a pyroclastic flow and is when "High-speed avalanches of hot ash, rock fragments, and gas move down the sides of a volcano during explosive eruptions or when the steep edge of a dome breaks apart and collapses. Thesepyroclastic flows, which can reach 1500 degrees F and move at 100-150 miles per hour, are capable of knocking down and burning everything in their paths." A similar hazard is known as a pyroclastic surge which is more energetic and has a dilute mixture of searing gas and rock fragments. They can move over ridges easily whereas flows tend to follow valleys (Myers & Brantley, 1995).
Hazards that are not a direct result of the initial blast can be classed as secondary. Mud and debris flows are known as lahars and are initiated by large landslides of water-saturated debris, heavy rainfall eroding volcanic deposits, sudden melting of snow or ice near a vent or the breakout of water from glaciers, crater lakes or from lakes dammed by eruptions (Tilling, Topinka, & Swanson, 1990). These are also very destructive and range greatly in size from several centimetres in size to kilometres and in speed from less than a metre per second to tens of metres per second.
Most of the time an earthquake proceeds a volcanic eruption due to the imminent release of the pressures that have built up inside. An earthquake can be extremely dangerous in itself, so when coupled with an eruption it can be devastating. The main hazard is shaking and ground rupture which can lead to severe damage of buildings and in turn cause loss of life. They are largely dependent on the local geological and geomorphological conditions which can either amplify or reduce wave propagation (Perkins & Boatwright, 1995). For example, a city built on a river bed is far more vulnerable due to the phenomenon of liquefaction which amplifies the size of the waves due to soil temporarily losing its strength and transforming into a liquid. Damage to electrical power lines or gas mains can also cause fires to break out and in some cases they may be extremely difficult to put out due to water mains bursting which would incur a loss of pressure.
One of the most important processes involved in reducing the risks imposed by a volcano is monitoring. According to (Brantley & Topinka, 1984) "Volcano monitoring involves a variety of measurements and observations designed to detect changes at the surface of a volcano that reflect increasing pressure and stresses caused by the movement of magma, or molten rock, within or beneath it." There are many measurements that are taken in order to build up a large picture of the volcano and ultimately predict to the nearest accuracy possible when an eruption is going to take place. The movement of the ground is closely recorded because increased movement can indicate an upcoming eruption due to the movement of magma underground. Standard levelling surveys are used to obtain changes in the elevation, the tilt is measured and electronic distance measurement is also used. When no earthquakes or measurable ground movement occurs there are geophysical properties which can be measured including electrical conductivity, magnetic field strength and the force of gravity. Once again, changes in any of these values can indicate the movement of magma. Changes in the composition or emission rate of sulphur dioxide and other gases from a volcano can also indicate a variation in magma supply rate or a change in magma type. Modified from (Wright & Pierson, 1992)
In addition to monitoring, detailed hazard maps are drawn which show the areas that are likely to be effected during an eruption event. Figure 3 is a simplified version for the Mount St. Helens volcano. These maps are extremely useful because they allow resources to be allocated to the parts that need it the most. For example any settlements in immediate danger from the volcano must be evacuated first, and so on.
The final piece in the hazard reduction puzzle is communication. No matter how precise the information regarding an eruption is, it is useless unless this information is successfully conveyed to the people at risk and they are in a position where they understand and can take action. The following is what the U.S. Geological Survey Volcano Hazards Program does in order to try and achieve this:
(U.S. Geological Survey, 2009).
It is clear that volcanoes pose a huge threat to people's safety. However, when a high amount of monitoring, planning and communication takes place it is usually possible to predict eruptions to a level accurate enough to save lives. The main limiting factor is the money available to spend (or the money willing to be spent) on all of these things by a country's government. This means that for example, people in the USA and Canada can feel relatively safe about the threat to them from most of their volcanoes whereas people living in some countries of Africa cannot.
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