Case study for Natural Hazards - VOLCANOES By William Wong
Distribution Volcanoes can be described as being tectonic hazards that occur in many parts of the world. The distribution of volcanoes is closely linked with the positioning of the tectonic plate boundaries across the globe. Today there are about 500 active volcanoes in the world. The world map of volcanoes in your atlas shows that the most volcanic activity occurs along the West coasts of North and South America, (along the Rockies and Andes) and the coasts of many Far East countries (in areas like Japan, China etc).
The positioning of these volcanoes coincide with major plate boundaries. E.g. the volcanoes along the West coast of North America are the result of the Juan de Fuca plate converging with the North American plate. Another example is the volcanoes formed in the Andes Mountains are a result of the convergence of the Nazca and South American plates. Other areas that have high volcanic activity are along the plate boundaries of the Eurasian and African plates.
Most of the volcanic activity on Earth occurs on the island of Iceland. Which is due to the formation of the Mid-Atlantic ridge, which is where two tectonic plates are diverging away from each other. Features Volcanoes release large amounts of energy from beneath the lithosphere, (Earthís crust and upper part of the mantle) and asthenosphere, (semi-molten mantle).
During volcanic eruptions molten rock or magma rises up from the asthenosphere towards the surface as itís propelled by gases surrounding it. Some eruptions can occur through cracks in the Earthís surface called fissures. As magma reaches the surface, itís called lava. Common features of major volcanic eruptions include: pyroclastic flows which is material ejected by volcanoes in a fragmented form. The pyroclastic flows are clouds of ash, stones etc that move down the side of volcanoes and carry any debris in its way. A famous pyroclastic flow occurred when Mt Pinatubo erupted in 1991. Mud volcanoes, which may form where hot water mixes with mud and surface deposits. Solfataras, that are created when mainly sulphurous gases escape onto the surface. Geysers, occuring when water in the lower crust is heated by rocks and turns to steam, pressure increases and the steam and water explode onto the surface. Fumaroles, formed when superheated water turns to steam as its pressure drops when it emerges from the ground. Formation: - there are three main ways in which volcanoes can be formed.
At constructive plate margins This is where two tectonic plates are diverging away from each other, which causes new crust to be created at the boundary between these two plates. A well known example is where the North and South American plates are pulled away from the Eurasian and African plates due to convection currents. This has caused the formation of the Mid-Atlantic ridge.
When two plates diverge, initially, a rift valley may occur. Magma rises from the mantle filling in the gaps between the two plates, which can cause submarine volcanoes to occur, (volcanoes under water). These volcanoes may rise above sea level thus forming islands. Two examples of this are the island of Surtsey, (south of Iceland) and Easter Island, (on the East Pacific Rise, which lies on the African rift valley). Other volcanoes may form on top of the land such as Kilimanjaro and Kenya formed on the Great African Rift Valley.
At destructive plate margins Destructive plate margins occur where continental and oceanic plates converge together. The continental crust has a rock density much lower than the oceanic crust. As these two plates meet the oceanic crust is submerged underneath the continental crust, as it is denser. This causes a subduction zone to form where deep sea trenches result. Examples include the Peru-Chile trench and the Japan trench. As the oceanic plate is submerged, it begins to melt due to increased heat and friction, which can lead to the possibility of earthquakes occurring beneath the surface. The melting of the plate results in magma building up, which eventually rises up to the surface. If the magma rises on continental land then a long chain of fold mountains may result like the Rockies and Andes. If the magma forces its way up on offshore, then Ďisland arcsí form. The West Indies and Japan are just two examples of island arcs.
Hot-spots These are small areas of the crust with an unusually high heat flow and are found away from the plate boundaries. The magma slowly rises from the mantle rocks and pushes the crust upward creating volcanoes, which may rise above sea level. A good example of this is the chain of Hawaiian Islands. If the islands are submerged under water again they are then known as guyots.
The Human Response to Volcanic Hazards Environmental Controls:-
Not much can be done to stop the effects of a volcanic explosion. The only primary effect that can be moderated by human intervention is the lava flow. This has only been done by either using explosives to divert its course or using water spray to slow it down. There are a few other techniques to deal with lava flows planned. Such as barriers in Hilo, Hawaii. Barriers are used to protect areas from the secondary hazard of lahars.
Hazard-Resistant Design It is not possible to make a building lava or pyroclastic flow proof. These volcanic hazards will destroy any structure in their path. Roofs need to be strong and designed to shed ash, with steep-sloping sides. Damage can be limited however, by building cheap houses which can be replaced for a low cost.
Modify Vulnerability

Community Preparedness Most volcanic events are preceeded by clear warning signs coming from the volcano. If the community is prepared, lives can be saved. Although the degree to which a community is prepared helps determine the number of lives saved. Evacuation is the most important and common hazard prevention method used today. Ash and other harmful gases and material emitted from the volcano can continue to fall to the ground long after the volcano has finished erupting. They can be of a great health risk, so in many areas, the civilians are provided with masks to aid safer breathing. E.g. After the Mount Pinatubo eruption in 1991. There are problems associated with this. It damages the local economy in the short term and often finding suitable relocation areas is hard especially in island nations such as Montserrat.
Predictions and Warnings Various physical processes can be monitored and variations in them signal an eruption. The record of past events is used in the prediction of future ones in deciding what the risks were etc. At present only 20 % of volcanoes are being monitored and most of theses are in MEDCís such as USA and Japan. An example of a closely monitored volcano is Sakurajima, in South Kyushu, Japan.
Case Studies

(a) The Sakurajima Volcano, Japan. The Sakurajima Volcano is located near to the city of Kagoshima (population Ĺ million). With many people nearby, it is important that the volcano is monitored so the people can be warned of the hazard before actually happens. There was a number of methods of monitoring: 1. Aircraft and satellites measured heat, gas and ground movements. 2. An observation borehole monitored the underground temperature and the mountains movements as well as change in quality of hot spring water and underground gas composition. 3. The measurement of local magnetic fields. 4. Remote sensing of the chemical composition of escaping gases.
(b) Mount Pinatubo, Philippines, 1991. The eruption of Pinatubo in '91 was the largest volcanic eruption in the world in over 50 years. Before '91, it hadn't erupted for 600 years. It is a subduction zone volcano and is surrounded by pyroclastic flow and lahar deposits from previous eruptions. By April 1991, a team from the PHIVOLCS (Philippine Institute of Volcanology and Seismology) were monitoring the volcano, and by the end of the month they were joined by a team from the USGS (United States Geological Survey), who set up the Pinatubo Volcano Observatory (PVO). Using geological evidence of past eruptions, they produced a map of the volcanic hazard. They then informed the local authorities of the potential risk. They then used videos and they devised a six point scale to simplify communications, as the area had no volcanic previous experience. This human response undoubtedly saved lives. In this eruption almost 500 ft of the volcano was 'blasted away'. During the eruption 350 people were killed, and the secondary impacts increased the number to over 800. Lahars alone killed 77 people. The most vulnerable people to the eruption were the tribal Aetas people who lived on the slopes of the volcano. Many refused to leave their holy mountain, and died during the eruption. Many others who went to the evacuation camps also died because, due to their isolation before the eruption, they were susceptible to diseases and refused to take medicines, 94% of deaths in evacuation camps were Aetas. It is estimated around 2 million people were affected by the eruption which caused total losses of US$711.4 million, mainly due to the loss of about 80,000 ha of farmland under ash and the destruction of infrastructure. The eruption not only destroyed the harvest of 1991, but it also made planting for 1992 impossible.
(c) Iceland. Iceland is located on the Mid-Atlantic Ridge. Iceland has large amounts of volcanic activity, and is estimated that on Iceland a third of all lava that has erupted on land in the last 500 years can be found. The island of Surtsey is volcanic. In November 1963 the island of Surtsey emerged from the sea 130m from the seabed. A year later another eruption guaranteed the survival of Surtsey. In January 1973 ash and lava started to pour out a 2km long fissure the lava eruption then became concentrated on the volcano Helgafell. Nearby Heimaey had to be evacuated. The activity continued for six months and by the time it stopped houses had burnt and others buried under 5m of ash. The area also lost its livelihood with the blocking of the harbour.
Hazards and Responses - Victoria Bishop
Geography: An Integrated Approach - David Waugh