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The word volcano comes from the island called Vulcano, off the south-west coast of Italy. Due to frequent eruption on that island Romans considered it to be the forge of the God of fire Vulcan. The word has since come to mean any vent in the earth's crust through which magma can reach the surface. The term volcano is also used to describe the landform built by eruptions from the vent and which meaning is implied depends on the useage of the word. These landforms can be mountains and islands formed during eruptions.
Volcanoes are openings in the surface of the earth from which molten rock, magma, and gases escape which consist of a fissure in the earth's crust above which a cone of volcanic material has accumulated. At the top of the cone is a bowl-shaped vent called a crater. The cone is formed by the deposition of molten or solid material that flows or is ejected through the vent from the interior of the earth.
Volcanoes are easily classified in a causal classification scheme as their whole existence is due to great movement and change deep within, and nearer the surface of, the earth. They are Geomorphic and Geological Hazards, a term which can also be described as either Tectonic or Land Instability Hazards.
Hazards can also be classified by their spatial occurrence and this is also a good way of classifying volcanoes. Due to their nature and cause volcanoes occur at weaknesses in the earth's outer most layer, the crust. The crust is made up of several pieces, plates, and it is at the boundaries of these plates that most volcanoes will occur. Here there is a weakness in the crust and magma can force its way to the surface. Because of this volcanoes can be classified as globally occurring but limited to the boundaries of the tectonic plates.
Volcanic activity is almost as diverse as human nature and so many humanlike terms are use to describe a volcano's state or mood. These include : alive, active, restless, awakening, erupting, dormant, sleeping, dead, and extinct. The most commonly used of these are active, dormant and extinct. Active means the volcano is restless and may erupt at any time, dormant means the volcano has not erupted for a while and is unlikely to while extinct implies that the volcano will not erupt again. These last two descriptions should not be taken at face value as a volcano can erupt at any time.
Volcanoes are like a safety valve for the earth, releasing the build up of pressure beneath the earth's surface . The strength of a volcanic eruption depends on the type of magma and the amount of gases trapped in it. If the vent is blocked by a plug of hardened lava, the trapped gases escape with a deafening explosion. There are often signs that a volcano is going to erupt. The ground start to shake and the sides of the volcano bulge out as magma collects inside it. There is a smell of sulphur as gases escape through the cracks in the rocks. Surges of red hot lava can flood out of the volcano's crater at speed of up to 180 metres per second - half the speed of sound. Lava will flow from the volcano as long as there is enough pressure to force it to the surface.
Most volcano-related deaths are associated with highly explosive eruptions involving pyroclastic flows, lahars, flooding and tsunamis. Pyroclastic flows are high-density mixtures of hot, dry rock fragments and hot gases that move away from the vent at high speed. Lahars, mudflows caused by the earth's violent movement, account for 10% of deaths. On the other hand eruptions of molten lava and ashfalls (tephra) are likely to cause a greater threat to property.
As described previously volcanoes can be described as active, dormant or extinct. Extinct obviously means that the volcano will never erupt again but the adjectives active and dormant give no idea of the time scale involved. Generally, all volcanoes that have erupted within the last 2,500 years should be regarded as at least potentially active. This may seem a long time but volcanoes can have lives of up to 1 million years. In an average year around 20 -30 volcanoes actually erupt world-wide, of the 500 currently active. This means that the eruptive, hazardous phases are short in comparison to the lengthly times of inactivity. It is this infrequency that is one of a volcanoes most dangerous features and can make any average figures in statistics misleading.
The period between a volcanoes eruption is called a volcano's repose time. On Muana Loa Volcano the average repose time is about 3 to 4 years but have been as short as a few months or as long as 25 years. Once again one of a volcano's most deadly weapons can be seen - infrequency.
Like earthquakes, the distribution and behaviour of volcanoes is controlled by the global positioning of plate tectonics and active volcanoes exist in every major continental land mass, except Australia. Volcanoes are found in three tectonic settings.
Firstly they are heavily concentrated at the interplate margins. About 80% of the world's active volcanoes are located in the subduction zones of destructive plate boundaries. Here the plates are moving toward each other and one is forced under the other. Volcanoes here are known as subduction volcanoes and are the most explosive type.
Second, rift volcanoes occur at constructive plate boundaries where tectonic plates are diverging. They are generally less explosive with lava pouring out more slowly than the eruptions of subduction volcanoes, especially when they occur on the deep ocean floor.
Third, hot spot volcanoes are located in the middle of tectonic plates where a crustal weakness allows molten material to penetrate from the earth's interior. The Hawaiian islands in the middle of the Pacific Plate are a good example.
As I have just mentioned volcanoes can occur under the sea as well as on land. Indeed the largest number of volcanoes are found under the sea floor. The ocean floor is very thin and can be easily pierced by the magma which lies underneath , especially along the lines of weakness at plate margins.
More than half the world's volcanoes occur in a belt around the Pacific Ocean, known as the Ring of Fire.
Most volcanoes are composite landforms built up partly of lava flows and partly of fragmental materials. Italy's Etna, in Sicily, and Vesuvius, near Naples, are examples of composite cones. In successive eruptions, the solid materials fall around the vent on the slopes of the cone, while lava streams issue from the vent and from fissures on the flanks of the cone. Thus, the cone is built up of layers of fragmental materials and flows of lava, all inclined outward away from the vent. Some enormous, crater-like basins, called calderas, at the top of long-dormant or extinct volcanoes, are eventually occupied by deep lakes, such as Crater Lake in Oregon (USA). Some calderas are the result of cataclysmic explosions that destroy the erupting volcano; the volcanic islands of Thera, Greece, and Krakatau, Indonesia, and Crater Lake are in this category.
In a violent eruption of a volcano, the lava is highly charged with steam and other gases which continuously escape from the lava's surface with violent explosions and rise in a turbid cloud. This cloud frequently discharges showers of rain. Large and small portions of the lava are shot upward, forming a fiery fountain of glowing drops and fragments, which are classified as bombs, cinders, or ash, depending on their size and shape. These objects or particles fall back in showers on the external slopes of the cone or into the crater, from which they are again and again ejected. Lightning often plays through the cloud, especially if the cloud is heavily charged with dust particles. The lava rises in the vent and finally flows over the rim of the crater or oozes, as a pasty mass, through a fissure in the side of the cone. This may mark the so-called crisis, or crucial point, of the eruption; after a final ejection of fragmental material, the volcano may then return to a dormant state.
I have illustrated what volcanoes are and how they can erupt but what are the consequences for people nearby?
Volcanic hazards can be classified into primary and secondary hazards.
Primary Hazards :
Pyroclastic Flows, Air-Fall Tephra, lava flows, volcanic gases.
These are associated with the products ejected by the volcanic eruption.
The most explosive volcanic eruptions are accompanied by pyroclastic flows. Pyroclastic flows are high-density mixtures of hot, dry rock fragments and hot gases that move away from the vent that erupted them at high speeds. They may result from the explosive eruption of molten or solid rock fragments, or both. Very little can survive in the path of a pyroclastic surge and these flows have been responsible for more than 70% of all deaths in volcanic eruptions this century. People exposed to surges are immediately killed by a combination of severe external and internal burns and asphyxiation through inhalation of the searing hot air. Buildings are destroyed.
Air-fall Tephra (Ashfalls) comprises all the fragmented material which is ejected by the volcano and subsequently falls to the ground. The degree of this hazard can vary from global weather disturbances as huge amounts of ash circulate in the atmosphere to reduced visibility and deaths due to choking - less than 5% of deaths are from ashfalls. Ashfalls can also disrupt radio communication and destroy crops.
Lava flows are streams of molten rock that either extrude quietly from a vent or are fed by lava fountains. Lava flows destroy everything in their path, but most move slowly enough that people can move out of the way. Those that have a low viscosity (are quite runny) can reach speeds of up to 15 m/s and pose a great threat to human life.
Volcanic Gases are released by explosive eruptions and lava flows. Despite the toxic nature of these gases they have only rarely been the direct cause of a major disaster and death.
Secondary Hazards :
Ground Deformation, Lahars, Landslides, Tsunamis
Ground deformation occurs as volcanoes swell due to the magma welling up within and as slopes bend as this expelled out onto them. Such deformation is not a hazard in itself but can cause various mass movement hazards.
Lahars are volcanic mudflows. They occur in the flanks of volcanoes, especially in the wet tropics. Apart from pyroclastic flows they are the greatest threat to human life. They can occur in any volcanic event when large quantities of water are present on the steep sides of a volcano.
Landslides and debris avalanches are a common feature of volcanic-related ground failure. As mentioned an eruption can cause ground deformation which can cause landslides.
Finally, Tsunamis can occur. These are much rarer than the other possible consequences of volcanoes. They are huge tidal waves which are mainly caused by earthquakes but can also occur due to volcanic eruptions. By the time it reaches the shore, it may become a towering wall of water 15 m (50 ft) high or more, capable of destroying entire coastal settlements.
As with most environmental hazards the impact of volcanic eruptions depends heavily on the local population density and building type. The flanks of a volcano often attract settlement due to rich agricultural soil which are created by falling volcanic ash and volcanic rock. It is the high population density caused by this that can explain why two-thirds of all volcanically related deaths have been recorded in Indonesia. In 1815 a massive eruption shook the Tambora volcano. The height of the mountain was reduced by 1,400 m, leaving a caldera 12 km in diameter. About 12,000 people died immediately after the event, although a further 80,000 persons eventually perished through the disease and famine which followed the destruction of crops by volcanic ashfall over a wide area of Indonesia.
Volcanic eruptions are the source of multiple hazards. World-wide they have been responsible for killing, on average, about 650 people per year in the 20th century up to 1982 and it is estimated that more than a million people have been killed by volcanic eruptions in the past 2,000 years. The death toll for the last 100 years is around 100,000 deaths and damage costs have been put at $10 billion for the same period. Most deaths occur on few eruptions. For example 29,000 people died in the port of Saint Pierre on the island of Martinique in the Caribbean after an eruption in 1902. There are only 2 known survivors from the whole town.
The historical record however probably does not provide a reasonable guide to future risk from volcanic hazards. There are 2 reasons for this : First, the population of the earth is continues to increase rapidly, thereby putting more people at risk from even moderate scale eruptions. Second, evidence shows that many volcanic eruptions in prehistoric times were much larger - 10 to 100 times greater in volume - than the huge eruption in Indonesia described earlier. Based on present knowledge, it appears that eruptions of roughly 100 cubic kilometres occur every 10,000 years and those of 1,000 cubic kilometres about every 100,000 year. If this is correct, infrequent but extremely large volcanic eruptions will occur in the future.
A second case study showing the scale of volcanic hazards is in Mt. Pinatubo, in the Philippines. Located on Luzon Island in the Philippines, 4,000 feet high, Mt. Pinatubo erupted on the 12th of June 1991. Approximately 200 people died and 100,000 were made homeless by the multiple eruptions. Seismologists issued warnings and over 12,000 residents were ordered to evacuate. At 8:41 am the volcano erupted spewing a 15 mile high mushroom cloud of ash and cloud into the air. Rivers of searing gas, ash and 1,800 °F molten rock gushed down slopes at speeds up to 60 mph. 150,000 refugees were created because of the eruption. Once again the size of disruption and potential hazards that volcanoes posses can be seen.
There is no known method of preventing volcanic eruptions and no known defence from the primary threat from pyroclastic flows. Comparatively little can be done about ashfalls but strengthening of buildings can help prevent their collapse on the occupants due to ash accumulating on the roof. Lava flows moving at comparatively slow speeds are the primary volcanic hazard over which most physical control can be exerted. There are three identified measures for diverting and controlling lava flows. They are :
Bombing or the use of ground explosives can be used. Bombing used high on the volcano may cause the spread and halt the advancing lava by depriving the flow of supply. Bombing of the sides of the channels that are created by lava flows allows the lava to spread over a large area thus slowing the flow and reducing the distance the lava flows.
Artificial Barriers can be used to divert lava streams away from valuable property. Barriers must be constructed from resistant material, such as massive rocks, with a broad base and gentle slopes. This method is most appropriate for thin runny lava which exert small amounts of thrust.
Water Sprays cool the lava rapidly thus solidifying it. This rapid solidifying of the lava causes the flow to slow and creates a new barrier for the lava to overcome. This method was first used experimentally in the 1960's.
Some protection from lahars has been attempted using artificial barriers such as I have just talked about.
As with many natural hazards the only totally effective way of reducing or preventing the hazard is to remove all human presence from the area in which the hazard may occur. Of course with volcanoes this is not viable and very unlikely to happen due to the advantages of living on the rich agricultural soils of the volcano. This means that a volcano hazard is very difficult to prevent as it is impossible to prevent an actual eruption. The best way of reducing the danger of the hazard is to monitor volcanoes to give warning time for evacuation, and there are increasing efforts to encourage the local population in seismically active areas to become more involved with disaster preparedness to prevent catastrophe.
Major volcanic eruptions do not happen spontaneously. Some volcanoes show quite consistent patterns of activity while others are much more erratic. Eruptions are preceded by various environmental changes which accompany the rise of magma toward the surface. The best way to predict eruptions is the monitoring of these changes. Monitoring of these changes include looking at :
Having illustrated these methods of trying to predict volcanic activity it is important to remember that there is no fully reliable system of monitoring and predicting volcanic eruptions.
Volcanoes are a difficult phenomena to suppress but are clearly visible. Because of this they should not form a hazard to humans as we know where they are, and that we cannot tame them, and we can stay away. However, because of the advantages of living to a volcano, predominantly good agricultural soils, many people ignore the dangers and put themselves in a hazardous position. This can lead to disaster as I have shown. Really volcanoes should not be too much of a hazard as they are so visible, but once again human nature can be seen to be putting itself in danger.
Sources
The University of Maryland Web Site