Volcanic Hazards: Too Close for Comfort
Geography 301: Natural Hazards
Professor J.K. Mitchell
Prepared by: Hans N. Lechner
December 17, 2002
The planet seems to be getting more hazardous. It has been estimated that the number of disasters rose six percent between 1962 and 1992 (Bankoff, 2001). Of course there can be no hazard without people, so the rise in hazards is most likely the result of an increasing population. The rapid population growth facing the world will almost certainly increase society's vulnerability to natural hazards (Chester et al., 2002). Major volcanic eruptions, while often a spectacular geophysical events, are relatively infrequent compared to floods, cyclones, and earthquakes. Infrequent as eruption incidents may be, volcanic hazards are increasing (Smith, 2001). Again, this can be attributed to the increasing populations residing within close proximity to volcanoes. If volcanoes are becoming more hazardous because of growing populations, then the question society should be asking is how do we determine who is more vulnerable? It is my argument that volcanic events pose an increasing threat to humans and the things we value because human social systems are growing larger and closer to volcanic event zones thus increasing the level of exposure. By examining the distribution, magnitude and frequency of three selected volcanic events: lava flows, pyroclastic ejecta, and lahars this paper will try and establish an event zone. This paper will then illustrate several examples where human settlement has developed or encroached within the event zone, and attempt to identify a vulnerability paradigm that can best be applied to volcanoes as a hazard to human lives and property.
There should be no doubt that the population is growing. As human numbers increase our settlements sprawl ever farther onto the landscape. Currently, the world's population is estimated well over 6 billion and with a growth rate of 2-3 percent is projected to reach nearly 8 billion by 2020 (Espenshade, 1994). Smith (2001) estimates that 90 percent of the population growth is occurring in the Less Developed Countries (LDC's) and Chester et al. (2002) expects roughly 80 percent of the population to be living in the LDC's by 2025. A world map depicting human settlement compared to the global distribution of volcanoes clearly illustrates that humans are living near and around volcanoes (Figure 1, not included). The unfortunate coincidence seems to be the fact that most subduction zone volcanoes occur within 200 miles of the coastline, which is also where a majority of the world population growth is occurring. Figure 1 also shows that populations in east and Southeast Asia and western South and Central America are growing along the coast near volcanic areas.
Volcanoes are found around the globe with the largest concentration occurring along the Pacific Rim. This distribution pattern is the primarily the result of the heavier Pacific Oceanic Plate subducting under the more buoyant Eurasian, American and Australian Continental Plates. As the oceanic plate sinks deeper into the mantle of the earth melting occurs and large plumes of magma ascend back towards the crust. This process results in the formation of subduction-zone volcanoes and has given the world what is known as the Ring of Fire. Volcanoes also form in rift zones where plates are separating, and at hot spots where magma erupts to the surface through a weak spot in the earth's crust.
Volcanoes can have direct or indirect effects on culture and society. Volcanic eruptions and associated phenomena cause damage. Lava flows can destroy buildings, infrastructure and agriculture, while lahars and pyroclastic flows can erase entire towns from existence. Volcanic ash and tephra can take lives, destroy buildings and change the global climate. However, volcanoes also have positive effects. They can add to the area of an island, increase the fertility of soil, and provide a destination for international tourists.
Blong (1984) has compiled much of the data provide by geoscientists and created a table that distinguishes the level of frequency of nine volcanic events considered as hazardous (table 1). The table lists ballistic projectiles, tephra falls, and atmospheric effects as separate hazards that occur approximately 60 percent of the time followed by seismic activity, lava flows as the other most frequently occurring volcanic hazards. However, when the spatial range of these hazards is considered, their adverse affects do not necessarily follow the above trend. Ballistic projectiles are considered a common adverse hazard within 10 km of the event but are not even considered beyond that, whereas tephra fallout is a very frequent hazard under 10 km and is still a common hazard at 100-500 km distance from the event. By using the table adapted from Blong we can determine how far the event zone reaches. From here we can begin to estimate where, on a spatial level, human society becomes vulnerable. Unfortunately, this table is not an indicator of magnitude of the volcanic event; therefore, we must look at the individual phenomena and the unique risk each poses.
It is estimated that there are about 600 active volcanoes on earth at this moment, which have clamed over 70,000 lives from 1900 through 1990 (McQuire and Kilburn, 1995; Blong, 1984). There are a number of volcanic phenomena that take lives and cause damage and most of the knowledge that we have of these phenomena comes from earth scientists who have looked at these events as a natural process with little concern of the social implications. Although this paper is an attempt to look at volcanic hazards and vulnerability issues from a social scientist’s perspective it is important that we have a basic understanding of the physical concepts and terminology. The primary goal of this is to present the various types of volcanic hazards so that a connection may be made between the behavior and range of the volcanic event and the human interaction with it.
There are two main tectonic scenarios involved in the formation of volcanoes: those that are centered over a fissure and those generated by a pipe or vent in the crust (Blong, 1984). The former commonly occur at divergent plate margins, such as mid-oceananic ridges and continental rift zones and occasionally over a stationary hot spot in the lithosphere. This type of volcano is typically associated with basaltic magmas that erupt as sheets of lava and form gently sloping shield volcanoes such as the Hawaiian Islands and Medicine Lake in California (Chester, 1993; Blong, 1984; USGS, Dec. 13, 2002). It should be noted that this type of volcano will sometimes produce cones along the fissure such as those found in east Africa. The latter type volcano, always forms a cone and is associated with a thicker, more silicaceous magma that can generate much more explosive eruptions (Chester, 1993; Blong, 1984; USGS, Dec. 13, 2002). Subduction-zone volcanoes form large composite cones created by successive eruptions. Mt. St. Helens and Mt. Fuji are two examples. Their high andesitic magma composition usually produces explosive and devastating eruptions. Chester (1993) estimates that 80 percent of the world’s volcanoes lie above a subduction zone.
Magma composition is extremely important, not only for understanding the type of volcano that will be produced but also the dynamics of the hazards that are commonly associated with it. Magmas of high iron composition are less viscous and allow for trapped gasses to escape with ease; conversely, magmas high in silica are much more viscous and keep gasses trapped within. When these thick magmas reach the surface, or an area of lower pressure, the gasses suddenly have less difficulty escaping. This tends to result in explosive events (Bullard, 1979). The high iron content in basaltic magmas lends to their relative fluidity, higher flow rates and lower explosivity; whereas the high silica and andesite contents create slow moving, thick lavas with a higher explosive potential. According to Blong (1984) about 90 percent of lava flows are derived from basaltic magmas and the remainder are from andesitic and silicaceous magmas.
No two volcanoes are alike. Each one displays its own unique characteristics and experiences various types of eruptions. The French geologist A. Lacroix devised a classification system, which is explained in Bullard (1979) and modified in Blong (1984, p.5), that recognizes several eruption types: Icelandic, Hawaiian, Strombolian, Vulcanian, Phreatic, Surtseyan, Plinain, Peleean, Bandaian, and Katmaian. The names are primarily derived from volcanoes that exemplify these eruption characteristics. Icelandic (basalt floods) releases widespread flows of low viscosity lava and small amounts of tephra and bombs. Hawaiian eruptions typically display lava fountains, bombs and projectiles, with thin flows of lava. Strombolian eruptions eject fluid lava clots, scoria, bombs and glassy tephra, which can produce cinder cones and release moderately thick lava. Vulcanian eruptions display moderate to violent blasts of viscous blocks and can produce thick lava. Phreatic eruptions are usually an explosive release of non-magmatic material. Surtseyan, or phreatomagmatic, are violently explosive eruptions of fragmented material caused when lava encounters water. Plinian eruptions spasmodically disperse large amounts of tephra and sometimes pumice. Peleean eruptions are associated with the collapse of a Plinian or Vulcanian eruption column, dome or cone, which commonly causes a nuee ardente or pyroclastic flow. Bandian eruptions occur when part of a volcanic edifice collapses and produces a massive landslide. Lastly, a Katmaian eruption is the voluminous production of ignimbrites.
It is not uncommon for volcanoes to display more than one of the above eruption types. Blong (1984) points out how Krakatau first displayed Vulcanian activity before the powerful Plinian and Peleean phases.
When one imagines volcanic hazards lava flows – glowing rivers of molten rock raging down the slopes of a volcano – are often times what come to mind. While lava flows may be spectacular, and quite frequent events they are not as hazardous as would be imagined. The relatively slow movement of lava usually allows ample time for people to evacuate themselves and their possessions from the path of danger. Of course, roads structures, farmland and forests cannot be evacuated and are, therefore, at a higher risk.
There are three different types of lava flows: pahoehoe, aa, and block lava. Pahoehoe, which is relatively fluid lava, can flow fairly easy on a gentle slope. The latter two are far more viscous with a flow front that is characterized by large chunks, fragments, or blocks of cooled lava that flow in front and on top of the lava. Because aa and block lavas are much thicker than pahoehoe flows they require a much steeper slope for flow movement. With distance pahoehoe lavas commonly become aa or block lavas (Blong, 1984). These three flow types are quite common with Hawaiian type volcanism.
It is the composition of lavas that influence their rate of flow and average distance as well. For instance, basaltic lavas at Mauna Loa Volcano have average flow speeds of 16-40 km per hour with the longest flow recorded reaching over 50 km (Blong, 1984; Bullard, 1979). At Paricutin Volcano in Mexico flow velocities exceeded 66 km per hour on a steep slope and slowed after spreading onto a gentler grade (Bullard, 1979). When the Nyiragongo lava lake drained in 1977 the lava traveled over 5km with an average flow velocity of 30 km per hour and a maximum of over 100 km per hour (Blong, 1984; McQuire and Kilburn, 1997).
Pyroclastic events are the second hazard we shall look at. Unlike lava flows this hazard is far less predictable and much more frequent (Blong, 1984). Some form of pyroclastic material is ejected from nearly every kind of volcanic eruption. This category will include volcanic projectiles such as bombs and blocks, pyroclastic flows, and tephra fallout. The term pyroclastic refers to volcanic material ejected into the air, which can also be referred to as tephra. However, for simplicity sake let us refer to tephra as small airborne particles such as cinders, ash and dust.
It should not be hard to imagine the threat posed by a glowing avalanche racing down a mountain or red-hot rocks and ash falling from the sky. The possibility of buildings, property and lives being crushed or ignited by pyroclastic activity is relatively high within a short distance of the volcanic vent.
Bombs and blocks are particles over 64mm, lapilli are particles of 2 mm to 64 mm and tephra and ash is under 2 mm. These types of events pose a serious hazard to property, structures, flora, fauna and human lives simply as destruction from above. Pyroclastic features are commonly associated with explosive type eruptions that occur when magma and gas are driven upward resulting in decompression and rapid expansion. Vulcanian, Surtseyan and Plinian eruptions can produce this type of activity (Blong, 1984). Usually this explosive action will launch a projectile from the volcanic vent. If the projectile is solid when ejected then it is considered a block, but if it is launched as a semi-solid clot of magma then it is a bomb (USGS, Dec. 13, 2002).
The following examples of volcanic projectiles are provided by Blong (1984). During the 1943 eruption of Paricutin, Mexico, a bomb measuring 50 inches in diameter was blasted 850 feet into the air, and in Karkar, 1979 a bomb crater 7m in diameter was found 1.5km away from the source. The 1937 eruption of Asama launched a one-meter bomb a distance of 3.5km. In 1737 a 50mm projectile was found 11km from the vent.
The A.D. 79 eruption of Vesuvius began with moderate to powerful ejection of ash, bombs and blocks, before part of the crater collapsed causing the nuee ardente that destroyed Herculaneum and the massive tephra fallout that buried Pompeii and killing between 2000 and 16,000 people by asphyxiation and burial (Bullard, 1979; Chester, 1993; McQuire and Kilburn, 1997).
Tephra falls do pose a risk to lives; however, pyroclastic flows are the chief cause of deaths in volcanic eruptions (Chester, 1993). Pyroclastic flows are usually hot dry masses of tephra and gas that form at volcanic vents and flow outward, sometimes at great speed and to a considerable distance (Chester et al. 2002; Kittleman, p. 52, 1979). A collapsing eruption column or explosive horizontal blast can generate a pyroclastic flow (Chester, 1993; Kittleman, 1979). Most move at high speeds close to the ground up and down slopes or along drainage systems (Chester, 1993). The term nuee ardente, or glowing avalanche, is commonly applied to hot pyroclastic flows. There are a variety of flow types; however, we will remain fairly general in our description here.
Pyroclastic flows have been known to reach distances exceeding 100km (Chester et. al, 2000). The 1991 eruption of Mt. Pinatubo produced a pyroclastic flow that deposited 30 to 200m of material over 15km away (Tayag and Punongbayan, 1994). A flow from the May 18th eruption of Mt. St. Helens had speeds over 57 km per hour and was able to move up slopes about 500 m high (Blong, 1984). The volcanic eruption at Bezymianny, Kamchatka, in 1956 sent an ash-flow more than 20km from the source; and the most famous example is that which occurred during the eruption of Mt. Pelee, Martinique, which sent a nuee ardente with a temperature close to 700 degrees Celsius into the city of St. Pierre 6km away claiming about 30,000 lives (Kittleman, 1979).
Tephra fallout is probably the most common and widespread of all volcanic hazards. It is a product of the rising eruption column and plume that drifts downwind (Blong, 1984). Depending on the relative size and density of particles tephra can affect areas well over 10,000km away (Chester et al., 2000). As the plume rises and disperses away from the vent, the larger and heavier particles dropout and cause a decline in the concentration of the cloud, which also reduces the thickness of the resulting deposits. Tephra fallout, which is usually generated by an explosive eruption, has been known to cause damage up to 500km away from the source; however, the thickness of the tephra layer is what determines the degree of damage (Thorarinsson, 1979). Areas closest to the volcano are under direct threat of being crushed by the weight of tephra accumulation, a situation that can be exacerbated by rain and the additional weight of water. Thorarinsson (1979) uses the eruption on Heimaey in 1973, in which the specific weight of freshly fallen tephra was 600 kg per square meter on a flat surface if the layer is one meter thick.
The 1992 eruption of Cerro Negro, Nicaragua, deposited 2.5 centimeters of tephra on the city of Leon over 20km away (Connor et al., 2001). In Usu, 1977, a tephra accumulation exceeded 200mm at a distance of 8.5km (Blong, 1984). And most recently, the 1991 eruption of Mt. Pinatubo ejected an eruption column that reached 40km into the atmosphere and deposited material 200km to the north (Tayag and Punongbayan, 1994).
Lahars or volcanic mudflows are another hazard that is responsible for a large number of deaths and destruction. Sigurdsson and Carey (1986) estimate that 10 percent of all volcano related deaths are the result of lahars. The term lahar refers to a hydrologic event, or mass movement, that incorporates water and volcanic material such as ash debris. The water may come from a number of sources, such as crater lakes, melting glaciers and snow, or precipitation. A pyroclastic flow, which follows a stream valley, can change into a lahar if it incorporates enough stream water.
Lahars are quite a dynamic volcanic phenomenon with a variety of flow types depending on the water to particle ratio. Chester (1993) explains that when the water ratio exceeds 91 percent flow dynamics are Newtonian and can move on gentle slopes with a small amount of shear stress; however, when particle ratios exceed nine percent by volume the Newtonian properties are altered and the flow progress becomes more plastic in behavior – like wet concrete – with a high density and high yield strength. Therefore, the initiation of the flow will depend on factors such as cohesion, slope angle, mass, and moisture content. "Because fluid densities and viscosities vary not only from lahar to lahar but also downstream in one lahar, velocities and shear stresses will cover quite a range (Blong, p. 4 6, 1984)."
Because lahars respond as a fluid most follow the path of least resistance and flow down the stream valleys that drain the slopes of the volcano. Therefore, areas at risk of lahars can easily be identified. However, lahars can travel great distances at high velocities and inundate large areas. Chester et al. (2000) estimates that lahars can pose a hazard as far as 100km from the initiating event Furthermore, an actual eruption need not occur to produce a devastating lahar. Secondary lahars, which are those initiated by events unrelated to an eruption can continue to pose a threat long after volcanic activity has ceased.
The moderate Plinian eruption of Nevado del Ruiz, Columbia in 1985 generated five major lahars after hot tephra was deposited on several valley glaciers causing rapid melting and mixing with loose debris (Sigurdsson and Carey, 1986). One lahar averaged 38km per hour before reaching the town of Armero and killing 25,000 people. On Mt. Pinatubo lahar activity began with the first major eruptions in June flowing down the mountain’s major river valleys and continued through August before ceasing in October (Tayag and Punongbayan, 1994). When the first typhoons hit in June and July of 1992, small and moderate secondary lahars began to occur.
We shall take a moment to briefly discuss some of the other volcanic hazards that we have not yet received mention: gasses, seismic activity, tsunami, and climatic effects. Volcanic gasses do pose a threat to lives as was seen at Lake Monoun in 1984 and Lake Nyos in 1986. Clouds of carbon dioxide that had accumulated at the bottom of the cater lake until a some surface disturbance allowed the gas to bubble out of the lake and down slope into nearby villages. The two events together are responsible for the death of over 1700 people (McQuire and Kilburn, 1997). However, this type of hazard rarely poses a threat over 30km away, so it can be considered a minute hazard. Seismic activity, while important in volcano monitoring, usually has very shallow focus depth and is very rarely felt beyond 30km as well. However, a magnitude 7.2 earthquake was recorded at Kilauea and was related to an eruption (Blong, 1984). Because tsunamis and climatic effects have such a wide range – tens of thousands kilometers and possible global for climate – it creates difficulty in determining vulnerability based on proximity to the volcanic event.
Determining the Degree of Hazard
Volcanic hazards are unlike others such as floods and hurricanes because they are generated from a single point source that can easily be identified. Therefore, the various phenomena associated radiate outward from that point and the range of the various hazards produced and the populations at risk can easily be identified. The levels of exposure for a given community can be easily determined. Figure 2 (not included) from Chester et al (2000) shows the proximity of several urban areas to a given volcano and the potential range of hazards. Part A identifies 24 large urban areas and the relative distance from the nearest volcano. Part B illustrates the distance of the various hazards associated with volcanic eruptions.
If we quantify the spatial extent of the given data for three volcanic events discussed we can estimate fairly accurately that the highest degree of exposure is within 100km range. Figure 2 also shows that 18 cities with populations over 200,000 are exposed to these hazards. By making a comparison between table 1, from Blong (1984), and figure 2 we can begin to see how the risk frequency increases with proximity to the event and how many urban areas are exposed.
Lava flows present a common hazard but have relatively slow velocities and follow paths determine by topography. Therefore, incineration and burial of buildings, property, and agricultural land are the primary hazards. Rarely do lava flows claim lives. Also, because of the short range of lava flows – rare after 30km – very few cities are exposed to this hazard.
On the other hand, the various hazards associated with pyroclastic ejecta produce a greater spatial distribution of communities exposed. Closer to the volcano, bombs pose a common and damaging hazard. Bombs frequently smash and ignite structures and forests within 10km from the source. Pyroclastic flows and nuee ardentes, while not as frequent, are responsible for a high number of deaths. The high velocities and intense temperatures create an extreme hazard up to 30km and occasionally a 100km away. Unfortunately, because pyroclastic flows are not always determine by topography their range and extent are difficult to predict. Tephra fallout is a very common and far-reaching hazard. However, because the density and particle size is greatest near the volcano the risk to exposed communities varies with distance. Tephra fallout accumulations frequently crush structures within 100km and commonly affect agricultural areas up to 500km. Of course, smaller and lighter particles can affect areas over 10,000km away. Using historic and paleo-eruption data along with climatic data (prevailing wind, precipitation, etc.), hazard probability maps for tephra fallout have been made with a good degree of accuracy (Connor et al., 2001).
Lahars, which are another extremely destructive event rarely create a hazard beyond 30km, but have been known to reach a distance of 100km. While infrequent, lahars are also responsible for incredible death tolls. Their high velocities and various triggers create a much greater degree of risk than, say, lava flows. However, like lava flows, lahars follow paths determine by topography and estimates of their extent can be made with a fair degree of accuracy.
Using the above information and table 1 and figure 2 it becomes obvious that exposure increases with proximity to the event and the potential for loss, or vulnerability, increases as well.
The above examples should indicate that there are a number of variables that influence the degree of the volcanic hazard: distance, velocity, temperature, frequency and time. These variables alter the degree of hazard by modifying the degree of exposure. To recall Mitchell (1990 p. 132):
" Hazard = f (Risk X Exposure X Vulnerability X Response).
Risk is defined as the probability of a damaging event or circumstance. Exposure is a measure of the population at risk. Vulnerability is the potential for experiencing loss. Response is the degree to which society acts to reduce, avoid or prevent loss."
If we treat Response as a separate variable that indicates economic and social structure and temporarily remove from the equation then we can effectively say: Volcanic hazards are increasing because populations are growing within an area near an active volcano that has the potential to generate a geophysical event that would probably cause damage to people and the things that they value.
By returning Response to the equation we should be able to say that a volcano has the potential to generate a geophysical event that would probably cause more damage to people or the things that they value who lack ability to protect themselves or recover from the event.
It can be agreed that hazards are created by the combination of the variables, which therefore makes them relative rather than absolute. However, if we can determine the degree of exposure by evaluating a community's position within the spatial range of a given geophysical event then we have empirical data that will modify the degree of risk and vulnerability; thereby, bringing a more absolute understanding to hazards.
Given the data provided we can say with relative certainty that 1.4 million people in Quito, Ecuador, are extremely vulnerable to the effects of pyroclastic ejecta (projectiles, flows, and tephra fallout), lahars and other volcanic hazards due to its proximity to the volcano Guagua Pichincha. Or, that Manila, Philippines, is not threatened by projectiles, or lava flows but vulnerable to pyroclastic flows, lahars, and tephra fallout. These two examples illustrate how proximity increases the degree of vulnerability.
In the study of natural hazards there are two prevailing approaches to understanding vulnerability: the 'behavioral' approach and the 'structural' approach (Smith, 2001). The behavioral approach, which was developed by Gilbert White and the Chicago School, examines natural hazards from the viewpoint that humans can make adjustments in an attempt to reduce their level of vulnerability; whereas, the structural approach, developed by anthropologists, views vulnerability as a result of constraints placed on individual communities by social, economic, or political forces (Chester, 1993; Smith, 2001).
Clearly this paper has taken the behavioral approach by arguing that exposure is exacerbated by proximity, thereby, increasing vulnerability. Therefore, the most effective adjustment a community can make in order to modify their level of vulnerability to volcanic hazards is to prohibit growth and development within the volcanic event zone or relocate out of the volcanic event zone. However, it is unfair to limit the degree of vulnerability strictly to the magnitude and frequency of an event. We must also ask the question, "why do people live within close proximity of a volcanic event?" Some groups are simply unable or unwilling to make the prescribed adjustment.
There are some comparative advantages to dealing with volcanic phenomena, which may explain why people are unwilling to leave. Blong (1984) gives several examples of comparative advantages. First, there is a resource; the fertility of volcanic soils is remarkable. The 1906 eruption of Vesuvius destroyed many of the nearby vineyards, but several months later the countryside was plush once again. In Papua New Guinea a group of Highlanders hold rituals to bring about a recurrence of tephra falls that will fertilize their lands. Volcanoes also provide tourist attractions. Hawaii Volcanoes National Park draws over 2.5 million visitors a year, while Mt. Lassen brings in 400,000. A tourist industry requires employees to reside nearby. There are aesthetic and recreational advantages as well as others. Some communities may choose to stay because they perceive the advantages as outweighing the risks.
Social and economic disadvantages also play a role in people's location (Blaike et al. 1994). Groups may continue to grow within the event zone due to poor perceptions or knowledge of the potential risks. Others may be unable to relocate do to unstable financial situations or heavy investments in a particular place. Impoverished and marginalized groups in LDC's perpetuate this situation (Smith, 2001).
Chester (1993) notes that over 86 percent of the world’s volcanoes are located in the LDC's. As noted earlier in the paper, by 2025 80 percent of the world’s population will also be found in the LDC's. Therefore, there exists a need to develop an effective measure of vulnerability and potential range of adjustments in relation to volcanic hazards.
The two predominant approaches to vulnerability create an imbalance in terms of the possible adjustments. What is needed is a framework that joins the two approaches together, an approach that can focus the behavioral view and the structural view into one. A new paradigm is needed that examines how effective adjustments are encouraged or inhibited by the social, economic, and political situations of a community.
The international Decade for Natural Disaster Relief made an attempt to do just that. 16 volcanoes were selected from countries around the world that represented a wide range of social and economic situations. The idea was to provide a body of knowledge that could be applied to volcanic hazards in both economically more-developed and economically less-developed countries (Chester, 1993; Chester et al., 2001). The IDNDR study resulted in a stronger emphasis being placed on the intricacies of individual societies and their responses and adjustments to a damaging volcanic phenomenon (Chester et al., 2001).
While future studies of volcanic hazards are moving towards this marriage of the two approaches, I still maintain that the behavioral approach and its emphasis on field monitoring and scientific explanation leading to the development of prescribed adjustments is currently the most effective. Furthermore, it is my opinion that modifying the loss potential by avoiding the event-zone of specific volcanic phenomena is the most effective adjustment. Unfortunately, large and growing communities already live within this zone. Many of these communities are currently unable to make the necessary adjustments and as population and poverty levels increase many more communities will be further inhibited as well.
This paper has identified three of the most significant hazards associated with volcanoes: lava flows, pyroclastic ejecta, and lahars. By understanding the range of the event-zone we should see that the highest degree of exposure occurs within 100km of the volcano. I have suggested appropriate adjustments as prescribed by the behavioral paradigm as the most effective. Although I take the dominant approach I do believe that the structural approach has strong merits in the study of vulnerability and volcanic hazards; however, it is my belief that a synergism between the two approaches is necessary in order to better understand vulnerability issues. Hopefully, as the knowledge of volcanic hazards improves, through scientific monitoring and explanation the range of adjustments will change and the potential to modify to the level of vulnerability will increase proportionately. Communities are living near volcanoes and will continue to do so. As their populations grow so will their level of exposure; the question we should ask is, will they continue to become more vulnerable?
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