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PART VII Volcanic Hazards  Volcanic eruptions are one of the major natural hazards on Earth. The hazard from a volcanic eruption depends on the type of volcano, the time since the last eruption of that volcano, the geographical location, the local climate and the time of year. The longer a volcano is dormant between eruptions, the larger the eruption tends to be. Thus while Kilauea (Hawaii) and Etna (Sicily) are active volcanoes that erupt almost continuously, the direct risk to the human population from their eruptions is negligible. The risk to property and the environment is not diminished however. In this section, the various types of hazard posed by volcanoes are discussed mostly from the point of view of their effect on people and their environment. The processes that give rise to these hazards are discussed elsewhere in Part III (Effusive Volcanism) and Part IV (Explosive Volcanism).The location and timing of a volcanic event affects the risk, as wind direction and strength in the upper atmosphere varies with latitude and season, so that ash from an explosive eruption might be transported round the globe at one time, but would remain confined to a small region at another time. The area affected by volcanic activity may range up to several tens of square kilometers, but it varies depending on the eruption type. For example, lavas may flow at between 1 m to 30 km per hour for distances of several tens of km. Explosive eruptions may eject large quantities (0.1 to 100's of km3) of volcanic ash into the atmosphere and much of this falls out locally. The collapse of an explosive eruption column may form a glowing cloud of volcanic debris (nuees ardentes) or pyroclastic flows. Most eruptions are accompanied by sulfur dioxide gas release, and this may escape into the upper atmosphere to form a volcanic aerosol that can have global consequences, or it may combine with water vapour at lower elevations and form aerosols that cause local acid rain. Eruptions occurring on volcanic islands or close to the sea may produce tsunami (tidal waves) with devastating effects reaching tens or even hundreds of km. Passenger airliners that encounter volcanic ash clouds are subject to complete engine failure, which can have catastrophic effects. The first chapter of this section, Volcanic Ash Hazards to Aviation, documents several case histories of eruption-induced aviation incidents and explains why this hazard is becoming increasingly important. The release of gases to the atmosphere is one of the hazards associated with volcanic eruptions. During large eruptions several million tons of gases are released and in some events this has been found to be of global significance. Etna volcano (Sicily) emits about 13 Mt of CO2 per year, equivalent to a 1000 MW coal-fired power station in terms of greenhouse gas production. Etna also produces some 1.4 Mt of SO2 per year. The 1991 eruption of Pinatubo (Philippines) ejected 15 Mt of SO2 and this caused surface temperatures to fall globally by about 0.1° C. The aerosol formed from this gas contributes to global cooling by absorbing incoming solar radiation. The next chapter, Volcanic Aerosol and Global Atmospheric Effects, documents unusual climatic features caused by volcanic eruptions as far back as the 17th century BC. The following chapter, Hazards from Pyroclastic Flows and Surges, illustrates the power of pyroclastic flows and surges to devastate whole communities. The speed and great extent of pyroclastic flows makes them by far the greatest and most lethal volcanic hazard. Lava flows represent a much more sedate volcanic eruption process, but the effects, as the chapter Lava Flow Hazards demonstrates, are still significant. Volcanic eruptions can trigger secondary events if there are significant quantities of groundwater, snow, or ice involved. Risk may then extend beyond the immediate region of the eruption. A relatively minor eruption of Nevado del Ruiz (Colombia; 1985) resulted in the loss of some 23,000 lives as the glacier on the top of the volcano melted. Water flowed down the river valleys removing the loose material from the banks and the muddy torrent rushed downslope at 30 km per hour to cover the village of Armero to a depth of 3 m. A similar muddy torrent (lahar) resulted in the death of about 900 people after the eruption of Mount Pinatubo in 1992. Lahar and Jökulhlaup Hazards illustrates how these devastating events can continue to occur even without further volcanic activity, sometimes for decades after the eruption has ceased. The next chapter, Hazards of Volcanic Gases, illustrates the effects that various volcanic gases have on people, but perhaps more importantly when they fall as acid aerosols on crops, vegetation and communities downwind. Persistent gas release may cause asphyxiation by inhalation locally and asthma more regionally. Acid rain may produce skin disorders. All these phenomena are devastating to wildlife and farmland for the duration of the eruption and sometimes far longer. Most of the world's volcanoes are below the sea and many are located in coastal areas. Explosion or collapse of a volcano can therefore in many cases cause a large body or water to move suddenly, creating a tsunami. The chapter Volcanic Tsunamis presents several mechanisms that are thought to produce tsunamis and shows that in the last 250 years, some 25% of volcano related fatalities are attributed to tsunamis. Volcanic earthquakes have different characteristics from the deep and tectonic seismic events that occur along great faults. They are considerably less devastating and may continue at a detectable level for weeks or months prior to eruption. As the chapter Volcanic Seismicity demonstrates, they are the most important window on the processes going on inside a volcano before eruption and their study has considerable scientific value and application to risk mitigation. As human populations continue to grow and expand, many people live close to active or potentially active volcanoes. The chapter Impacts of Eruptions on Human Health presents a graphic account of the nature of injuries and the cause of death from various types of eruptions. On a more global scale, the next chapter discusses the origin and fate of carbon and sulfur from volcanic sources and the implications for climate. Following this, the chapter Ecology of Volcanoes considers the various effects of volcanic activity on plant and animal communities. It is striking how resilient life is in the face of volcanic activity. Eruptions do not completely eradicate life and many species survive; even newly formed volcanic islands soon become home to a wide range of opportunists. Looking back through the geological record, there is considerable evidence for huge eruptions. By scaling up the observed effects of recent eruptions, the global consequences of large eruptions are awesome. The concluding chapter of this section, Volcanism and Biotic Extinctions, provides compelling evidence for a correlation between mass extinctions and the largest basalt lava flows on Earth. Whether these flood basalts, mass extinctions, and asteriod impacts are related in some way is a matter of ongoing debate. Thus volcanic hazards have always impinged on life and the environment and an understanding of their past and present effects is vital for effective monitoring, prediction and mitigation. Hazel Rymer | |