Erupting volcanoes only become a risk when they affect something of value, i.e. by causing injury or death to people or damage to property or resources. Risk is an indication of the possible impact of a particular hazard, a lava flow or pyroclastic flow for example. It is usually assessed on the basis of the number of human lives that may be lost because of the hazardous event. In 1000 years of record keeping, volcanic eruptions have killed less than 300,000 people. Wars and other hazardous events have taken many millions of human lives. Volcanic eruptions are far more likely to affect the economic base of a region than to kill people. The economy of an area can be affected by the destruction of either resources or infrastructure. Even eruptions in remote areas can adversely affect industries such as logging and fisheries.

Assessments of volcanic hazards made for individual volcanoes provide a basis for understanding the type of activity (hazards) that might be expected at that volcano in the future, given its previous history. From these studies and other factors, such as population density, risk can be estimated. These assessments, carried out for the American portion of the volcanoes of the Cascade Mountains by the United States Geological Survey, provide a basis for long-range planning in areas likely to be affected by a volcanic event. They are, however, limited by the level of knowledge about an individual volcano. The risk may be significantly underestimated because of incomplete knowledge about the eruptive history of an area. The record for most Canadian volcanoes is too poorly known to make accurate hazard assessments. However, enough is known to delineate areas most at risk from volcanic hazards in Canada. People can make themselves aware of these risks, and how to prepare for eruptions, using information provided by the Provincial Emergency Program.

Growing populations increase the risk posed by volcanoes both in Canada and abroad. For example, neither Mount Pinatubo, in the Philippines, nor Nevado del Ruiz, in Columbia, were considered high-risk volcanoes. Loss of life from the 1991 eruption of Mount Pinatubo was minimized because excellent monitoring provided enough time to evacuate people from critical areas. Nevado del Ruiz was monitored, but warnings failed to reach the people. When it erupted on 13 November 1985, 25,000 people were killed by mudflows - the greatest volcanic disaster since the 1902 eruption of Mount Pelé in the Caribbean killed 28,000 people. A poignant point brought out in later studies of the Nevado del Ruiz eruption was the observation that in 1845, a similar mudflow wiped out 1400 people, virtually the town's entire population. In 1985, 30,000 people lived in the same area. Twenty times more people died in the 1985 eruption than in the earlier event. In a similar vein, in the 1814 eruption of the Philippine volcano of Mayon, pyroclastic flows killed 1200 people, most of the inhabitants living in the area - 800,000 people now live in the same region.

Lava flows pose little threat or significant risk unless the eruption occurs in winter or beneath or near glacial ice. If lava flows over significant amounts of snow, it may lead to some melting, producing debris flows that extend past the end of the lava flow. In addition, water entering a basaltic vent can produce explosions much larger than those normally associated with a basaltic eruption. Consequently, the presence of water, snow, or ice will increase the risk of significant impact from an eruption. Subglacial volcanism has also produced large, destructive jökulhlaups.

Pyroclastic flows, surges, lava flows, and ballistic projectiles constitute a significant risk only to communities and resources close to the volcano. As a rule of thumb, studies suggest that areas within 50 km of an erupting volcano may be severely affected by these and other volcanic hazards. Beyond this limit, the main hazards (and thus risk) are from tephra and debris flows, the effects of which can be localized because of prevailing wind patterns and topography.

Figure 47. Radar image tracking tephra Composite satellite radar image of tephra from the 17 September 1992 eruption of Mount Spurr in Alaska. The satellite radar tracked the ash cloud all the way across Canada to Greenland. (courtesy of D. Schneider, Alaska Volcano Observatory, United States Geological Survey) larger image [JPEG, 64.8 kb, 443 X 465, notice]

Tephra poses the greatest risk to western Canada. In the past 12,000 years, volcanoes in the Cascade magmatic arc have erupted over 200 times. Several of these eruptions deposited significant quantities of tephra in southern British Columbia, most of it fine, ash-sized particles in a layer a few millimetres in depth (Figure 8). Alaskan volcanoes in the Aleutians have erupted even more often, dusting and blanketing parts of northern Canada with ash (e.g. White River Ash, Figure 8). During the 1992 eruption of Mount Spurr in Alaska (Figure 8), ash blown southeastward over Canada by upper-atmospheric winds crossed over the Yukon Territory into British Columbia and Alberta, then blew over Ontario and Quebec before fading over the North Atlantic.

Despite the proximity of active and erupting volcanoes in the United States, significant accumulations of tephra (over 10 cm) are very rare. We can, however, expect small amounts of ash much more frequently. The 18 May 1980 eruption of Mount St. Helens deposited a 1 mm thick layer of ash from southeastern British Columbia to Manitoba. During its 1992 eruptions, Mount Spurr deposited enough ash in the Yukon Territory to close the Alaska Highway for a few hours due to reduced visibility.

This hazard is a risk only close to a volcano. Large rock failures (or landslides) are a hazard at many of Canada's volcanic centres, even those long extinct. Many of Canada's volcanoes are in extremely rugged regions of high relief and are underlain by unstable rocks. These conditions have already led to many landslides. One of these, at Mount Meager in southern British Columbia, killed four people. Such rock failures can generate secondary hazards when they temporarily dam local rivers, which may later lead to dam failure and catastrophic flooding.

This hazard poses a risk to some communities situated along the banks of rivers or streams draining directly from a volcano. During one Mount Meager eruption, 2350 radiocarbon years ago, ash was carried as far as Alberta and pyroclastic flows extended 7 km down the Lillooet River. Upstream of the eruption, volcanic debris dammed the Lillooet River, forming a lake. Catastrophic failure of this dam created a flood. The flood waters carried 10 m blocks of volcanic debris several kilometres downstream and inundated large parts of the valley downstream from the eruption. Eruptions almost anywhere along the valley corridor between Vancouver and Pemberton may generate lahars that will cut rail and road links, as do the nonvolcanically induced landslides and debris flows that are common in that area.