Figure 31. Hazards associated with volcanic eruptions Schematic figure illustrating hazards associated with different types of volcanic eruptions. Most commonly, not all hazards shown will accompany a single eruption from a volcano, although a single eruption can produce more than one type of hazard at the same time. The rock classification at the bottom emphasizes that magmas with greater amounts of SiO2 (weight per cent) are more likely to produce large, explosive eruptions (modified from Myers et al., 1998). larger image [GIF, 31.0 kb, 474 X 586, notice]

Erupting volcanoes can generate many primary hazards including lava flows, pyroclastic flows, pyroclastic surges, volcanic bombs, ash clouds, landslides, debris flows, and clouds of poisonous gas (Figure 31). The style of eruption and type of hazard generally depend largely on the size of the eruption, the composition of the erupting magma, and the environment in which the eruption occurs.

Figure 32. Volcanoes in proximity with air routes Diagram showing major air routes over western Canada. The volcanoes marked with stars are described in more detail in the Catalogue of Canadian Volcanoes larger image [GIF, 23.0 kb, 360 X 471, notice]

For example, a small, subaerial, basaltic eruption in a remote part of British Columbia (e.g. close to Mount Edziza in northwestern British Columbia, Figure 32) would constitute a minimal hazard compared with a large, explosive, dacitic eruption from Mount Meager, in southwestern British Columbia. However, even in remote areas such as Mount Edziza, an eruption can have a considerable impact if it occurs under a glacier-clad summit. Debris flows (lahars) or floods could form from the rapidly melting snow and ice. These floods, carrying large amounts of water and debris, might destroy salmon runs in the surrounding rivers and threaten local villages along these rivers.

The size of a volcanic eruption is quantified using a scale called the Volcanic Explosivity Index (VEI), which takes into account the volume of material erupted, the height of the eruption cloud, the duration of the main eruptive phase, and other parameters to assign a number from 0 to 8 on a linear scale. For example, the 18 May 1980 eruption of Mount St. Helens, which destroyed 632 km² of land, expelled 1.4 km³ of magma, and produced an eruption column that rose to 24 km, was assigned a VEI of 5. On the other hand, the last large eruption from the Yellowstone caldera, which occurred 600,000 years ago and expelled over 1000 km³ of magma, would be assigned a VEI of 7. However, the vast majority of volcanic eruptions have VEIs from 0 to 2.

Several secondary hazards are also associated with volcanoes. Secondary hazards are those that are not associated with an eruption, but rather result from the environment created by the volcano. They include mudflows, debris flows, landslides, ground water contamination, and surface water contamination (Figure 31). All these hazards may persist at a volcano for decades after an eruption and even long after the volcano is considered extinct. Stratovolcanoes are particularly susceptible to debris flows because they are commonly very steep and are found in regions of rugged terrain. Heat from deep-seated cooling magma causes hydrothermal alteration of the overlying rocks. In this type of alteration, hot water attacks the volcanic rocks, turning them into clay. The presence of clay weakens the rocks and increases the risk of landslides.

Figure 33. Basalt lava flow A small basalt lava flow on Mt. Etna, showing a thin, broken-up crust. The temperature of the glowing lava was close to 1000°C. (Photo by S. Sparks (U. of Bristol)) larger image [JPEG, 21.1 kb, 400 X 250, notice]

Figure 34. Pahoehoe lava crust A ropy (pahoehoe) lava crust preserved on a solidified basalt flow on Mt. Etna. When a continuous, unbroken, flexible crust forms on a lava, the continued motion of the lava beneath the crust rumples it into folds, just like crumpling up a carpet. (Photo by M. Stasiuk (Geological Survey of Canada)) larger image [JPEG, 31.7 kb, 400 X 259, notice]

Lava flows commonly accompany volcanic eruptions, especially basaltic eruptions. They are among the least hazardous processes associated with a volcanic eruption, although they can destroy any immobile object in their path, including houses, roads, and volcano observatories! Flows travel slowly, a few kilometres an hour to a fraction of a kilometre an hour, and people and animals can normally move easily out of the way. Lava flows, however, can cause many secondary hazards. They can start forest fires and will usually destroy any structures in their path. The expulsion of poisonous gases can also accompany the eruption of a lava flow, as happened in 1783 at the Laki volcano in Iceland. A lava flow can dam rivers, change their courses, kill resident fish, and even act as a barrier to migrating fish. This happened along the Nass River in northern British Columbia during the eruption of the Tseax cone. Of special concern in western Canada is the possibility of lava erupting onto or beneath glacial ice. Such an event might cause large-scale melting of the ice and generate a catastrophic flood; such floods are referred to by their Icelandic name, 'jökulhlaups'.

Figure 35. Lava tube A "skylight" in the roof of a lava tube on Mt. Etna. Once a thick enough crust forms over a lava, it cannot be deformed by the flowing lava and so the lava flows independently in a tunnel made of its own crust, called a lava tube. In some places the crust is weak enough that parts of it collapse in, forming windows down to the flowing lava that are good places to make measurements, as the volcanologists are doing here. (Photo by S. Sparks (U. Bristol)) larger image [JPEG, 16.4 kb, 400 X 274, notice]

Figure 36. Mt. St. Helens A pyroclastic flow (lighter colour part of ash column near its base) forming by collapse of part of an explosive eruption column at Mt. St. Helens in 1980. Flows formed in this way are among the most dangerous as they have a large amount of momentum, and hence are extremely fast-moving. (Photograph by D. Swanson (U.S. Geological Survey)) larger image [JPEG, 27.2 kb, 264 X 390, notice]

Figure 41. Pyroclastic deposit The deposit from a pyroclastic flow on Montserrat in 1996. Once a pyroclastic flow runs out of momentum, the material that makes it up settles to the ground to form a sinuous deposit of ash and blocks. (Photo by M. Stasiuk (Geological Survey of Canada)) larger image [JPEG, 33.4 kb, 400 X 261, notice]

Pyroclastic flows are dense avalanches of hot gas, hot ash, and blocks (tephra) that cascade down the slopes of the volcano during an eruption (Figure 31). They are most commonly associated with explosive eruptions, and form when the towering column of ash rising above the volcano collapses. They also form when a less energetic eruption 'boils' over the edge of a crater or caldera or when a lava flow or dome on a steep slope disintegrates. Pyroclastic flows originating from column collapse can have considerable momentum and travel great distances. Speeds between 50 and 150 km/h have been measured and distances of 30 km are not unusual. Although extremely rare, the largest eruptions have produced pyroclastic flows that extend 100 km from the volcano. All pyroclastic flows are extremely destructive, destroying buildings, trees, or any objects that are in their path by impact, burial or fire. People caught in a pyroclastic flow have little chance of survival. Pyroclastic-flow deposits are not abundant at dormant volcanoes in Canada, but have been found at Mount Meager, Hoodoo Mountain, and Mount Edziza (Figure 32; see the Catalogue of Canadian Volcanoes).

Figures 37-40. Pyroclastic flow A series showing the movement of a small pyroclastic flow shed from the margins of the growing dome (in cloud) on Montserrat in 1997. The photos were taken only a few seconds apart. The flow looks like ground-hugging grey smoke, but consists of a mixture of gas, ash, and dense hot lava blocks. The shroud of ashy cloud conceals the dense interior, and in fact sometimes separates from the dense part to form a less dense, more mobile current called a pyroclastic surge. (Photos by M. Stasiuk (Geological Survey of Canada)) larger image [JPEG, 189.0 kb, 400 X 1041, notice]

Pyroclastic surges are dense clouds of hot gas and rock debris, that are generated when water and hot magma interact. They are more violent and travel much faster than pyroclastic flows; surges have been clocked at over 360 km/h. They are extremely destructive because of their high density and high speeds. People and structures in their path have little hope for survival. White Horse Bluffs, in British Columbia, was built from successive surge deposits, but in general, these deposits are difficult to preserve and what little evidence may exist is easily eroded away.

Figure 42. Pyroclastic surge A pyroclastic surge flowing over the surface of the sea on Montserrat in 1996. The surge originated when a dense pyroclastic flow shed from the growing lava dome flowed down the volcano flank to the sea. The dense part of the flow stopped at the beach, but the lighter-than-water, denser-than-air, ash cloud separated and flowed across the water for about 100 meters. (Photo by M. Stasiuk (Geological Survey of Canada)) larger image [JPEG, 28.5 kb, 400 X 260, notice]

Figure 43. Destruction from a pyroclastic surge The destroyed Tar River Estate house, a short distance downhill from the growing lava dome on Montserrat, in 1997. The house was wrecked by a pyroclastic surge that hit with similar power to a hurricane wind, but at temperatures of 200 to 600°C. The surge left the concrete walls and larger trees standing, but blasted and burned away small pieces of wood and leaves. For all its destructive power, the surge left a deposit only a few millimeters thick. (Photo by M. Stasiuk (Geological Survey of Canada)) larger image [JPEG, 33.9 kb, 400 X 257, notice]

Figure 44. Mt. St. Helens in 1980 Explosive eruption column of Mt. St. Helens in 1980, shortly after the initial landslide and lateral blast. The column holds ash, pumice and dense lava blocks and may rise up to a few tens of kilometres height. (Photograph by D. Swanson (U.S. Geological Survey)) larger image [JPEG, 22.0 kb, 256 X 390, notice]

Tephra is finely comminuted (broken) volcanic rock and accompanies nearly all explosive volcanic eruptions. In very energetic, explosive eruptions, tephra is carried upward into the upper atmosphere and the finest tephra (ash) can be carried by the jet stream for hundreds and thousands of kilometres. Ballistic projectiles (e.g. volcanic blocks and bombs) are the larger blocks of desegregated and comminuted magma that are most commonly thrown short distances from the vent (Figure 31), but that are know to have been thrown several kilometres from the vent. In less energetic eruptions, most of the tephra falls within a few kilometres of the vent and the accompanying ballistic projectiles, a correspondingly shorter distance.

Figure 45. Mt. Pinatubo Satellite image looking down on the top of the 1991 explosive eruption column of Mt. Pinatubo, Philipines, shown punching straight through clouds of a typhoon. As an ash column rises, it eventually becomes neutrally buoyant and spreads out to form a mushroom cloud that moves the ash laterally away from the volcano. (Photograph by W. Duffield (U.S. Geological Survey)) larger image [JPEG, 18.2 kb, 257 X 390, notice]

Because it is often widely dispersed, volcanic ash may be a significant health hazard and create economic problems over a wide area. Ash can pollute water supplies and disrupt transportation; thick accumulations of heavy ash can cause buildings or other structures to collapse. Inhaled ash can aggravate respiratory conditions such as asthma and bronchitis. Coarser particles can lodge in the nose (causing irritation) and the eyes (causing corneal abrasions). Silicosis has been attributed to long-term exposure to volcanic ash. Ash, however, is rarely a direct cause of death unless the fallout is so severe that suffocation results.

Ash can damage mechanical and electrical equipment. It is abrasive and, at great distances from the volcano, fine enough to work its way into bearings or other moving parts of equipment and machinery, causing damage or mechanical failure. Computer equipment is particularly sensitive to this type of damage. Electrical power can be disrupted because transformers conduct heat poorly and will overheat and explode when covered by only a few millimetres of ash.

Figure 46. The mushroom cloud The leading edge of the mushroom cloud from an explosive eruption of Mt. St. Helens in 1980. The ash is carried laterally away from the volcano for hundreds of kilometres at the altitude of commercial jets, forming a major hazard to air traffic. (Photo by D. Swanson (U.S. Geological Survey)) larger image [JPEG, 11.4 kb, 400 X 250, notice]

Ash can affect aircraft flying at high altitudes. The adverse impact of ash on high-performance jet engines was well publicized for the first time in 1982 when a Boeing 747 jet on a flight over Indonesia encountered ash from Galunggung volcano. Although the plane was severely damaged and all four engines failed, it was able to make a successful emergency landing in Jakarta. Since then, the number of plane-volcanic ash incidents has grown and the adverse effects of ash have been studied more carefully. Ash sucked into high-performance jet engines abrades the outer parts of the engine and can melt inside at the high engine operating temperatures. The abrasion severely damages the engine fan blades and the melting ash disrupts the mixing of air and fuel in the combustion chamber, leading to engine shutdown. The ash also abrades the exterior of the aircraft, 'frosting' cockpit windows and landing lights. Ingested ash can badly damage aircraft navigation and hydraulic systems. The threat from airborne ash in Canadian airways constitutes the most important short-term impact of volcanoes on the Canadian public (Figure 32).

Ash also has deleterious effect on fish because it abrades their gills. Studies of steelhead and salmon in streams west of Mount St. Helens showed that fish populations suffered severely after the 1980 eruptions. There were many other lethal, secondary effects, such as the loss of fish-spawning habitat and riparian cover. Thick accumulations of ash and mudflows can kill the riparian cover (the normal trees and bushes that grow along steams and rivers), which keeps stream waters cool and liveable for fish.

Thick accumulations of ash can significantly affect forestry and agriculture. Ash can rip the leaves from trees and bury small plants. Ash-covered trees and crops are difficult to work because of the potential damage to harvesting and cultivation equipment and vehicles caused by the ash. Wind and moving equipment also redistribute the ash, prolonging the problems. However, the long-term impact of ash can actually be beneficial, as ash enriches the soil and, as with tephra from the 1980 Mount St. Helens eruption, can act as a mulch, increasing soil moisture and promoting growth.

This hazard, known by several names, involves the collapse of part of the volcano. If collapse is associated with an eruption, it can result in a larger and potentially more explosive eruption than might otherwise be expected. Such was the case in the 18 May 1980 eruption of Mount St. Helens, where catastrophic depressurization of the volcano caused by a massive landslide resulted in an eruption of significantly different character than what had been predicted. As magma is pushed into a volcano from the magma chamber below via the magma conduit, the volcano inflates, much like blowing up a balloon (Figure 31). During inflation, the volcano's slopes become oversteepened and unstable. At this stage, ground shaking associated with stream venting and magma ascent can trigger a collapse. The collapse of a large part of the edifice is termed a 'sector collapse'.

Landslides and debris avalanches originating on volcanoes also constitute a significant secondary hazard unrelated to volcanic eruptions. As a landslide moves downslope, the large blocks break into smaller and smaller pieces, which, when combined with water from melting snow or ice, can transform the landslide into a debris avalanche, carrying it farther than a similar-sized rock avalanche. The slopes of volcanoes, steepened by explosions or later erosion, are often weakened by hydrothermal alteration, making them susceptible to failure long after the volcano becomes dormant.

'Lahar' is an Indonesian word for debris flow. These slurries of water and rock particles behave like wet concrete and are associated with volcanic eruptions. They consist of particles of many different sizes, ranging from flour-sized particles to blocks as large as houses. Lahars are extremely destructive; they are, however, topographically controlled and usually follow river valleys where they are confined to valley bottoms. They are most common and most voluminous as a result of explosive eruptions on snow-clad volcanoes. Pyroclastic flows can instantaneously melt ice and slow, creating large-volume lahars.

Debris flows can be a significant secondary volcanic hazard. They can occur days, weeks, or years after an eruption. Explosive eruptions and, to a lesser extent, effusive eruptions can denude areas around a volcano and disrupt the drainage pattern, which can lead to long-term flooding problems around the volcano. In addition, vast areas around the volcano may be covered by loose, unconsolidated tephra, which is easily mobilized and can be washed away by heavy rainfall, forming mudflows and debris flows.

Volcanoes produce large quantities of gases, mostly water (H2O), but also significant amounts of carbon dioxide (CO2), hydrogen sulphide (H2S), sulphur dioxide (SO2), and hydrofluoric acid (HF) as well as some chlorine (Cl) and nitrogen (N) compounds. Each of these gases has caused loss of life and damage. Sulphur dioxide vented from the Laki volcano in Iceland in 1783 damaged crops and killed livestock and people. Survivors faced starvation and many more died as a result. Sulphur dioxide causes other problems as well. Exposure to this gas can lead to more numerous incidents of acute asthma and bronchitis. When SO2 reacts with water in the atmosphere, it forms sulphuric acid and acid rain. Acid rain formed from volcanic clouds can damage crops and, in Hawaii, the increased acidity of water collected in cisterns leaches heavy metals into drinking water. The long-term health affects of this problem are not completely known. In Iceland, fluorine killed and disfigured livestock after the 1845 and 1970 Hekla eruptions. When CO2 gas, which is heavier than air, is released by a volcano, it collects in low areas; concentrations can become high enough to be deadly. The only known fatalities at a Canadian volcano are reported to have been from poisonous gases during the eruption of the Tseax cone.

A secondary hazard at volcanoes is contamination of groundwater and surface water. By virtue of the composition and physical attributes of their component rocks, volcanoes are much more susceptible to weathering than many other types of rocks. Weathering leads to increased particulate matter in streams draining the edifices and higher concentration of elements easily leached by percolating groundwater. Thermal springs are often associated with volcanoes. The water in some of these springs is acidic. Hot, acid waters will enhance the breakdown (or leaching) of the rock and will often carry dissolved metals. Because of this, streams and rivers draining volcanic areas may have a significantly different trace-element chemistry than nearby streams draining other regions underlain by more chemically stable rock types.