Proactive disclosure Print version ![Print version Print version](/web/20061103041235im_/http://www.gsc.nrcan.gc.ca/esst_images/_printversion2.gif) ![ÿ](/web/20061103041235im_/http://www.gsc.nrcan.gc.ca/esst_images/_spacer.gif) | ![ÿ](/web/20061103041235im_/http://www.gsc.nrcan.gc.ca/esst_images/_spacer.gif) | ![Strong and safe communities Strong and safe communities](/web/20061103041235im_/http://www.gsc.nrcan.gc.ca/esst_images/2002iscom_e.jpeg) Natural Resources Canada > Earth Sciences Sector > Priorities > Strong and safe communities > Volcanoes of Canada
Volcanoes of Canada Reducing the risks
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.
Hazard and risk assessments |
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.
Population growth, hazard growth |
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.
Ballistic projectiles and other flowage phenomena |
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 tephraComposite 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) Figure 47. Radar image tracking tephraComposite 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)](/web/20061103041235im_/http://www.gsc.nrcan.gc.ca/volcanoes/images/fig47_e_.jpg) Figure 47. Radar image tracking tephraComposite 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)
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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.
Landslides, sector collapse, and debris avalanches |
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.
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