Volcanoes do not seem to be part of the Canadian reality today.
However, there are many volcanoes in western Canada. The entire
western Cordillera (British Columbia and Yukon) remains geologically
active. The possibility of an eruption, even a large explosive
one, cannot be ruled out. Quiet as they are, the West Coast volcanoes
are part of the "Pacific ring of fire".
In fact, in southwestern British Columbia and adjacent parts of
the United States, an offshore tectonic plate is forced under the
edge of the continental plate, causing molten rock to rise to the
surface and form volcanoes. Volcanoes in the northern end of Cascade
chain include among others Mount Garibaldi and Mount Meager, both
north of Vancouver, and Mount St. Helens, Mount Baker and Mount
Rainier, all in Washington State.
West of Vancouver Island, there is also an undersea ridge called
the Juan de Fuca Ridge, where the earth's crust is being stretched
and pulled apart, and where new magma wells up forming new oceanic
crust.
Distribution of the Main Volcanic Complexes in Canada and the
World
[D] Click for larger version, 117 KB Figure 1. Map of the Distribution of Main Volcanic Complexes in the World
On this map, the red areas represent the location of volcanoes
in the world, and the yellow areas, the edges of crustal plates.
The circle of volcanoes that surrounds the Pacific Ocean is called
the Pacific Ring of Fire of which British Columbia is part. In
the Western Cordillera of British Columbia, there are numerous
volcanoes that have been active within the last two million years.
The most recent volcanic eruption in Canada was Lava Forks volcano,
about 150 years ago. Tseax volcano erupted about 400 years ago
and led to many fatalities among the Nisga'a people.
How do volcanoes work?
Volcanoes erupt, in part, because of pressure from dissolved gas
with the liquid rock called magma. Magma rises through cracks
and channels in the earth's crust and pools in magma chambers
below the volcano. When ready to erupt, it moves upward into and
out of the volcano through a vent. If the magma is fairly fluid,
gases can escape readily and the eruption will be in the form
of a lava flow. However, if the magma is more viscous (meaning
that it flows more slowly), the lava can impede the gradual escape
of the gases and an explosive eruption might occur, depositing
volcanic ash over a wide area.
Recent Examples of Volcanic Activity in Canada
[D] Click for larger version, 11 KB Photograph of Mount St. Helens, Washington, United States
Mount St. Helens erupted violently in 1980. Mount St. Helens is
300 kilometres south of Canada.
[D] Click for larger version, 13 KB Photograph of Mount Baker
Closer to Canada, Mount Baker, which is cooling from a minor eruption
in the early 1800's, still lets off steam near its top.
[D] Click for larger version, 24 KB Photograph of Mount Meager, British Columbia
Some 2400 years ago (not long ago in geological time), Mount Meager,
180 kilometres north of Vancouver, erupted in a huge explosion that
dumped volcanic ash as far away as southern Alberta.
What are the dangers?
Although southwestern British Columbia and the Yukon have not experienced
a major volcanic eruption for more than 1000 years, the potential
for future activity remains. A volcano may sleep for many centuries
while gas pressure slowly builds in its subterranean magma chambers.
However, before an eruption the increasing pressure creates many
small earthquakes which are detected at seismograph stations and
warn of the eruption to come.
Lava flows and burning hot projectiles are hazards only in the
immediate vicinity of a volcano. Mudflows and clouds of volcanic
ash are also dangerous. When a large volcano erupts, the snow
and ice on its slopes can melt rapidly and sweep down the mountain
in a slurry of meltwater and debris. These mudflows can follow
valleys and river beds for hundreds of kilometres, carrying away
anything in their path. When Mount St. Helens erupted, its mudflows
followed the Toutle and Cowlitz rivers as far south as the Columbia
River, 120 kilometres downstream.
In an explosive eruption, volcanic ash may be hurled high enough
into the atmosphere to be carried with the prevailing winds. The
city of Yakima, Washington, 130 kilometres northeast of Mount St.
Helens, received 30 centimetres of ash over it after the May 18,
1980 eruption of Mount St. Helens.
On Canadian territory, large zones are covered by tephra. Tephra
means anything thrown out by an erupting volcano such as volcanic
ash. The map below (Figure 2) shows the distribution of tephras
deposited during the Holocene period i.e. since about 12 000 years
ago. Those deposits occured during the seven following events:
- 1 - St. Helens, 508 years ago
- 2 - White River, 1200 years ago
- 3 - White River, 1500 to 1900 years ago
- 4 - Bridge River, 2400 years ago
- 5 - St. Helens, 3400 years ago
- 6 - Mazama, 6800 years ago
- 7 - Glacier Peak, 11 200 years ago
[D] Click for larger version, 10 KB Figure 2. Map of the Tephra Deposition, Holocene Period
Volcanoes and Lava Flows Seen from Canada
[D] Click for larger version, 8 KB Photograph of Mount Baker, United States
Mount Baker, in the United States, can be seen from many places in the Lower Mainland and in southern Vancouver Island. This
mountain is 25 kilometres south of the border.
[D] Click for larger version, 15 KB Photograph of Mount Garibaldi, British Columbia
Just north of Vancouver, Mount Garibaldi can be viewed from the
road leading to the resort municipality of Whistler. From the viewpoint
on Highway 99, five kilometres south of Squamish, the volcano's
prominent twin peaks are easily recognized.
Lava flows, recognizable
by their well-defined columnar jointing, can be spotted from several places
along the roads to Whistler. In Brandywine Falls Provincial Park, just
south of Whistler, Brandywine Creek has carved a canyon through lava flows
35 000 years old forming a spectacular waterfall at the head of the canyon.
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