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Natural Resources Canada > Earth Sciences Sector > Priorities > Strong and safe communities > Volcanoes of Canada
Volcanoes of Canada Types of volcanoes
Volcanoes and volcanic vents can have a variety of shapes. A volcanic
vent is the hole from which the magma emerges from beneath
the Earth's surface. Vents mark the birthplace of the
volcano and are most commonly roughly circular. Some,
however, are long cracks in the ground and are called
'fissures'. Volcanoes are given specific names depending
on their shape (or morphology). Stratovolcanoes and shield
volcanoes are the largest types of volcanoes, but have
distinctly different shapes because of differences in
the chemistry of the erupting magma. Shield volcanoes
and stratovolcanoes erupt many times over thousands to
hundreds of thousands of years whereas cinder cones, lapilli
cones, and maars, which are smaller volcanoes with different
morphologies, usually have a short life, erupting only
once.
Figure 15. A typical stratovolcanoMount Baker is a typical stratovolcano, with steep sides and a pointed summit.
(Photo by C.J. Hickson (Geological Survey of Canada))
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Classically shaped volcanoes, such as Mount Fuji in Japan with its
sharp, pointed top and sloping, concave sides, are what
comes to mind when we think of stratovolcanoes. Stratovolcanoes
form from repeated eruptions of viscous, slow-moving lavas
(most commonly andesite and dacite) and are common in
subduction zones. Because the lavas are viscous, they
do not move quickly and commonly flow only a few kilometres
from the vent. Explosive eruptions are often associated
with these volcanoes. No examples of stratovolcanoes resembling
Mount Fuji are found in Canada. Many of Canada's volcanoes
may have started out as Mount Fuji-like stratovolcanoes,
but erosion by glacial ice and water has destroyed most
of their original cone shape, leaving them as jagged,
irregular peaks. Many of the more active volcanoes in
Alaska and many of the Cascade volcanoes south of the
Canadian-American border (e.g. Mount Baker, Figure 15)
are 'classic' stratovolcanoes.
Figure 16. Shield volcanoesThe Illgachuz Range in western British Columbia is a shield volcano several million years old.
(Photo by C.J. Hickson (Geological Survey of Canada))
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Shield volcanoes commonly are not as dramatic-looking as stratovolcanoes,
but are often much larger. The Hawaiian Islands are a
series of large, gently sloping shield volcanoes. They
have broad summits and generally concave flanks. The base
of Mauna Loa, a shield volcano on the island of Hawaii,
is over 80 km in diameter. Shield volcanoes form by repeated
eruptions from a common vent area of usually basaltic
lavas low in SiO2. These lavas have low viscosity and
can flow for tens to hundreds of kilometres. Examples
of shield volcanoes in Canada include Mount Edziza, in
central British Columbia, whose base is composed of several
overlapping, large, basaltic shield volcanoes, and the
Ilgachuz Range of central British Columbia (Figure 16),
which erupted special kinds of low-viscosity magmas to
form a broad shield volcano whose youngest lava flows
are several million years old.
Calderas form when large volumes of magma erupt all at once. The
large hole left after the magma is ejected collapses,
forming a circular to oval depression on the Earth's surface
(Figure 17, Figure 18).
These holes are roughly similar in appearance to a
crater formed by meteorite impact. Caldera-forming eruptions
are the largest and most deadly types of volcanic eruptions.
Fortunately, they are also very rare. Only one or two
occur every several hundred thousand years. One of the
most active regions for these eruptions is the western
United States. Yellowstone Park is part of a 45 km diameter
caldera formed 600,000 years ago. At least a few calderas
may exist buried beneath the ice caps of volcanoes in
Canada, including at Mount Silverthrone in southwestern
British Columbia and at Mount Edziza in north-central
British Columbia.
Figure 17. CalderaThe circular feature in the centre of the photograph is the summit caldera on Muana Loa volcano in Hawaii. Calderas on Hawaiian type volcanoes, (basaltic) are caused by lava eruptions on the flank of the volcano, draining the magma chamber and causing the summit to subside.
(Photograph courtesy of D. Peterson)
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Figure 18. Las Canadas caldera, Tenerife, Canary Islands.In the distance is the 350 m high wall of the 16 km diameter caldera, formed by enormously explosive eruptions of about 100 cubic km of magma between 170 thousand and 1.3 million years ago. Calderas such as these can be so large as to be only recognizable from space. The eruptions cause rapid evacuation of the magma chamber and collapse of the overlying volcano summit. Such "super-eruptions" can have global climatic impacts. Few such events have occurred in recent geological time in Canada, but they have in California (Long Valley) and Oregon (Crater Lake).
(Photo by M. Stasiuk (Geological Survey of Canada).)
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Other types of volcanoes are generally smaller, but mimic their
larger cousins in shape if not in size. Cinder cones form
around the vent where magma reaches the surface and are
generally round to oval in plan view with a crater or
depression at the top (Figure 19).
They are made of 'cinder', which is ash- to bomb-sized,
solidified magma largely composed of vesicles (or solidified
gas bubbles). Small, fist-sized, vesicular pieces of cinder
can be referred to as 'scoria' (Figure 11).
As the magma reaches the surface, gases held in suspension
within it are released rapidly - much like the bubbles
that fizz out of a carbonated beverage the moment it is
opened. Cinder cones are abundant in western Canada (e.g.
Wells Gray and Mount Edziza) and are easily accessible
in some areas such as Atlin and Nazko, British Columbia.
Figure 19. Cinder coneEve cone is a young, well preserved cinder cone at Mount Edziza, British Columbia.
(Photograph by C.J. Hickson (Geological Survey of Canada))
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Figure 20. An Eruptive VentA basaltic eruptive vent on Mt. Etna. As the lava reaches the surface, gas held within it is released explosively to form a lava fountain, similar to a geyser. Pieces of the lava are jetted up, cool in the air and fall around the vent to form a cone of lava pieces, or cinder cone. Some of the lava is not broken up and flows from the vent to the left as a lava flow.
(Photo by S.Sparks (U. Bristol))
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| Maars, diatremes & diamonds |
Maars are volcanoes formed by the violent interaction of red-hot
magma coming into contact with groundwater. The instantaneous
conversion of the liquid groundwater to steam drives a
violent explosion that forms a hollowed-out crater, similar
to that made by a meteorite impact. The only maar-like
volcano known in Canada is found in Wells Gray Provincial
Park, east-central British Columbia. Another type of volcano,
unique to a relatively rare type of rock, is a diatreme.
A diatreme is a complex and chaotic pipe of a special
type of very low viscosity magma (such as kimberlite)
filled with rock fragments. The eruption forces the column
of magma and rock fragments to the surface from deep within
the Earth's crust. Kimberlite diatremes or pipes have
been found across Canada and the world and recently in
the Northwest Territories. They are important economically
because kimberlites are the world's main source of gem-quality
diamonds. The kimberlite pipes found in the Northwest
Territories and in Alberta may soon make Canada one of
the world's major producers of gem-quality diamonds.
A distinctive type of volcano is produced when a volcanic
eruption occurs beneath a glacier. This occurs most commonly
today in Iceland and Antarctica. Both of these areas contain
large glaciers (or ice caps) and active volcanoes. However,
as recently as 10,000 years ago, much of western Canada
was also covered with thick sheets of ice. In fact, large
parts of Canada have been repeatedly covered by thick
glacial ice over the last few million years. As a result,
western Canada also has an abundance of volcanoes whose
unusual, flat-topped (Figure 21)
or conical (Figure 22)
forms are the direct result of subglacial eruptions.
Figure 21. TuyaHyalo Ridge, in Wells Gray Provincial Park, is a tuya, a typical, flat-topped, steep-sided, subglacial volcano.
(Photograph by C.J. Hickson)
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Figure 22. Subglacial moundThe 3 Caribou tuya, in north-central British Columbia, is a subglacial mound.
(Photograph by C.J. Hickson)
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Figure 23. Stages in the development of subglacial volcanoesThe initial eruptions produce pillow lavas and hyaloclastite as the overlying glacial ice melts (stage 1). If the eruption stops before the volcano is higher in elevation than the surrounding water, a subglacial mound results (stage 2b). If the eruption continues, a subaerial lava flow may form a flat cap on the volcano, producing the classic tuya form (stage 3b).
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Dr. W. H. Mathews, a professor at the University of British
Columbia, Vancouver, was one of the first people in the
world to describe in detail these unique types of volcanoes
while mapping in northern British Columbia in the 1940s.
His work led him to recognize a sequence of interactions
between a growing, subglacial volcano and the overlying
ice that ultimately would produce the distinctive shapes
of the subglacial volcanoes. He proposed that as a volcano
erupted beneath a glacier, the heat from the volcano would
immediately start to melt the overlying ice (Figure 23,
stage1).
The resulting water quickly cooled the lava and produced forms
called 'pillows'; aggregates of pillows form 'pillow lavas'
(Figure 24, Figure 25).
In places, the pillows broke up and rolled down the slopes
of the submerged volcano, forming other types of volcanic
deposits called 'pillow breccia' (Figure 26),
'tuff breccia', and 'hyaloclastite' (Figure 27).
Pillow lavas and other types of subaqueous (formed beneath
water) volcanic deposits can be seen forming today in
Hawaii where lava flows enter the crystal blue water of
the Pacific Ocean.
Figure 24. Single pillow lavaA cross-section through a single lava pillow, surrounded by yellow-ish hyaloclastite. Pillows are recognized as solid lava pieces with pillow-shaped forms, a chilled glassy margin and radial cooling joints.
(Photo by C.J. Hickson (Geological Survey of Canada))
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Figure 25. Stacks of pillow lavaStacks of pillow lavas commonly form where large lava flows enter a lake or an ocean.
(Photograph by C.J. Hickson (Geological Survey of Canada))
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Figure 26. Pillow brecciaFragments of pillow lava are commonly found with hyaloclastite. Together the hyaloclastite and pillow fragments are referred to as 'pillow breccia'.
(Photo by C.J. Hickson (Geological Survey of Canada))
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Figure 27. HyaloclastiteHyaloclastite is a glass-rich breccia that forms when volcanoes erupt under water or under ice. Most fragments are lapilli to ash sized.
(Photo by C.J. Hickson (Geological Survey of Canada))
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| Subglacial volcano shapes |
If the volcano was not hot enough to melt through the overlying
ice cap, a subglacial mound would be all that remained
of the volcano once the ice had retreated (e.g. Figure 22,
Figbure 23, stage 2b). However, if the volcano was hot enough to melt
through the overlying ice cap, more-normal-looking, subaerial
(formed in the air) lava flows might flow out on top of
the pillow lavas and cover them up. Once the glaciers
had retreated or melted away, the steep-sided and in places
flat-topped volcanoes would reappear with a unique shape
as a result of their confinement within glacial ice (Figure 23,
stage 3b). Dr. Mathews referred to these flat-topped,
subglacial volcanoes as 'tuyas' after Tuya Butte in northern
British Columbia where he first studied these unique volcanic
forms. Similar mountains in Iceland are called 'table
mountains' because of their flat tops.
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