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.

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) are 'classic' stratovolcanoes.

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, 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. 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.

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. 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'. 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.

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 covered with thick sheets of ice, and 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 shapes are the result of eruptions under (subglacial) or alongside (ice-marginal) glacial ice. Subglacial eruptions can produce both flat-topped and conical forms. Ice-marginal eruptions can produce steep cliffs and unusual patterns of cooling joints.

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 erupts beneath a glacier, the heat from the volcano immediately starts to melt the overlying ice.

The resulting meltwater quickly cools the lava and produces forms called 'pillows'; aggregates of pillows form 'pillow lavas'. In places, the pillows break up and roll down the slopes of the submerged volcano, forming other types of volcanic deposits called pillow breccia (a rock composed of pillow fragments) and hyaloclastite (a rock composed of fragments of volcanic glass). 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.

If the eruption stops before the volcano is higher in elevation than the surrounding water, a subglacial mound results. If the volcano produces sufficient heat to melt through the overlying ice cap, it may eventually build itself up to above the level of the ice or water surface, and subaerial (formed in the air) lava flows may cover the pillow lavas and hyaloclastite. Once the glacier has retreated or melted away, a steep-sided, flat-topped volcano will remain. 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.

Different eruptive conditions (steep versus gentle topography, viscous versus runny lava, thin versus thick ice, and so on) can produce variations on these basic forms. In British Columbia’s Garibaldi Volcanic Belt, three common variations are present:

1. Repeated lava flows into well-drained ice cavities produce subglacial domes – steep-sided, finely-jointed mounds of lava.
   

   

   Subglacial dome at Ember Ridge Northwest.
   

   (Photograph by P. Adam.)

   
   

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2. If eruptions in one of these well-drained cavities continue after the surface of the ice is breached, lava flows will stack up, filling an ice-walled chimney above the vent, resulting in formation of a flow-dominated tuya; these differ from classical tuyas in that they have little or no pillows and hyaloclastite, consisting almost entirely of lava flows.
   

   

   The Table, a flow-dominated tuya.
   

   (Photograph by P. Adam.)

   
   

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3. If lava from a subaerial eruption is ponded against glacial ice, it melts some of the ice but also quickly solidifies, forming a barrier behind which more lava can pile up. Such ice-marginal lava flows are anomalously thick and are either bounded by steep cliffs with fine-scale cooling joints and lots of volcanic glass, or are bounded by cliffs formed by the collapse of the unstable original margins of the lava flow.
   

   

   The Barrier, an ice-marginal lava flow.
   

   (Photograph by M.C. Kelman.)

   
   

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