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Natural Resources Canada > Earth Sciences Sector > Priorities > Strong and safe communities > Volcanoes of Canada
Volcanoes of Canada Building blocks of volcanoes
All volcanoes start out the same way, as a liquid that forms
beneath the surface of the Earth. These subsurface liquids
are made up mostly of the following eight elements: oxygen
(O), silicon (Si), aluminum (Al), iron (Fe), magnesium
(Mg), calcium (Ca), sodium (Na), and potassium (K). Compounds
referred to as 'volatiles' (or gases), such as sulphur
dioxide, carbon dioxide, and water, are also commonly
present in these liquids in minor amounts. The subsurface
liquids, or silicate melts (because they contain abundant
silica), are referred to as 'magmas' until they reach
the surface, when they become 'lavas'. Lava flows are
most commonly streams of very hot silicate liquids that
can carry crystals that formed underground or grow as
the lava cools. Lavas can vary from 500°C to more
than 1200°C and can erupt in many different ways depending
on variations in their chemical composition, especially
in the relative amounts of silicon and water.
Figure 5. Schematic view of volcanoSchematic figure of a stratavolcano illustrating some volcanic terms and rock names. Rock names are based on the weight per cent of silicon dioxide (SiO2) in the rock.
(courtesy of the Cascade Volcano Observatory, United States Geological Survey)
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Volcanoes are built of rocks that record the history of eruptions
at a particular volcano. Because their chemical composition
varies tremendously, volcanic rocks have many different
names. These names have been chosen to convey as much
information as possible about the rock in a single word.
Because volcanic rocks are composed mostly of the elements
Si and O, for convenience in describing them, volcanologists
combine these two elements and refer to them as 'silica'
(SiO2). These two elements account for over 45% (by weight)
of the rocks; thus, the names of volcanic rocks vary depending
on the amount of SiO2 present in the rock. For example,
basalt, the most common volcanic rock in Canada (and on
the surface of the Earth), contains between 45% and 52%
SiO2 by weight (wt. %; Figure 5).
Large volumes of basalt are found throughout Canada. In
western Canada, basalt covers much of the Chilcotin and
Cariboo plateaus of central British Columbia. Andesite,
another kind of volcanic rock that contains more SiO2
than basalt (52-63 wt. % SiO2; Figure 5),
is not as common at Canada's youngest volcanoes, but could
potentially erupt in the Garibaldi area of southern British
Columbia or from Mount Baker, immediately south of the
border in northwestern Washington State. Basalt and andesite
usually flow easily and most commonly form lava flows.
Volcanoes that erupt dacite (63-68 wt. % SiO2; Figure 5)
and rhyolite (>68 wt. % SiO2; Figure 5)
lavas are even less common, but are potentially more explosive
and dangerous than volcanoes that erupt basalt and andesite
lavas. Because dacite and rhyolite magmas are thick and
viscous (i.e. 'sticky', like honey or molasses), they
do not flow easily and are prone to explode when they
are rich in volatiles. Dacitic and rhyolitic lavas that
contain little gas build up around the vent area as rounded,
muffin-shaped mounds of lava called 'domes'.
Figure 6. Lava domeSoufriere Hills volcano, Montserrat (West Indies), with active lava dome at summit (December 1996). A similar eruption occurred 2350 years ago at Mount Meager, British Columbia.
(Photograph by M. Stasiuk (Geological Survey of Canada))
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Figure 7. Lava dome detailActively growing lava dome on Montserrat, extruding a stubby lava "spine" through an mantling apron of lava talus and ash (December 1996).
(Photograph by M. Stasiuk (Geological Survey of Canada))
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Two large eruptions of dacite magma, at Mount Meager in southern
British Columbia and at Mount Churchill in eastern Alaska
(a few kilometres west of the Yukon Territory border),
produced significant deposits of volcanic ash across much
of western Canada in the last 3000 years.
Figure 8. Distribution of tephra depositsApproximate distribution of Holocene (0-10,000 years) ash deposits in western North America. Also shown is the enormous airfall ash deposit produced by Long Valley caldera (California), one of the largest volume eruptions in North America in the last million years. |
| Eruption styles depend on composition |
Whether the magma is basaltic, andesitic, dacitic, rhyolitic,
or a myriad of other, more obscure compositions, different
names are used to describe it once it has erupted. Depending
on its chemical composition (and other factors such as
the volatile content), magma will either flow out of the
volcano's vent (Figure 5)
passively and be 'coherent' (e.g. lava), or it will spew
from the vent violently and be fragmental (pyroclastic).
As they cool, coherent lava flows often develop very regular
fractures called 'columnar joints' (Figure 9)
that are easily recognized and very distinctive. Lava
flows may also produce some fragmental material, referred
to as 'flow breccia', that forms as the lava cools, but
continues to move.
Figure 9. Columnar jointsColumnar joints in a solidified lava flow. These regular, polygonal joints form as a result of contraction as the lava cools.
(Photo by C.J. Hickson (Geological Survey of Canada))
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Figure 10. Pahoehoe and aaA pahoehoe lava surface (foreground) and an active aa flow front (background) in Hawaii.
(Photo by C.J. Hickson (Geological Survey of Canada))
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The surface of the lava can also have different forms. Ropy,
shell-like surfaces are called 'pahoehoe' and broken,
vesicular surfaces, 'aa' (Figure 10).
If, however, the magma breaks into fragments during an
explosive eruption, the fragments are called 'tephra'
or 'scoria' (Figure 11).
These fragments are described on the basis of their size
and the volume percentage of vesicles (i.e. air bubbles;
Figure 12, Figure 13) they contain.
Figure 11. TephraScoriaceous tephra from the Nazko cone, central British Columbia. The smaller pieces (less than 6.2 cm long) are lapilli sized and the larger pieces are block or bombsized. For comparison, the geological hammer is approximately 30 cm long.
(Photograph by C.J. Hickson)
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Figure 12. VesiclesVesicles in solidified lava. After a lava flow comes to rest but before it solidifies, the gas bubbles in it rise upward and collect beneath the solidified surface crust to form a vesicle-rich zone at the top, and a denser zone in the central part of the lava.
(Photo by C.J. Hickson (Geological Survey of Canada))
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Magma fragments whose largest dimension is over 64 mm
are referred to either as 'blocks' or 'bombs', depending
on their shape (Figure 11, Figure 14).
Blocks are angular and composed of magma that solidified
before being thrown from the vent. Bombs, on the other
hand, are ejected while still partly molten and can show
signs of aerodynamic shaping during flight (Figure 14).
Magma fragments with a maximum dimension under 64 mm and
over 2 mm are referred to as 'lapilli' (Figure 11).
Figure 13. Pumice fragmentScanning electron microscope image of a pumice fragment showing its very porous, vesicular nature.
(Photo by C.J. Hickson (Geological Survey of Canada))
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Figure 14. Volcanic bombsVolcanoes commonly shoot out large blobs of hot magma called 'volcanic bombs'. Because the magma is very hot, as it sails through the air, it can develop a streamlined shape like that of the 'spindle bomb' shown here.
(Photo by C.J. Hickson (Geological Survey of Canada))
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'Ash', the term used for fragments that are less than 2 mm in
maximum dimension, is the smallest type of tephra. 'Cinder'
and 'scoria' are terms used to describe tephra that is
moderately vesicular (<75% vesicles by volume) and
most commonly basaltic to andesitic. The term 'pumice'
is reserved for tephra that is highly vesicular (>75%
vesicles by volume; Figure 11).
Pumice most often forms from dacitic or rhyolitic magmas
and is commonly so light that it will float on water;
large pieces are easily carried.
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