As the Earth cooled from a gaseous ball, its interior formed several layers (Figure 1). The innermost layers, or core and lower mantle, do not directly generate volcanic eruptions. It is the outermost layers, or solid crust and asthenosphere, from which volcanoes form. According to the theory of plate tectonics, the Earth's crust is divided into large blocks or plates that slide around on the planet's outermost layer (Figure 2). The theory has been almost universally accepted largely because it explains many geological events and patterns in a simple and unified way. The study of volcanoes also benefited significantly from this theory. Maps showing the boundaries of tectonic plates and volcanoes show that most volcanoes occur near the edges of tectonic plates (Figure 2). Even more significantly, specific types of volcanoes are most commonly found along the same type of plate edge (referred to as 'tectonic environment'). When one plate moves, by necessity it must push against the neighbouring plate, pull away from the neighbouring plate, or move parallel to the neighbouring plate. These three types of relative motion form three distinct types of plate edges or boundaries. When two plates collide, one is usually forced under the other to form a subduction zone (Figure 3). When two plates pull apart, hot material from beneath the plates rises to fill the intervening gap, forming a spreading ridge in the oceans or a rift valley on the continents. The boundary of two plates that move past each other in a parallel way is referred to as a 'transform boundary'. Volcanoes form in all three of these tectonic environments, especially the first two (subduction zones and spreading ridges).

Figure 3. A subduction zone and a spreading ridge Schematic block diagram showing major features of a subduction zone and spreading ridge, such as are found in southwestern British Columbia. The North American Plate is travelling westward and colliding with the smaller Juan de Fuca Plate. The collision creates earthquakes and volcanoes. The chain of volcanoes that forms is called an 'arc'. Mounts St. Helens, Rainier, Baker, Garibaldi, and Meager belong to the Cascade magmatic arc. larger image [GIF, 24.8 kb, 618 X 413, notice]

In a subduction zone, one piece of crust is forced down into the mantle below the overriding crust (Figure 3). In most subduction zones, the crust being forced into the mantle is oceanic crust, which is generally denser than continental crust and commonly covered in thick, wet sediments and ooze. As these sediments and ooze are carried into the Earth's mantle with the oceanic crust, rising heat and pressure dry them out in a process called 'dehydration'. The water expelled from the descending crust rises and changes the chemical composition of the overlying mantle. The presence of water actually lowers the melting temperature of the overlying mantle rocks and causes them to melt. Melting mantle forms magma that rises through the crust to erupt on the surface, forming a volcano. In Canada, subduction-zone volcanoes are found only in extreme southwestern British Columbia, as part of the Cascade volcanic arc (e.g. Mounts Garibaldi and Meager; see Figure 3, Figure 4, and the Catalogue of Canadian Volcanoes).

Figure 4. Volcanoes & their tectonic settings in Western Canada Quaternary (triangles) and Holocene (stars) volcanoes in western Canada and their tectonic settings. Holocene volcanoes are discussed in detail in the Catalogue of Canadian Volcanoes. larger image [GIF, 21.0 kb, 336 X 445, notice]

In other parts of the Earth, instead of colliding, plates move away from one another. Commonly, this occurs in the oceans, creating an oceanic ridge. Along the ridge, rising magma creates new oceanic crust. These ridges are formed mostly of basaltic lava. The Juan de Fuca Ridge off the west coast of Vancouver Island is just such a spreading ridge (Figure 3). On continents, the crust can also be stretched and torn apart, forming regional weaknesses and , in places, large valleys. One such valley is the East African rift zone; in western Canada, the northwestern corner of the British Columbia is undergoing such stretching and weakening of the crust. In fact, some of Canada's youngest volcanoes formed in the weakened zone. They are mostly basalt and are commonly small cinder cones (e.g. Lava Fork or Tseax cone; see Figure 4 and the Catalogue of Canadian Volcanoes), though much larger volcanoes also exist, such as Mount Edziza (see Figure 4 and the Catalogue of Canadian Volcanoes).

Yet another type of volcano-forming tectonic environment is found where hot, upwelling mantle forms magmas beneath an expanse of crust. The upwelling mantle material, focused on a single spot, creates a volcano. The crust continues to move, however, and a series or chain of volcanoes forms. The oldest volcanoes are found in the direction towards which the crust is moving and youngest, in the direction from which it is coming. These chains of volcanoes, called 'hot-spot volcanoes', are often large, basaltic, shield volcanoes. A famous example is the Hawaiian Islands. The oldest volcanoes have sunk beneath the waves west of Kauai, and Lohi, the newest volcano in the Hawaiian chain, is slowly rising from the ocean floor off the east coast of the island of Hawaii, although it has yet to break the surface of the ocean. Nazko cone (see Figure 4 and the Catalogue of Canadian Volcanoes), in central British Columbia, forms the eastern end of a hot-spot chain of volcanoes that can be traced back 14 million years to now long-eroded volcanoes along the coast of British Columbia.