Here, an offshore tectonic plate is forced under a continental plate, causing molten rock to rise to the surface and form a volcano. Volcanoes in the northern end of this chain include Mount Garibaldi and Mount Meager, both north of Vancouver, and Mount St. Helens, Mount Baker and Mount Rainier, all in Washington State.
Volcanoes erupt, in part, because of pressure from dissolved gas.
Inside the volcano, liquid magma rises through cracks and channels in the earth's crust. If the magma is fairly fluid, gases can escape readily and the eruption will be in the form of a lava flow. Mount St. Helens is 300 kilometres south of Canada. Closer to Canada, Mount Baker, which is cooling from a minor eruption in 1870, still lets off steam near its top. A volcano may sleep for many centuries while gas pressure slowly builds in its subterranean magma chambers.