By volume, the two largest Cascade volcanoes are the broad shields of Medicine Lake Volcano and Newberry Volcano, which are about 145 cubic miles (600 km 3) and 108 cubic miles (450 km 3) respectively. Twelve volcanoes in the arc are over 10,000 feet (3,000 m) in elevation, and the two highest, Mount Rainier and Mount Shasta, exceed 14,000 feet (4,300 m). Volcanism in the arc began about 37 million years ago however, most of the present-day Cascade volcanoes are less than 2,000,000 years old, and the highest peaks are less than 100,000 years old. The Cascade Arc includes nearly 20 major volcanoes, among a total of over 4,000 separate volcanic vents including numerous stratovolcanoes, shield volcanoes, lava domes, and cinder cones, along with a few isolated examples of rarer volcanic forms such as tuyas. It is also the site of Canada's most recent major eruption about 2,350 years ago at the Mount Meager volcanic complex. Two most recent were Lassen Peak in 1914 to 1921 and a major eruption of Mount St. The Cascade Volcanoes have erupted several times in recorded history. The Cascade Volcanoes are part of the Pacific Ring of Fire, the ring of volcanoes and associated mountains around the Pacific Ocean. Many large, long-runout landslides originating on Cascade volcanoes have inundated valleys tens of kilometers from their sources, and some of the inundated areas now support large populations. Consequently, Mount Rainier is one of the Decade Volcanoes identified by the International Association of Volcanology and Chemistry of the Earth's Interior (IAVCEI) as being worthy of particular study, due to the danger it poses to Seattle and Tacoma. Because the population of the Pacific Northwest is rapidly increasing, the Cascade volcanoes are some of the most dangerous, due to their eruptive history and potential for future eruptions, and because they are underlain by weak, hydrothermally altered volcanic rocks that are susceptible to failure. All could be potentially affected by volcanic activity and great subduction-zone earthquakes along the arc. Some of the major cities along the length of the arc include Portland, Seattle, and Vancouver, and the population in the region exceeds 10,000,000 people. Although taking its name from the Cascade Range, this term is a geologic grouping rather than a geographic one, and the Cascade Volcanoes extend north into the Coast Mountains, past the Fraser River which is the northward limit of the Cascade Range proper. The arc has formed due to subduction along the Cascadia subduction zone. The Cascade Volcanoes (also known as the Cascade Volcanic Arc or the Cascade Arc) are a number of volcanoes in a volcanic arc in western North America, extending from southwestern British Columbia through Washington and Oregon to Northern California, a distance of well over 700 miles (1,100 km). While the volcanoes are generally considered either extinct or dormant, there are at least four volcanoes that have shown seismic activity since 1985. The most active has been Mount Meager massif, a group of volcanic peaks within southwestern British Columbia. Though there have not been any eruptions for more than 2,000 years, Mount Meager was responsible for Canada’s largest volcanic eruption. The other volcanoes with active seismic activity are Mount Garibaldi, Mount Cayley, and Silverthrone Caldera.Oregon, Washington, California, British ColumbiaĤ6★1′1.9″N 121☄5′35.6″W / 46.850528°N 121.759889°W / 46.850528 -121.759889 The volcanic activity has also formed a variety of intrusive igneous rocks, which form as magma cools. The Canadian Cascades were formed as the ancient oceanic Farallon Plate melded into the earth’s mantle and pushed up a volcanic arc. This has caused a substantial fault area, which can cause earthquakes of magnitude 7.0 and beyond. The First Nations people of Vancouver Island tell stories of the 1700 Cascadia Earthquake, which caused a tsunami that struck the west coast of North America and even traveled as far as Japan, flooding both coasts. This earthquake is estimated to have been a magnitude 9.0.
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