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 . The arc formed due to subduction along the Cascadia subduction zone. 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.
Some of the major cities along the length of the arc include Portland, Seattle, and Vancouver, and the population in the region exceeds 10 million. All could be potentially affected by volcanic activity and great subduction-zone earthquakes along the arc. 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. 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. Many large, long-runout landslides originating on Cascade volcanoes have engulfed valleys tens of kilometers from their sources, and some of the areas affected now support large populations.
The Cascade Volcanoes are part of the Pacific Ring of Fire, the ring of volcanoes and associated mountains around the Pacific Ocean. The Cascade Volcanoes have erupted several times in recorded history. Two most recent were Lassen Peak in 1914 to 1921 and a major eruption of Mount St. Helens in 1980. It is also the site of Canada's most recent major eruption about 2,350 years ago at the Mount Meager massif.
Geology
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. 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. Twelve volcanoes in the arc are over in elevation, and the two highest, Mount Rainier and Mount Shasta, exceed . By volume, the two largest Cascade volcanoes are the broad shields of Medicine Lake Volcano and Newberry Volcano, which are about respectively. Glacier Peak is the only Cascade volcano that is made exclusively of dacite. The history of the cascade volcanoes can be separated into three major chapters which are discussed below.