Cascade Volcanoes
The Cascade Volcanoes are a number of volcanoes in a continental 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 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, in 410 BCE 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.File:Lassen-Peak-Large.jpg|thumb|Lassen Peak and Devastated Area from Cinder Cone
West Cascades period
The time between 37 million and 17 million years ago is known as the West Cascades period, this era is characterized as being when the volcanoes in this region were exceptionally active. During this time the arc was situated a little farther west than it is today. One volcano that was active during this time was the Mount Aix Volcanic Complex, which erupted more than of tephra and pyroclastic debris over the span of just three eruptions. Lavas representing the earliest stage in the development of the Cascade Volcanic Arc mostly crop out south of the North Cascades proper, where uplift of the Cascade Range has been less, and a thicker blanket of Cascade Arc volcanic rocks has been preserved. In the North Cascades, geologists have not yet identified with any certainty any volcanic rocks as old as 35 million years, but remnants of the ancient arc's internal plumbing system persist in the form of plutons, which are the crystallized magma chambers that once fed the early Cascade volcanoes. The greatest mass of exposed Cascade Arc plumbing is the Chilliwack batholith, which makes up much of the northern part of North Cascades National Park and adjacent parts of British Columbia beyond. Individual plutons range in age from about 35 million years old to 2.5 million years old. The older rocks invaded by all this magma were affected by the heat. Around the plutons of the batholith, the older rocks recrystallized. This contact metamorphism produced a fine mesh of interlocking crystals in the old rocks, generally strengthening them and making them more resistant to erosion. Where the recrystallization was intense, the rocks took on a new appearance dark, dense and hard. Many rugged peaks in the North Cascades owe their prominence to this baking. The rocks holding up many such North Cascade giants, as Mount Shuksan, Mount Redoubt, Mount Challenger, and Mount Hozomeen, are all partly recrystallized by plutons of the nearby and underlying Chilliwack batholith.Widespread dormancy period
The West Cascades period came to an end 17 million years ago when the Columbia River flood basalts began erupting in eastern Washington and Oregon. For a reason unknown to scientists the initiation of the flood basalts seemingly caused a significant dip in volcanic activity in the cascade chain lasting for over 8 million years. During this time the volcanoes were stripped down to their cores by weathering and erosion because they were not active enough to rebuild. This low point lasted from 17 to 9 million years ago and came to end when the Columbia flood basalts waned.High Cascades period
As production of the Columbia River flood basalts slowed 9 million years ago the Cascade volcanoes became active again. The volcanic arc also drifted farther east to its present location. When the Columbia basalts stopped entirely 6 million years ago the Cascades of central Oregon spectacularly flared up. This flare up lasted between 6.25 and 5.45 million years ago and is known as the Deschutes Formation. During this 800,000 year span approximately of pyroclastic material was expelled in 78 distinct eruptions. It has been hypothesized that a heightened flux of basalt, possibly induced by tectonic slab-rollback, was focused beneath the volcanic arc and into the shallow crust by minor amounts of crustal extension. This extension allowed for the high flux of basalt to be stored at shallow levels beneath a new arc locus within fertile crust, resulting in the silica-rich volcanism we see in the Deschutes Formation. After this pulse of activity the cascades retreated to the levels of activity we are more familiar with today.For the remaining 5 or so million years the ancestors of many of the modern day Cascade volcanoes were built. Around half a million years ago a generation of older volcanoes died and many of the stratovolcanoes that we see today began their growth such as Glacier Peak and Mt. Shasta, Mt. Rainier and Mt. Hood, Mt. Adams, and Mt. Mazama.
Modern arc
The volcanoes of the Cascade Arc share some general characteristics, but each has its own unique geological traits and history. Lassen Peak in California, which last erupted in 1917, is the southernmost historically active volcano in the arc, while the Mount Meager massif in British Columbia, which erupted about 2,350 years ago, is generally considered the northernmost member of the arc. A few isolated volcanic centers northwest of the Mount Meager massif such as the Silverthrone Caldera, which is a circular wide, deeply dissected caldera complex, may also be the product of Cascadia subduction because the igneous rocks andesite, basaltic andesite, dacite and rhyolite can also be found at these volcanoes as they are elsewhere along the subduction zone. At issue are the current estimates of plate configuration and rate of subduction, but based on the chemistry of these volcanoes, they are also subduction related and therefore part of the Cascade Volcanic Arc. The Cascade Volcanic Arc appears to be segmented; the central portion of the arc is the most active and the northern end least active.The Garibaldi Volcanic Belt is the northern extension of the Cascade Arc. Volcanoes within the volcanic belt are mostly stratovolcanoes along with the rest of the arc, but also include calderas, cinder cones, and small isolated lava masses. The eruption styles within the belt range from effusive to explosive, with compositions from basalt to rhyolite. Due to repeated continental and alpine glaciations, many of the volcanic deposits in the belt reflect complex interactions between magma composition, topography, and changing ice configurations. Four volcanoes within the belt appear related to seismic activity since 1975, including: Mount Meager massif, Mount Garibaldi and Mount Cayley.
The Pemberton Volcanic Belt is an eroded volcanic belt north of the Garibaldi Volcanic Belt, which appears to have formed during the Miocene before fracturing of the northern end of the Juan de Fuca Plate. The Silverthrone Caldera is the only volcano within the belt that appears related to seismic activity since 1975.
The Mount Meager massif is the most unstable volcanic massif in Canada. It has dumped clay and rock several meters deep into the Pemberton Valley at least three times during the past 7,300 years. Recent drilling into the Pemberton Valley bed encountered remnants of a debris flow that had traveled from the volcano shortly before it last erupted 2,350 years ago. About of rock and sand extended over the width of the valley. Two previous debris flows, about 4,450 and 7,300 years ago, sent debris at least from the volcano. Recently, the volcano has created smaller landslides about every ten years, including one in 1975 that killed four geologists near Meager Creek. The possibility of the Mount Meager massif covering stable sections of the Pemberton Valley in a debris flow is estimated at one in 2,400 years. There is no sign of volcanic activity with these events. However, scientists warn the volcano could release another massive debris flow over populated areas any time without warning.
In the past, Mount Rainier has had large debris avalanches, and has also produced enormous lahars due to the large amount of glacial ice present. Its lahars have reached all the way to Puget Sound. Around 5,000 years ago, a large chunk of the volcano slid away and that debris avalanche helped to produce the massive Osceola Mudflow, which went all the way to the site of present-day Tacoma and south Seattle. This massive avalanche of rock and ice took out the top of Rainier, bringing its height down to around. About 530 to 550 years ago, the Electron Mudflow occurred, although this was not as large-scale as the Osceola Mudflow.
While the Cascade volcanic arc includes volcanoes such as the Mount Meager massif and Mount Garibaldi, which lie north of the Fraser River, the Cascade Range is considered to have its northern boundary at the Fraser.