Cascade Volcanoes - 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 10,000 ft (3,000 m) in elevation, and the two highest, Mount Rainier and Mount Shasta, exceed 14,000 ft (4,300 m). By volume, the two largest Cascade volcanoes are the broad shields of Medicine Lake Volcano and Newberry Volcano, which are about 145 mi³ (600 km³) and 108 mi³ (450 km³) respectively. Mount Garibaldi and Glacier Peak are the only two Cascade volcanoes that are made exclusively of dacite.

Over the last 37 million years, the Cascade Arc has been erupting a chain of volcanoes along the Pacific Northwest. Several of the volcanoes in the arc are frequently active. 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 Mount Meager 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 Mount Meager such as the Silverthrone Caldera, which is a circular 20 km wide, deeply dissected caldera complex, may also be the product of Cascadia subduction because andesite, basaltic andesite, dacite and rhyolite can be found at these volcanoes and elsewhere along the subduction zone. At issue are the current plate configuration and rate of subduction but based on chemistry is for these volcanoes to be subduction related and are 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.

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.

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 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, Mount Garibaldi and Mount Cayley.

Mount Meager 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 travelled 50 kilometers from the volcano shortly before it last erupted 2350 years ago. About 1,000,000,000 m³ 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 32 kilometers 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 Mount Meager covering stable sections of the Pemberton Valley in a debris flow is estimated at about one in 2400 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 anytime 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 the 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 1,600 feet (500 m) of Rainier, bringing its height down to around 14,100 feet (4,300 m). 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 (a geological term) includes volcanoes such as Mount Meager and Mount Garibaldi, which lie north of the Fraser River, the Cascade Range (a geographic term) is considered to have its northern boundary at the Fraser.

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