Most of the eruptions have occurred at small cinder cones and produce lava flows that cover a few hundred acres to a few square miles. A few larger volcanoes including Mount Hood, the Three Sisters, and Mount McLoughlin, alternate between lava flows and explosive eruptions of ash, pumice, and broken lava.
Even rarer are huge eruptions like the Mazama see Unit 19 explosion that created Crater Lake and blanketed surrounding states with ash.
Some of these young volcanoes extend beyond the Cascade Range. Portland is one of the few cities in the world where young volcanoes lie within the urban area. Most of the largest peaks in the Cascade Range are still considered active volcanoes.
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 , is the southernmost historically active volcano in the arc, while Mount Meager in British Columbia, which erupted about 2, 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.
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. 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 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 , 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, 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 years ago.
Two previous debris flows, about 4, and 7, 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 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 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. To learn more, read about volcanic hazards on this website. Volcanic threat is the combination of hazards the dangerous or destructive natural phenomena produced by a volcano and exposure the people and property at risk from the volcanic phenomena.
Based upon eruption history and distance to population centers, a volcanic threat assessment designated nine volcanoes in Washington and Oregon with a "high" or "very high" rating. Helens, Newberry, Three Sisters.
Download the update to the U. Geological Survey national volcanic threat assessment. Mount Hood dominates the skyline outside of Portland, Oregon on a clear day. An major eruption of Mount Hood would pose a great hazard to the regional economy. Skip to main content. Search Search. Cascades Volcano Observatory.
0コメント