- Cascade Volcanoes
Mount Hoodreflected in Trillium Lake
region_type = Provinces/States
region3= British Columbia
The Cascade Volcanoes (also known as the Cascade Volcanic Arc or the Cascade Arc) are a number of
volcanoes in a volcanic arcin western North America, extending from southwestern British Columbiathrough Washingtonand Oregonto Northern California, a distance of well over 700 mi (1,100 km). The arc has formed due to subductionalong 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 Riverwhich 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 10,000,000. All could be potentially affected by volcanic activity and great subduction-zone earthquakesalong the arc. Because the population of the Pacific Northwestis rapidly increasing, the Cascade volcanoes are some of the most dangerous, due to their past eruptive history, potential eruptions and because they are underlain by weak, hydrothermally altered volcanic rocks that are susceptible to failure. 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.
The Cascade Volcanoes are part of the
Pacific Ring of Fire, the ring of volcanoes and associated mountains around the Pacific Ocean. All of the known historic eruptions in the contiguous United Stateshave been from the Cascade Volcanoes. Two most recent were Lassen Peakin 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 Meagervolcanic complex. [VNUM|1=1200-18-|2=Meager]
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 Rainierand Mount Shasta, exceed 14,000 ft (4,300 m). By volume, the two largest Cascade volcanoes are the broad shields of Medicine Lake Volcanoand Newberry Volcano, which are about 145 mi³ (600 km³) and 108 mi³ (450 km³) respectively. Mount Garibaldiand Glacier Peakare 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. [ [http://www.washington.edu/burkemuseum/geo_history_wa/Cascade%20Episode.htm The Cascade Episode] Retrieved on 2007-10-03] 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 Peakin California, which last erupted in 1917, is the southernmost historically active volcano in the arc, while Mount Meagerin 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 calderacomplex, may also be the product of Cascadia subduction, but geologic investigations have been very limited in this remote region. About 5-7 million years ago, the northern end of the Juan de Fuca Platebroke off along the Nootka Faultto form the Explorer Plate, and there is no definitive consensus among geologists on the relation of the volcanoes north of that fault to the rest of the Cascade Arc. When the Cascade Volcanic Arc resumed 4-5 million years ago after reorganization of the Explorer Plate, there were some apparent changes along the northern end. Where the northern end of the arc originally extended due north from the modern-day location of Glacier Peak- into the Chilliwack Batholithand the Pemberton Volcanic Beltin Canada, it now headed northwest into the Mount Baker- Garibaldi Volcanic Belt. This apparently reflects a steepening of the subduction zone on the northern end of the Juan de Fuca Plate. At the same time, the Juan de Fuca Plate assumed a more easterly-directed sense of motion relative to the continent. However, the Pemberton Volcanic Belt is usually merged with the Garibaldi Volcanic Belt and so is usually part of the arc. Between 18 and 5 million years ago, the southern end of the Juan de Fuca Plate broke off to form the Gorda Plate, and continues to subduct beneath North America. 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 Rangehas 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 Parkand adjacent parts of British Columbiabeyond. 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.
Pemberton Volcanic Beltis an eroded volcanic beltnorth of the Garibaldi Volcanic Belt, which appears to have formed during the Miocenebefore fracturing of the northern end of the Juan de Fuca Plate. The Silverthrone Calderais the only volcano within the belt that appears related to seismic activity since 1975.
Garibaldi Volcanic Beltis the northern extension of the Cascade Arc. Volcanoes within the volcanic beltare mostly stratovolcanoes along with the rest of the arc, but also include calderas, cinder cones, and small isolated lavamasses. The eruption styles within the belt range from effusive to explosive, with compositions from basaltto rhyolite. Due to repeated continental and alpine glaciations, many of the volcanic deposits in the belt reflect complex interactions between magmacomposition, topography, and changing iceconfigurations. Four volcanoes within the belt appear related to seismic activity since 1975, including: Mount Meager, Mount Garibaldiand Mount Cayley. Mount Meageris the most unstable volcanic massif in Canada. It has dumped clayand rock several meters deep into the Pemberton Valleyat least three times during the past 7,300 years. Recent drilling into the Pemberton Valley bed encountered remnants of a debris flowthat 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 debrisat 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 Rainierhas 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 Tacomaand 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 Volcanoes (a geological term) includes volcanoes such as
Mount Meagerand 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. However this terminology is not universally adhered to; in particular the phrase "the volcanoes of the High Cascades" is sometimes understood to include the peaks north of the Fraser, and sometimes not. Fact|date=February 2007
Native Americans have inhabited the area for thousands of years and developed their own myths and
legends concerning the Cascade volcanoes. According to some of these tales, Mounts Baker, Jefferson, Shasta and Garibaldi were used as refuge from a great flood. Other stories, such as the Bridge of the Gods tale, had various High Cascades such as Hood and Adams, act as god-like chiefs who made warby throwing fire and stone at each other. St. Helens with its pre-1980 graceful appearance, was regaled as a beautiful maiden for whom Hood and Adams feuded. Among the many stories concerning Mount Baker, one tells that the volcano was formerly married to Mount Rainier and lived in that vicinity. Then, because of a marital dispute, she picked herself up and marched north to her present position. Native tribes also developed their own names for the High Cascades and many of the smaller peaks, the most well-known to non-natives being Tahoma, the Lushootseedname for Mount Rainier.
Legends associated with the great volcanoes are many, as well as with other peaks and geographical features of the arc, including its many hot springs and waterfalls and rock towers and other formations. Stories of Tahoma - today
Mount Rainierand the namesake of Tacoma, Washington- allude to great, hidden grottos with sleeping giants, apparitions and other marvels in the volcanoes of Washington, and Mount Shastain California has long been well-known for its associations with everything from Lemurians to aliens to elves and, as everywhere in the arc, Sasquatchor Bigfoot.
In the spring of 1792 British navigator
George Vancouverentered Puget Soundand started to give English names to the high mountains he saw. Mount Bakerwas named for Vancouver's third lieutenant, the graceful Mount St. Helensfor a famous diplomat, Mount Hoodwas named in honor of Samuel Hood, 1st Viscount Hood(an admiralof the Royal Navy) and the tallest Cascade, Mount Rainier, is the namesake of Admiral Peter Rainier. Vancouver's expedition did not, however, name the arc these peaks belonged to. As marine trade in the Strait of Georgiaand Puget Soundproceeded in the 1790s and beyond, the summits of Rainier and Baker became familiar to captains and crews (mostly British and American over all others, but not exclusively).
With the exception of the 1915 eruption of remote
Lassen Peakin Northern California, the arc was quiet for more than a century. Then, on May 18, 1980, the dramatic eruption of little-known Mount St. Helensshattered the quiet and brought the world's attention to the arc. Geologists were also concerned that the St. Helens eruption was a sign that long-dormant Cascade volcanoes might become active once more, as in the period from 1800 to 1857 when a total of eight erupted. None have erupted since St. Helens, but precautions are being taken nevertheless, such as the Mount Rainier Volcano LaharWarning System in Pierce County, Washington. [ [http://volcanoes.usgs.gov/About/Highlights/RainierPilot/Pilot_highlight.html Pilot Project: Mount Rainier Volcano Lahar Warning System] Retrieved on 2007-10-06] Hot springs in the Canadian side of the arc, were originally used and revered by First Nationspeople. The springs located on Meager Creekare called "Teiq" in the language of the Lillooet Riverand were the farthest up the Lillooet Riverthe spirit-beings/wizards known as "the Transformers" reached during their journey into the Lillooet Country, and a "training" place for young First Nations men who would privite themselves at the at the springs to acquire power and knowledge. In this area, also, was found the blackstone chief's head pipe that is famous of Lillooet artifacts; found buried in volcanic ash, one supposes from the 2350 BP eruption of Mount Meager.
Cascadia subduction zone
The Cascade Volcanoes were formed by the
subductionof the Juan de Fuca, Explorer and the Gorda Plate(remnants of the much larger Farallon Plate) under the North American Platealong the Cascadia subduction zone. This is a 680 mi (1,094 km) long fault, running 50 mi (80 km) off the west-coast of the Pacific Northwestfrom northern Californiato Vancouver Island, British Columbia. The plates move at a relative rate of over 0.4 inches (10 mm) per year at a somewhat oblique angle to the subduction zone.
Because of the very large fault area, the Cascadia subduction zone can produce very large earthquakes, magnitude 9.0 or greater, if rupture occurred over its whole area. When the "locked" zone stores up energy for an earthquake, the "transition" zone, although somewhat plastic, can rupture. Thermal and deformation studies indicate that the locked zone is fully locked for 60 kilometers (about 40 miles) downdip from the deformation front. Further downdip, there is a transition from fully locked to aseismic sliding.
Unlike most subduction zones worldwide, there is no
oceanic trenchpresent along the continental marginin Cascadia. [http://www2.nature.nps.gov/geology/usgsnps/province/cascade2.html Pacific Mountain System - Cascade volcanoes] ] Instead, terranes and the accretionary wedge have been uplifted to form a series of coast ranges and exotic mountains. A high rate of sedimentation from the outflow of the three major rivers ( Fraser River, Columbia River, and Klamath River) which cross the Cascade Range contributes to further obscuring the presence of a trench. However, in common with most other subduction zones, the outer margin is slowly being compressed, similar to a giant spring. When the stored energy is suddenly released by slippage across the fault at irregular intervals, the Cascadia subduction zone can create very large earthquakes such as the magnitude 9 Cascadia earthquake of 1700.
Eruptions of the Cascades
There have been 11 eruptions in the past 4,000 years, and 7 in just the past 200 years. The Cascade volcanoes have had more than 100 eruptions over the past few thousand years, many of them
explosive eruptions. However, certain Cascade volcanoes can be dormant for hundreds or thousands of years between eruptions, and therefore the great risk caused by volcanic activity in the regions is not always readily apparent.
When Cascade volcanoes do erupt,
pyroclastic flows, lavaflows, and landslides can devastate areas 10 or more miles away; and huge mudflows of volcanic ashand debris, called lahars, can inundate valleys more than convert|50|mi|km downstream. Falling ash from explosive eruptions can disrupt humanactivities hundreds of miles downwind, and drifting clouds of fine ash can cause severe damage to jet aircrafteven thousands of miles away.
All of the known historical eruptions have occurred in
Washington, Oregonand in Northern California. The two most recent were Lassen Peakin 1914 to 1921 and a major eruption of Mount St. Helens in 1980. Minor eruptions of Mount St. Helens have also occurred, most recently in 2006. [ [http://vulcan.wr.usgs.gov/Volcanoes/MSH/Eruption04/framework.html Mount St. Helens, Washington Eruption 2004 to Current] Retrieved on 2007-10-06] In contrast, volcanoes in southern British Columbia, central and southern Oregon are currently dormant. The regions lacking new eruptions keep in touch to positions of fracture zones that offset the Gorda Ridge, Explorer Ridgeand the Juan de Fuca Ridge. The volcanoes with historical eruptions include: Mount Rainier, Glacier Peak, Mount Baker, Mount Hood, Lassen Peak, and Mount Shasta.
Renewed volcanic activity in the Cascade Arc, such as the
1980 eruption of Mount St. Helens, has offered a great deal of evidence about the structure of the Cascade Arc. One effect of the 1980 eruption was a greater knowledge of the influence of landslides and volcanic development in the evolution of volcanic terrain. A vast piece on the north side of Mount St. Helens dropped and formed a jumbled landslide environment several kilometers away from the volcano. Pyroclastic flows and lahars moved across the countryside. Parallel episodes have also happened at Mount Shastaand other Cascade volcanoes in prehistoric times.
Major catastrophic eruptions
; 1980 Eruption of Mount St. Helens
1980 eruption of Mount St. Helenswas one of the most closely studied volcanic eruptions in the arc and one of the best studied ever. It was a Plinian style eruption with a VEI=5 and was the most significant to occur in the lower 48 U.S. states in recorded history. An earthquakeat 8:32 a.m. on May 18, 1980, caused the entire weakened north face to slide away. An ash column rose high into the atmosphereand deposited ash in 11 U.S. states. The eruption killed 57 people and thousands of animals and caused more than a billion U.S. dollars in damage.
; 1914–17 Eruptions of Lassen Peak
May 22, 1915, an explosive eruption at Lassen Peak devastated nearby areas and rained volcanic ashas far away as 200 miles (320 km) to the east. [http://pubs.usgs.gov/fs/1998/fs173-98/ USGS: Eruptions of Lassen Peak, California, 1914 to 1917] ] A huge column of volcanic ash and gas rose more than convert|30000|ft|m into the air and was visible from as far away as Eureka, California, 150 miles (240 km) to the west. A pyroclastic flowswept down the side of the volcano, devastating a convert|3|sqmi|km2|sing=on area. This explosion was the most powerful in a 1914–17 series of eruptions at Lassen Peak. ; 2350 BP Eruption of Mount Meager Mount Meagerproduced the most recent major eruption in Canada, sending ash as far away as Alberta. [http://gsc.nrcan.gc.ca/volcanoes/cat/feature_meager_e.php Catalogue of Canadian Volcanoes: Mount Meager] Retrieved on 2007-07-16] The eruption was similar to the 1980 eruption of Mount St. Helens, sending an ash column approximately 20 km high into the stratosphere. This activity produced a diverse sequence of volcanic deposits, well exposed in the bluffs along the Lillooet River, which is defined as the Pebble Creek Formation. The eruption was episodic, occurring from a vent on the north-east side of Plinth Peak. [http://www.bivouac.com/MtnPg.asp?MtnId=953 Mount Meager] in the Canadian Mountain Encyclopedia. Retrieved on 2007-07-16] An unusual, thick apron of welded vitrophyric breccia may represent the explosive collapse of an early lava dome, depositing ash several meters in thickness near the vent area. [http://www.eos.ubc.ca/~krussell/epapers/bv_hrs99.pdf Volcanology of the 2350 B.P. Eruption of the Mount Meager Volcanic Complex] Retrieved on 2007-07-16]
; 7700 BP Eruption of Mount Mazama
The 7700 BP eruption of
Mount Mazamawas a large catastrophic eruption in U.S. state of Oregon. It is estimated to have been 42 times larger than the 1980 eruption of Mount St. Helens. It began with a large eruption columnwith pumiceand ash that erupted from a single vent. The eruption was so great that most of Mount Mazama collapsed to form a calderaand subsequent smaller eruptions occurred as water began to fill in the caldera to form Crater Lake. Volcanic ash from the eruption was carried across most of the Pacific Northwestas well as parts of southern Canada.
; 13100 BP Eruption of Glacier Peak
; Silverthrone Caldera
Most of the
Silverthrone Caldera's eruptions occurred during the last ice ageand was episodically active during both Pemberton and Garibaldi Volcanic Beltstages of volcanism. The last eruption from Mount Silverthrone ran up against ice in Chernaud Creek. The lava was dammed by the ice and made a cliffwith a waterfallup against it.
; Mount Cayley
Mount Cayleylast erupted about 20,000 years ago. It contains several complex features which probably represent multiple eruptions under different conditions and are difficult to classify.
; Mount Garibaldi
Mount Garibaldiwas last active about 10,700 to 9,300 years ago from a cinder conecalled Opal Cone. It produced a 15 km long broad dacitelava flow with prominent wrinkled ridges. The lava flow is unusually long for a silicic lava flow.
; Mount Baker
During the mid-1880s,
Mount Bakerhad volcanic activity for the first time in several thousand years. Fumaroleactivity remains in Sherman Crater, close to the volcano's main peak, became more intense in 1975 and is still energetic. However there is still no proof that an eruption is about to happen at the volcano. [http://pubs.usgs.gov/fs/1997/fs165-97/ Living With Volcanic Risk in the Cascades] Retrieved on 2007-10-03]
; Glacier Peak
; Mount Rainier
Mount Rainierlast erupted between 1824 and 1854, but many eyewitnesses reported eruptive activity in 1858, 1870, 1879, 1882 and in 1894 as well. Mount Rainier has created at least four eruptions and many lahars in the past 4,000 years.
; Mount Adams
Mount Adams was last active about 1,000 years ago and has created few eruptions during the past several thousand years.
; Mount Hood
Mount Hoodwas last active about 200 years ago, creating pyroclastic flows, lahars, and a well-known lava domeclose to its peak called Crater Rock. Between 1856 and 1865, a sequence of steam explosions took place at Mount Hood.
; Newberry Volcano
A great deal of volcanic activity has occurred at
Newberry Volcano, which was last active about 1,300 years ago. It has one of the largest collections of cinder cones, lava domes, lava flows and fissures in the world.
; Medicine Lake Volcano
Medicine Lake Volcanohas erupted about 8 times in the past 4,000 years and was last active about 1,000 years ago when rhyoliteand daciteerupted at Glass Mountainand associated vents near the caldera's eastern rim.
; Mount Shasta
Mount Shastalast erupted in 1786 and has been the most active volcano in Californiafor about 4,000 years, erupting once every 300 years. The 1786 eruption created a pyroclastic flow, a laharand three cold lahars, which streamed 7.5 miles (12 km) down Shasta's east flank via Ash Creek. A separate hot lahar went 12 miles (19 km) down Mud Creek.
List of Cascade Volcanoes
Garibaldi Volcanic Belt
List of Cascade volcanoes
List of volcanoes in the United States of America
List of volcanoes in Canada
Volcanism in Canada
Geology of the Pacific Northwest
* [http://www.skimountaineer.com/CascadeSki/CascadeIntro.html Skiing the Cascade Volcanoes - Introduction to the Cascade Volcanoes]
* [http://vulcan.wr.usgs.gov/Volcanoes/Cascades/framework.html USGS Cascades Volcano Observatory]
* [http://www.volcano.si.edu/ Smithsonian Institution Global Volcanism Program]
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