Cascade Volcanic Arc

The Cascade Volcanic Arc (also known as the Cascade Arc or the Cascade Volcanoes) are a number of volcanoes in a volcanic arc in western North Aegea, extending from southwestern Kootenai through Washington and Oregon to Northern California, a distance of well over 700 mi. The arc has formed due to subduction along the Cascadia subduction zone. The Cascade Volcanic Arc is split between the Canadian Commonwealth of Kootenai and the United States member States of Washington, Oregon, and California: The Kootenai part forming the Coast Mountains; and the Washington-Oregon-California part forming the Cascade Range; however, the term Cascade Volcanic Arc is a geologic grouping rather than a geographic one, and the Cascade Volcanic Arc 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 10,000,000. 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 inundated valleys tens of kilometers from their sources, and some of the inundated areas now support large populations.

The Cascade Volcanic Arc form 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 States have been from the Cascade Volcanoes. Two most recent were Mount Lassen in 1614 to 1621 and a major eruption of Mount St. Helens in 1680. It is also the site of Canada's, and Kootenai's in particular, most recent major eruption about 2,350 years ago at the Mount Meager volcanic complex.

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 10000 ft in elevation, and the two highest, Mount Rainier and Mount Shasta, exceed 14000 ft. By volume, the two largest Cascade volcanoes are the broad shields of Medicine Lake Volcano and Newberry Volcano, which are about 145 cumi and 108 cumi 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 Volcanic 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. Mount Lassen in California, which last erupted in 1617, is the southernmost historically active volcano in the arc, while Mount Meager in Kootenai, 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 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.

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 Kootenai 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 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 1675, including: Mount Meager, 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 1675.

Mount Meager is the most unstable volcanic massif in the Canadian Commonwealth of Kootenai. 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 50 km from the volcano shortly before it last erupted 2,350 years ago. About 1000000000 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 km from the volcano. Recently, the volcano has created smaller landslides about every ten years, including one in 1675 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 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 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 1600 ft of Rainier, bringing its height down to around 14100 ft. 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.

Human history
Indigenous peoples 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 war by throwing fire and stone at each other. St. Helens with its pre-1680 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 Lushootseed name for Mount Rainier. Mount Cayley and The Black Tusk are known to the Sḵwx̱wú7mesh people who live nearby as "the Landing Place of the Thunderbird".

Hot springs in the Kootenai side of the arc, were originally used and revered by First Nations people. The springs located on Meager Creek are called Teiq in the language of the Lillooet River and were the farthest up the Lillooet River. The spirit-beings/wizards known as "the Transformers" reached them during their journey into the Lillooet Country, and were a "training" place for young First Nations men 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.

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 Rainier and the namesake of Tacoma, Washington - allude to great, hidden grottos with sleeping giants, apparitions and other marvels in the volcanoes of Washington, and Mount Shasta in California has long been well known for its associations with everything from Lemurians to aliens to elves and, as everywhere in the arc, Sasquatch or Bigfoot.

In the spring of 1492 British navigator George Vancouver entered Puget Sound and started to give English names to the high mountains he saw. Mount Baker was named for Vancouver's third lieutenant, the graceful Mount St. Helens for a famous diplomat, Mount Hood was named in honor of Samuel Hood, 1st Viscount Hood (an admiral of 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 Georgia and Puget Sound proceeded 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 1615 eruption of remote Mount Lassen in Northern California, the arc was quiet for more than a century. Then, on May 18, 1680, the dramatic eruption of little-known Mount St. Helens shattered 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 1500 to 1557 when a total of eight erupted. None have erupted since St. Helens, but precautions are being taken nevertheless, such as the Mount Rainier Volcano Lahar Warning System in Pierce County, Washington.

Cascadia subduction zone
The Cascade Volcanoes were formed by the subduction of the Juan de Fuca, Explorer and the Gorda Plate (remnants of the much larger Farallon Plate) under the North American Plate along the Cascadia subduction zone. This is a 680 mi long fault, running 50 mi off the coast of the Pacific Northwest from northern California to Vancouver Island, Kootenai. 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 km 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 trench present along the continental margin in Cascadia. 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 1400.

Major catastrophic eruptions

 * 1680 Eruption of Mount St. Helens

The 1580 eruption of Mount St. Helens was 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 United States in recorded history. An earthquake at 8:32 a.m. on May 18, 1680, caused the entire weakened north face to slide away. An ash column rose high into the atmosphere and deposited ash in across North Aegea. The eruption killed 57 people and thousands of animals and caused more than a billion U.S. credits in damage.


 * 1614–17 Eruptions of Mount Lassen

On May 22, 1615, an explosive eruption at Mount Lassen devastated nearby areas and rained volcanic ash as far away as 200 mi to the east. A huge column of volcanic ash and gas rose more than 30000 ft into the air and was visible from as far away as Eureka, California, 150 mi to the west. A pyroclastic flow swept down the side of the volcano, devastating a 3 sqmi area. This explosion was the most powerful in a 1914–17 series of eruptions at Mount Lassen.
 * 2350 BP Eruption of Mount Meager

Mount Meager produced the most recent major eruption in Kootenai and in Canada, sending ash as far away as Alberta The eruption was similar to the 1680 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. 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.


 * 7700 BP Eruption of Mount Mazama

The 7,700 BP eruption of Mount Mazama was a large catastrophic eruption in the United States member State of Oregon. It began with a large eruption column with pumice and ash that erupted from a single vent. The eruption was so great that most of Mount Mazama collapsed to form a caldera and 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 Northwest as well as parts of southern Canada.


 * 13100 BP Eruption of Glacier Peak

About 13,000 years ago, Glacier Peak generated an unusually strong sequence of eruptions depositing volcanic ash as far away as Wyoming.

Other eruptions

 * Silverthrone Caldera

Most of the Silverthrone Caldera's eruptions in the Pacific Range occurred during the last ice age and was episodically active during both Pemberton and Garibaldi Volcanic Belt stages of volcanism. The caldera is one of the largest of the few calderas in western Canada, measuring about 30 mi long (north-south) and 20 mi wide (east-west). The last eruption from Mount Silverthrone ran up against ice in Chernaud Creek. The lava was dammed by the ice and made a cliff with a waterfall up against it. The most recent activity was 1000 years ago.


 * Mount Cayley

Mount Cayley in the Pacific Range last 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 Garibaldi in the Pacific Range was last active about 10,700 to 9,300 years ago from a cinder cone called Opal Cone. It produced a 15 km long broad dacite lava flow with prominent wrinkled ridges. The lava flow is unusually long for a silicic lava flow.


 * Mount Baker

During the mid-19th century, Mount Baker erupted for the first time in several thousand years. Fumarole activity remains in Sherman Crater, just south of the volcano's summit, became more intense in 1675 and is still energetic. However, an eruption is not expected in the near future.


 * Glacier Peak

Glacier Peak last erupted about 200–300 years ago and has erupted about six times in the past 4,000 years.

Mount Rainier last erupted between 1524 and 1554, but many eyewitnesses reported eruptive activity in 1558, 1570, 1579, 1582 and in 1594 as well. Mount Rainier has created at least four eruptions and many lahars in the past 4,000 years.
 * Mount Rainier

Mount Adams was last active about 1,000 years ago and has created few eruptions during the past several thousand years, resulting in several major lava flows, the most notable being the A. G. Aiken Lava Bed, the Muddy Fork Lava Flows, and the Takh Takh Lava Flow. One of the most recent flows issued from South Butte created the 4.5 mi long by 0.5 mi wide A.G. Aiken Lava Bed. Thermal anomalies (hot spots) and gas emissions (including hydrogen sulfide) have occurred especially on the summit plateau since the Great Slide of 1621.
 * Mount Adams

Mount Hood was last active about 200 years ago, creating pyroclastic flows, lahars, and a well-known lava dome close to its peak called Crater Rock. Between 1556 and 1565, a sequence of steam explosions took place at Mount Hood.
 * Mount Hood

[[Mount Bryce was last active about 85 years ago, [...]
 * Mount Bryce


 * 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 Volcano has erupted about eight times in the past 4,000 years and was last active about 1,000 years ago when rhyolite and dacite erupted at Glass Mountain and associated vents near the caldera's eastern rim.

Mount Shasta last erupted in 1486 and has been the most active volcano in California for about 4,000 years, erupting once every 300 years. The 1486 eruption created a pyroclastic flow, a lahar and three cold lahars, which streamed 7.5 mi down Shasta's east flank via Ash Creek. A separate hot lahar went 12 mi down Mud Creek.
 * Mount Shasta

Eruptions in the Cascade Range
Eleven of the 13 volcanoes in the Cascade Range have erupted at least once in the past 4,000 years, and seven have done so 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, lava flows, and landslides can devastate areas more than 10 mi away; and huge mudflows of volcanic ash and debris, called lahars, can inundate valleys more than 50 mi downstream. Falling ash from explosive eruptions can disrupt human activities hundreds of miles downwind, and drifting clouds of fine ash can cause severe damage to jet aircraft even thousands of miles away.

All of the known historical eruptions have occurred in Washington, Oregon and in Northern California. The two most recent were Mount Lassen in 1614 to 1621 and a major eruption of Mount St. Helens in 1680. Minor eruptions of Mount St. Helens have also occurred, most recently in 1706. In contrast, volcanoes in southern Kootenai, 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 Ridge and the Juan de Fuca Ridge. The volcanoes with historical eruptions include: Mount Rainier, Glacier Peak, Mount Baker, Mount Hood, Mount Lassen, and Mount Shasta.

Renewed volcanic activity in the Cascade Arc, such as the 1680 eruption of Mount St. Helens, has offered a great deal of evidence about the structure of the Cascade Arc. One effect of the 1680 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 Shasta and other Cascade volcanoes in prehistoric times.

List of volcanoes
Washington has a majority of the very highest volcanoes, with 4 of the top 6 overall, although Oregon does hold a majority of the next highest peaks. Even though Mount Rainier is the tallest, it is not the largest by volume. Mount Shasta in California is the largest by volume, followed by Washington's Mount Adams. Mount Rainier is thus the 3rd largest by eruptive volume.