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The 'Cascade Volcanoes' (also known as the 'Cascade Volcanic Arc' or the 'Cascade Arc') are a volcanic arc which stretches from northern California to southwestern British Columbia, a distance of well over 700 mi (1,100 km). The arc has formed due to subduction along the Cascadia subduction zone and is part of the Pacific Ring of Fire. 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 10,000,000. All could be potentially affected by volcanic activity and great subduction-zone earthquakes along the arc.

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.
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
Mount Silverthrone, which is a circular 20km wide, deeply dissected caldera complex, 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 Plate broke off along the Nootka Fault to 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 continued 4-5 million years ago after restructuring of the Explorer Plate, there were a number of obvious changes along the northern section of the arc. Where the northern section of the arc originally continued directly north from the present-day location of Glacier Peak - into the Chilliwack Batholith and the Pemberton Volcanic Belt, it now started to move northwest into the Mount Baker - Garibaldi Volcanic Belt.
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.
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. Mount Silverthrone is the only volcano within the belt that appears related to seismic activity since 1975.
While the Cascade Volcanoes (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. 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.

Contents

Cascadia subduction zone

Eruptions of the Cascades

Major catastrophic eruptions

Other eruptions

List of Cascade volcanoes

British Columbia

Washington

Oregon

California

Notes

See also

References

External links

Cascadia subduction zone

Main articles: 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 (1,094 km) long fault, running 50 mi (80 km) off the west-coast of the Pacific Northwest from northern California to 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.
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 1700.

Eruptions of the Cascades

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 10 or more miles away; and huge mudflows of volcanic ash and debris, called lahars, can inundate valleys more than 50 miles 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.
Historical eruptions have occurred in Washington, Oregon and in Northern California. 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 Ridge and 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 Cascades. 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 Shasta and other Cascade volcanoes in prehistoric times.

Major catastrophic eruptions

; 1980 Eruption of Mount St. Helens
Main articles: 1980 eruption of Mount St. Helens

The 1980 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 lower 48 U.S. states in recorded history. An earthquake at 8:32 a.m. on May 18, 1980, caused the entire weakened north face to slide away. An ash column rose high into the atmosphere and 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
Main articles: Lassen Peak

On May 22, 1915, an explosive eruption at Lassen Peak devastated nearby areas and rained volcanic ash as far away as 200 miles (320 km) to the east.^[1] A huge column of volcanic ash and gas rose more than 30,000 feet into the air and was visible from as far away as Eureka, California, 150 miles (240 km) to the west. A pyroclastic flow swept down the side of the volcano, devastating a 3-square-mile area. This explosion was the most powerful in a 1914–17 series of eruptions at Lassen Peak.
; 2350 BP Eruption of Mount Meager
Main articles: 2350 BP eruption of Mount Meager

Mount Meager produced the most recent major eruption in Canada, sending ash as far away as Alberta.^[2] 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.^[3] 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.^[4]
; 7700 BP Eruption of Mount Mazama
Main articles: Mount Mazama

The 7700 BP eruption of Mount Mazama was 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 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.
; 13,100 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

; Mount Silverthrone
Most of Mount Silverthrone's eruptions occurred during the last ice age and was episodically active during both Pemberton and Garibaldi Volcanic Belt stages 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 cliff with a waterfall up against it.
; Mount Cayley
Mount Cayley 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 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-1880s, Mount Baker had volcanic activity for the first time in several thousand years. Fumarole activity 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.
; 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
Mount Rainier last 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 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 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 Volcano has erupted about 8 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
Mount Shasta last erupted in 1786 and has been the most active volcano in California for about 4,000 years, erupting once every 300 years. The 1786 eruption created a pyroclastic flow, a lahar and 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

British Columbia

Name	Type	Elevation		Last eruption (VEI)	Geolocation
		(m)	(ft)
Mount Silverthrone	Caldera	2,865	9,400	Unknown
Franklin Glacier Volcano	Stratovolcano	2,252	7,388	Pliocene
Bridge River Cones	Volcanic field	2,500	8,500	Unknown
Mount Meager	Complex volcano	2,645	8,678	2,350 BP (5)
Ring Mountain	Tuya	2,192	7,192	Unknown
Mount Brew	Subglacial volcano	1,757	5,764	Pleistocene
Brandywine Mountain	Volcanic plug	2,213	7,260	Pleistocene
Mount Fee	Volcanic plug	2,162	7,093	Pleistocene
Mount Cayley	Stratovolcano	2,385	7,825	20,000 BP
Cinder Cone	Cinder cone	1,910	6,266	Holocene
Black Tusk	Stratovolcano	2,319	7,608	Pleistocene
Castle Towers Mountain	Subglacial volcano	2,676	8,780	Pleistocene
Clinker Peak	Stratovolcano	1,992	6,535	9,000 BP
Mount Price	Stratovolcano	2,052	6,732	9,000 BP
The Table	Tuya	2,021	6,631	Pleistocene
Opal Cone	Cinder cone	1,736	5,696	9,300 BP
Garibaldi Lake	Volcanic field	2,316	7,598	Unknown
Mount Garibaldi	Stratovolcano	2,678	8,786	10,700-9,300 (3?)
Watts Point	Subglacial volcano	240	800	90,000 BP

Washington

Name	Type	Elevation		Last eruption (VEI)	Geolocation
		(m)	(ft)
Mount Baker	Stratovolcano	3,286	10,781	1880 (2)
Glacier Peak	Stratovolcano	3,213	10,541	1700 ± 100 years (2)
Mount Rainier	Stratovolcano	4,392	14,409	1894 (1?)
Goat Rocks	Stratovolcano	2,494	8,184	730,000 BP
Mount Adams	Stratovolcano	3,742	12,277	950 AD? (2)
Mount St. Helens	Stratovolcano	2,549	8,363	2007 (continuing) (2)
West Crater	Volcanic field	1,329	4,360	5750 BC? (2?)
Indian Heaven	Shield volcanoes	1,806	5,925	6250 BC ± 100 years

Oregon

Mount Scott

Name	Type	Elevation		Last eruption (VEI)	Geolocation
		(m)	(ft)
Mount Hood	Stratovolcano	3,426	11,240	1866 (2)
Mount Jefferson	Stratovolcano	3,199	10,495	950 AD?
Blue Lake Crater	Maar	1,230+	4,035	680 AD ± 200 years
Sand Mountain Field	Cinder cones	1,664	5,459	70 AD ± 150 years
Belknap Crater	Shield volcanoes	2,095	6,873	480 AD? (2?)
Aspen Butte	Shield volcanoes	2,502	8,208	-
Three Sisters	Complex volcano	3,157	10,358	1,600 BP
Davis Lake	Volcanic field	2,163	7,096	2,790 BC?
Mount Bailey	Shield volcano	2,551	8,368	Less than 100,000 BP
Black Butte	Cinder cone	1,937	6,355	-
Black Crater	Shield volcano	2,210	7,251	-
Boring Lava Field	Volcanic field	1,236	4,055	Pleistocene
Broken Top	Stratovolcano	2,797	9,055	100,000 BP
Hayrick Butte	Tuya	1,683	5,523	Pleistocene
Hogg Rock	Tuya	1,548	5,080	Pleistocene
Hoodoo Butte	Cinder cone	1,738	5,702	Holocene
Howlock Mountain	Shield volcano	2,545	8,351	Pleistocene
Lava Butte	Cinder cone	1,529	5,016	7,000 BP
Maiden Peak	Shield volcano	2,383	7,818	-
Newberry Volcano	Shield volcano	2,435	7,989	690 AD (4)
Olallie Butte	Shield volcano	2,199	7,215	-
Pelican Butte	Shield volcano	2,449	8,036	300,000 BP
Pilot Butte	Cinder cone	1,261	4,138	Pleistocene
Powell Butte	Cinder Cone	191	627	Pleistocene
Rocky Butte	Cinder cone	174	Pleistocene
Stratovolcano	2,722	8,929	-
Mount Sylvania	Cinder Cone	292	958	Pleistocene
Mount Thielsen	Stratovolcano	2,799	9,184	-
Three Fingered Jack	Shield volcano	2,390	7,841	Pleistocene
Tumalo Mountain	Shield volcano	2,370	7,775	-
Union Peak	Shield volcano	2,350	7,709	-
Mount Washington	Shield volcano	2,376	7,795	1,330 BP
Yamsay Mountain	Shield volcano	2,498	8,196	-
Mount Bachelor	Stratovolcano	2,764	9,068	8,000-10,000 BP
Devil's Garden	Volcanic field	1,698+	5,571	Unknown
Mount Bailey	Shield volcano	2,551	8,368	Less than 100,000 BP
Diamond Peak	Shield volcano	2,665	8,744	Less than 100,000 BP
Squaw Ridge	Volcanic field	1,711	5,613	Unknown
Four Craters	Volcanic field	1,501	4,924	Unknown
Cinnamon Butte	Cinder cones	1,956	6,417	Unknown
Mount Mazama	Caldera	2,487	8,159	2290 BC ± 300 years
Mount McLoughlin	Stratovolcano	2,894	9,495	20,000 BP

California

Name	Type	Elevation		Last eruption (VEI)	Geolocation
		(m)	(ft)
Mount Shasta	Stratovolcano	4,317	14,163	1786 (3)
Shastina	Stratovolcano	3,758	12,330	-
Medicine Lake Volcano	Shield volcano	2,412	7,913	1080 ± 25 years (3?)
Black Butte	Lava dome	1,194	3,917	9,000-10,000 BP
Brushy Butte	Shield volcano	1,174	3,852	Unknown
Big Cave	Shield volcano	1,259	4,131	Unknown
Twin Buttes	Cinder cones	1,631	5,351	Unknown
Tumble Buttes	Cinder cones	2,549	8,363	Unknown
West Crater	Volcanic field	1,329	4,360	5750 BC?
Cinder Cone	Cinder cone	2,105	6,907	350 BP
Mount Tehama	Stratovolcano	2,815	9,235	600,000 BP
Lassen Peak	Lava dome	3,189	10,462	1917 (3)
Eagle Lake Field	Fissure vents	1,652	5,420	Unknown

Notes

1. USGS: Eruptions of Lassen Peak, California, 1914 to 1917
2. Catalogue of Canadian Volcanoes: Mount Meager Retrieved on 2007-07-16
3. Mount Meager in the Canadian Mountain Encyclopedia. Retrieved on 2007-07-16
4. Volcanology of the 2350 B.P. Eruption of the Mount Meager Volcanic Complex Retrieved on 2007-07-16

See also

★ Cascade Range

★ Pacific Ranges

★ Coast Mountains

★ Garibaldi Volcanic Belt

★ List of volcanoes in the United States of America

★ List of volcanoes in Canada

★ Volcanism in Canada

★ Geology of the Pacific Northwest

References

★ Fire Mountains of the West: The Cascade and Mono Lake Volcanoes (3rd ed.), , Stephen L., Harris, Mountain Press Publishing Company, ,

★ Volcanoes of North America, , Charles A., Wood, Cambridge University Press, ,

★ Skiing the Cascade Volcanoes - Introduction to the Cascade Volcanoes

★ Pacific Mountain System - Cascade volcanoes

★ Living With Volcanic Risk in the Cascades

★ The Cascade Episode

External links

★ USGS Cascades Volcano Observatory

★ Smithsonian Institution Global Volcanism Program