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Cascade Range Volcanoes and Volcanics



Cascade Range

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From: Hoblitt, Miller, and Scott, 1987, Volcanic Hazards with Regard to Siting Nuclear-Power Plants in the Pacific Northwest: USGS Open-File Report 87-297
... 13 volcanic centers (1.. Mount Baker, 2.. Glacier Peak, 3.. Mount Rainier, 4.. Mount St. Helens, 5.. Mount Adams, 6.. Mount Hood, 7.. Mount Jefferson, 8.. Three Sisters, 9.. Newberry Caldera, 10.. Crater Lake (Mount Mazama), 11.. Medicine Lake, 12.. Mount Shasta, 13.. Lassen Peak) ... are the most prominent volcanic features of Quaternary age in the Cascade Range ...

From: Wood and Kienle, 1990, Volcanoes of North America: United States and Canada: Cambridge University Press, 354p., p.149, Contribution by Charles A. Wood.
Holocene volcanism in the Cascades extends from the Garibaldi Volcanic Belt in southern British Columbia to the Lassen volcanic complex in northern California. Pronounced differences in the nature of volcanism occur along the arc. In Washington there are five, generally large, widely spaced stratovolcanoes, with only one ( Mount Adams) having significant nearly basaltic volcanics. In marked contrast, Oregon has six generally smaller stratovolcanoes, but the entire state is traversed by a 40-50-kilometer-wide band of basaltic to andesitic lava shields, cinder cones, and smaller stratovolcanoes that the "Cascade" cones rise above. South of Crater Lake, the Cascade arc bends perceptibly toward the southeast, and continues along this trend to Lassen Peak. Both Lassen and Shasta are associated with eastward halos of mafic shields and lava fields which, near Shasta, culminate in the huge shield volcano of Medicine Lake.

High Cascades and Western Cascades

From: U.S. Forest Service Website, Deschutes and Ochoco National Forests, 2002
High Cascades

The High Cascades province is characterized by a north-trending belt of upper Miocene to Quaternary volcanic rocks that were erupted on the east margin of the upper Eocene to Miocene Western Cascades province. The late Pleistocene record of this volcanic activity is well preserved on the crest of the High Cascades. The best exposed record of the early Pleistocene, Pliocene and late Miocene Cascade volcanism is found in volcanic and volcaniclastic deposits on the east flank of the range and in the adjacent Deschutes Basin.

Upper Pliocene and Quaternary rocks of the High Cascades form a broad platform of chiefly basalt and basaltic andesite volcanoes that fill a structurally subsided zone in the older rocks of the High Cascades. Mount Hood, Mount Jefferson, Three Sisters-Broken Top, and Mount Mazama (Crater Lake) are the four major Quaternary volcanic centers along this platform. These major volcanic centers have erupted lava flows and pyroclastic material that ranges in composition from basalt to dacite and with the exception of Mount Hood have also erupted rhyolite.

Volcanism has continued until recent times with the eruptions in the Belknap Crater area. The Belknap Crater area is a 19 miles long en echelon zone of vents extending from the base of the North Sister northward to the Santiam Pass. The zone consists of the Belknap shield volcano and numerous cinder cones and lava flows. Eruptions have occurred between 4,000 and 1,500 years ago. Many of these eruptions produced lava flows down the ancient valley of the McKenzie River and Lost Creek which dammed the rivers and formed lakes and waterfalls.

Western Cascades

The Western Cascades province is characterized as an older, deeply eroded volcanic range lying west of the more recent snow-covered High Cascade range. They range in elevation from 1,700 feet on the western margin to 5,800 feet on the eastern margin. The Western Cascades began to form 40 million years ago with eruptions from a chain of volcanoes near the Eocene shoreline. Volcanic activity gradually shifted to the east in the Miocene and Pliocene.

The Western Cascades are made up almost entirely of slightly deformed and partly altered volcanic flows and pyroclastic rocks which range in age from late Eocene to late Miocene. These rocks have been heavily dissected by erosion and the only evidence remaining of the many volcanoes from which they were erupted are occasional remnants of volcanic necks or plugs which mark former vents. There are also minor Pliocene to Pleistocene intracanyon lavas derived from the High Cascades or rare local vents.

From: Swanson, et.al., 1989, Cenozoic Volcanism in the Cascade Range and Columbia Plateau, Southern Washington and Northernmost Oregon: AGU Field Trip Guidebook T106.
The Cascade Range has been an active arc for about 36 million years as a result of plate convergence. Volcanic rocks between 55 and 42 million years ago occur in the Cascades, but are probably related to a rather diffuse volcanic episode that created the Challis arc extending southeastward from northern to northwest Wyoming. Convergence between the North American and Juan de Fuca plates continues at about 4 centimeters per year in the direction of North-50-degrees-East, a slowing of 2-3 centimeters per year since 7 million years ago. According to most interpretations, volcanism in the Cascades has been discontinuous in time and space, with the most recent episode of activity beginning about 5 million years ago and resulting in more than 3,000 vents. In Oregon, the young terrane is commonly called the High Cascades and the old terrane the Western Cascades, terms that reflect present physiography and geography. The terms are not useful in Washington, where young vents are scattered across the dominantly middle Miocene and older terrane. ...

From: Wood and Kienle, 1990, Volcanoes of North America: United States and Canada: Cambridge University Press, 354p., p.169, Contribution by David R. Sherrod
The Cascade Range in Oregon is customarily divided into two physiographic subprovinces; Western Cascades and High Cascades. The High Cascades subprovince is built of rocks mainly younger than 3.5 million years and is the modern Cascade Range volcanic arc. In contrast, the Western Cascades encompass a deeply eroded pile of chiefly Oligocene to Pliocene volcanic and volcaniclastic rocks. ...

The High Cascades in the areas south of Mount Jefferson to Santiam Pass is a broad ridge built up by several shield volcanoes and numerous cinder cones. Most summits mark either relatively young vents or deeply eroded vent complexes. Reversed polarized basaltic andesite, basalt, and andesite lava older than 0.73 million years are exposed in the walls of U-shaped canyons, but most of the area is mantled my normally polarized rocks younger than 0.73 million years. Most of the area is within the Mount Jefferson Wilderness.

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Plate Tectonics

From: Brantley, 1994, Volcanoes of the United States: USGS General Interest Publication
Volcanoes are not randomly distributed over the Earth's surface. Most are concentrated on the edges of continents, along island chains, or beneath the sea forming long mountain ranges. More than half of the world's active volcanoes above sea level encircle the Pacific Ocean to form the circum-Pacific "Ring of Fire." In the past 25 years, scientists have developed a theory -- called plate tectonics -- that explains the locations of volcanoes and their relationship to other large-scale geologic features.

According to this theory, the Earth's surface is made up of a patchwork of about a dozen large plates that move relative to one another at speeds from less than one centimeter to about ten centimeters per year (about the speed at which fingernails grow). These rigid plates, whose average thickness is about 80 kilometers, are spreading apart, sliding past each other, or colliding with each other in slow motion on top of the Earth's hot, pliable interior. Volcanoes tend to form where plates collide or spread apart, but they can also grow in the middle of a plate, as for example the Hawaiian volcanoes. ...

In the Pacific Northwest, the Juan de Fuca Plate plunges beneath the North American Plate. As the denser plate of oceanic crust is forced deep into the Earth's interior beneath the continental plate, a process known as subduction, it encounters high temperatures and pressures that partially melt solid rock. Some of this newly formed magma rises toward the Earth's surface to erupt, forming a chain of volcanoes (the Cascade Range) above the subduction zone. ...

Map, Juan de Fuca Subduction, click to enlarge [Map,20K,InlineGIF]
Map, Juan de Fuca Subduction - Juan de Fuca Ridge - Cascade Range
-- Modified from: Brantley, 1994

From: Wood and Kienle, 1990, Volcanoes of North America: United States and Canada: Cambridge University Press, 354p., p.148-149, Contribution by Charles A. Wood.
This volcanic province has the best understood tectonic setting in the western USA because subduction of remnants of the Farallon Plate is apparently still driving continental arc volcanism. The Cascade region is not a typical subduction zone, however, for there is very little seismic evidence of active subduction (Weaver and Baker, 1988) and there is no trench (McBirney and White, 1982). In fact, the existence of volcanic activity in the Cascades is the best evidence for ongoing subduction.

The remaining part of the Pacific Plate currently converging with the American Northwest is the Juan de Fuca Plate, with small platelets at its northern (Explorer Plate) and southern Gorda Plate) terminations. The Explorer Plate separated from the Juan de Fuca approximately 4 million years ago and is apparently no longer being subducted (Hyndman, et.al., 1979); the Gorda split away between 18 and 5 million years ago (Riddihough, 1984). The present slow rate of convergence (3-4 centimeters per year) of the Juan de Fuca Plate is only about half its value at 7 million years (Riddihough, 1984), which probably explains the reduced seismicity, lack of a trench, and debatable decline in volcanic activity. ...

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Volcanic Background

From: Foxworthy and Hill, 1982, Volcanic Eruptions of 1980 at Mount St. Helens, The First 100 Days: USGS Professional Paper 1249
Mount St. Helens is one of a group of high volcanic peaks that dominate the Cascade Range between northern California and southern British Columbia. The distribution of these volcanic peaks in a broad band that roughly parallels the coastline is typical of the so-called "Ring of Fire", a roughly circular array of volcanoes located on islands, peninsulas, and the margins of continents that rim the Pacific Ocean.

Even before it began erupting in 1980, Mount St. Helens and at least six other volcanoes in the Cascade Range were know the be "active" - that is, to have erupted at least once during historical time. Few major Cascade volcanoes are known to have been inactive long enough to be considered "extinct" or incapable of further eruption. Most display some evidence of residual volcanic heat, such as fumaroles, hot springs, or hot ground where snow melt is unusually rapid. ... Dramatic eruptive activity in the Cascades has been rare so far in the 20th century. Until the recent eruptions at Mount St. Helens, the only Cascade volcano that had a major eruption during this century was Lassen Peak in California. A series of intermittent eruptions of steam and volcanic ash beginning in May 1914 and lasting until 1921 climaxed, during the 4 days from May 19 to 22, 1915, in a series of violent events comprising small lava flows, massive lava-triggered mudflows, and explosive eruptions of ash. ... From the time when Lassen Peak quieted until March 1980, the only other known increase in activity at a Cascade volcano occurred at Mount Baker, when a sudden increase in emanations of heat, steam, and other gases from a previously steaming old crater began on March 10, 1975. Although new fumaroles were formed and minor amounts of "volcanic dust" and sulfur were emitted, "the greatest undesirable natural results" that were observed at Mount Baker were "an increase in local atmospheric pollution and a decrease in the quality of some local water resources" (Bortleson and others, 1977, p.B1). Since 1976, however, even those effects have subsided to levels only slightly higher than those that prevailed before 1975.

Eruptions of Cascade volcanoes tend to be much more explosive than those of, for example, the well-known Hawaiian volcanoes. This explosive tendency is related to the chemical composition of magma that feeds the volcanoes and to the amount of gas contained in the magma. Magma from the more explosive volcanoes contains relatively large amounts of gas and silicon and produces rocks such as andesite, dacite, or rhyolite. Magma from the less explosive volcanoes contains smaller concentrations of gas and silicon and produces basalt as well as andesite. Some Cascade volcanoes, including Mount St. Helens, have had nonexplosive eruptions of andesite and basalt, as well as explosive eruptions, in the past.

The existence, position, and recurrent activity of the Cascade volcanoes are generally though to be related to the convergence of shifting crustal plates.

Holocene Volcanism in the Cascades

From: Wood and Kienle, 1990, Volcanoes of North America: United States and Canada: Cambridge University Press, 354p., p.148-149, Contribution by Charles A. Wood
Holocene volcanism in the Cascades extends from the Garibaldi Volcanic Belt in southern British Columbia to the Lassen volcanic complex in northern California. Pronounced differences in the nature of volcanism occur along the arc. In Washington there are five, generally large, widely spaced stratovolcanoes, with only one ( Mount Adams) having significant nearly basaltic volcanics. In marked contrast, Oregon has six generally smaller stratovolcanoes, but the entire state is traversed by a 40-50-kilometer-wide band of basaltic to andesitic lava shields, cinder cones, and smaller stratovolcanoes that the "Cascade" cones rise above. South of Crater Lake, the Cascade arc bends perceptibly toward the southeast, and continues along this trend to Lassen Peak. Both Lassen and Shasta are associated with eastward halos of mafic shields and lava fields which, near Shasta, culminate in the huge shield volcano of Medicine Lake.

Guffanti and Weaver (1988) used the locations of 2,821 vents shown on the maps of Luedke and Smith (1981, 1982) to divide the Cascades into five segments, with a sixth extending south-eastward across the High Lava Plains. They note a volcano gap between Rainier and Glacier Peak which coincides with the shallowest dip (11 degrees) of the Cascade subduction zone, and they also infer a change in the configuration of the subducting slab between Shasta and Lassen. Even though their segment boundaries differ from Hughes et.al. (1980), Guffanti and Weaver similarly find that a number of otherwise inexplicable features of Cascade volcanism are controlled by segmentation.

Some researchers (e.g., Christiansen and Lipman, 1972) have suggested that Sutter Buttes and the Sonoma and Clear Lake volcanics, south and southwest of Lassen, are older extensions of subduction-related Cascade volcanism. this seems unlikely. If Sutter Buttes were part of a series of older Cascade stratovolcanoes abandoned due to the northward migration of the south end of Juan de Fuca Plate, the "last Cascade volcano" hypothesis would be tenable. But northward, arc volcanoes are young and active. In fact, why do the Cascades have such an abrupt southern termination?

The Clear Lake and Sonoma volcanics are the <5 million year components of a northwesterly younging line of volcanic fields of Tertiary to Holocene age (Hearn, et.al., 1981). All these volcanics lie within the San Andreas fault system, which appears to have provided magma access to the surface. Hearn, et.al., point out that the timing of the volcanism suggests that it follows termination of subduction, as the Mendocino triple junction migrated northward. They also propose that the volcano alignment reflects an underlying hot spot. That suggestion seems inconsistent with the northward movement of the Pacific Plate which most of the volcanics ride. These volcanics are among the closest to a subduction plate boundary of any in the world and will repay closer tectonic investigation. similarly, a tiny sliver of basalt dated at 3.57 million years (Prowell, 1974, quoted in Luedke and Smith, 198) occurs 45 kilometers east of Santa Cruz, California, near the Calaveras and Hayward faults. Apparently leakage of basalts along the San Andreas fault system has occurred repeatedly.

From: Swanson, et.al., 1989, Cenozoic Volcanism in the Cascade Range and Columbia Plateau, Southern Washington and Northernmost Oregon: AGU Field Trip Guidebook T106.
The Cascade Range has been an active arc for about 36 million years as a result of plate convergence. Volcanic rocks between 55 and 42 million years ago occur in the Cascades, but are probably related to a rather diffuse volcanic episode that created the Challis arc extending southeastward from northern to northwest Wyoming. Convergence between the North American and Juan de Fuca plates continues at about 4 centimeters per year in the direction of North-50-degrees-East, a slowing of 2-3 centimeters per year since 7 million years ago. According to most interpretations, volcanism in the Cascades has been discontinuous in time and space, with the most recent episode of activity beginning about 5 million years ago and resulting in more than 3,000 vents.

In Oregon, the young terrane is commonly called the High Cascades and the old terrane the Western Cascades, terms that reflect present physiography and geography. The terms are not useful in Washington, where young vents are scattered across the dominantly middle Miocene and older terrane. ...

In Washington and Oregon, a striking contrast has existed for the past 5 million years in the style of volcanism in the Cascades relative to geography. North of Mount Rainier, young volcanism is concentrated in only a few isolated andesitic and dacitic composite cones (notably Glacier Peak, Mount Baker, and the volcanoes of the Garibaldi belt in British Columbia), whereas south of Mount Hood moderate-sized andesitic and dacitic composite cones are relatively unimportant features of a landscape dominated by small andesite and basalt vents. The area between Mounts Rainier and Hood is transitional; large andesite and dacite composite cones ( Rainier, Adams, St. Helens, Hood, and the extinct Goat Rocks volcano) occur together with fields and scattered vents of olivine basalt ( Indian Heaven, Simcoe Mountains, and the King Mountain fissure zone south of Mount Adams. ...

The southern Washington Cascades are seismically active. Most earthquakes occur along the 100-kilometer-long, north-northwest trending St. Helens seismic zone, where most focal mechanisms show dextral slip parallel to the trend of the zone and consistent with the direction of plate convergence. Other crustal earthquakes concentrate just west of Mount Rainier and in the Portland (Oregon) area. Few earthquakes occur north of Mount Rainier or south of Mount Hood.

From tomography, Rasmussen and Humphreys (1988) interpret the subducted Juan de Fuca plate as a quasi-planar feature dipping about 65 degrees to about 300 kilometers under the southern Washington Cascades. The plate is poorly defined seismically, however, owing to a lack of earthquakes within it. Guffanti and Weaver (1988) show that the present volcanic front of the Washington Cascades, defined by the westernmost young vents, parallels the curved trend of the subducting plate reflected by the 60 kilometer-depth contour. The front trends northwest in northern Washington -- where Glacier Peak, Mount Baker, and the volcanoes of southern British Columbia occur along a virtually straight line -- and northeast in southern Washington. A 90-kilometer gap free of young volcanoes between Mount Rainier and Glacier Peak is landward of that part of the subducting plate with the least average dip to a depth of 60 kilometers. South of Portland, the volcanic front is offset 50 kilometers eastward and extends southward into California, probably still parallel to the trend of the convergent margin.

Earthquakes and Seismicity

From: Swanson, et.al., 1989, Cenozoic Volcanism in the Cascade Range and Columbia Plateau, Southern Washington and Northernmost Oregon: AGU Field Trip Guidebook T106.
The Cascade Range has been an active arc for about 36 million years as a result of plate convergence. ... The southern Washington Cascades are seismically active. Most earthquakes occur along the 100-kilometer-long, north-northwest trending St. Helens seismic zone, where most focal mechanisms show dextral slip parallel to the trend of the zone and consistent with the direction of plate convergence. Other crustal earthquakes concentrate just west of Mount Rainier and in the Portland (Oregon) area. Few earthquakes occur north of Mount Rainier or south of Mount Hood.

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Cascade Range Volcanoes

Mount Adams, Washington

From: Scott, et.al., 1995, Volcano Hazards in the Mount Adams Region, Washington USGS Open-File Report 95-492.
Mount Adams, one of the largest volcanoes in the Cascade Range, dominates the Mount Adams volcanic field in Skamania, Yakima, Klickitat, and Lewis counties and the Yakima Indian Reservation of south-central Washington. The nearby Indian Heaven and Simcoe Mountains volcanic fields lie west and southeast, respectively, of the 1,250 square kilometers (500 square miles) Adams field. Even though Mount Adams has been less active during the past few thousand years than neighboring Mounts St. Helens, Rainier, and Hood, it assuredly will erupt again. Future eruptions will probably occur more frequently from vents on the summit and upper flanks of Mount Adams than from vents scattered in the volcanic fields beyond. Large landslides and lahars that need not be related to eruptions probably pose the most destructive, far-reaching hazard of Mount Adams.

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Mount Baker, Washington

From: Gardner, et.al., 1995, Potential Volcanic Hazards from Future Activity of Mount Baker, Washington, USGS Open-File Report 95-498
Mount Baker (3,285 meters; 10,778 feet) is an ice-clad volcano in the North Cascades of Washington State about 50 kilometers (31 miles) due east of the city of Bellingham. After Mount Rainier, it is the most heavily glaciated of the Cascade volcanoes: the volume of snow and ice on Mount Baker (about 1.8 cubic kilometers; 0.43 cubic miles) is greater than that of all the other Cascades volcanoes (except Rainier) combined. Isolated ridges of lava and hydrothermally altered rock, especially in the area of Sherman Crater, are exposed between glaciers on the upper flanks of the volcano: the lower flanks are steep and heavily vegetated. The volcano rests on a foundation of non-volcanic rocks in a region that is largely non-volcanic in origin.

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Crater Lake, Oregon

From: U. S. National Park Service, Crater Lake National Park Website, 2001
Crater Lake is located in Southern Oregon on the crest of the Cascade Mountain range, 100 miles east of the Pacific Ocean. It lies inside a caldera, or volcanic basin, created when the 12,000-foot-high Mount Mazama collapsed 7,700 years ago following a large eruption. Generous amounts of winter snow, averaging 533 inches per year, supply the lake with water. There are no inlets or outlets to the lake. Crater Lake, at 1,958 feet deep, is the seventh deepest lake in the world and the deepest in the United States. Evaporation and seepage prevent the lake from becoming any deeper. The lake averages more than five miles in diameter, and is surrounded by steep rock walls that rise up to 2,000 feet above the lake's surface. Following the collapse of Mount Mazama, lava poured into the caldera even as the lake began to rise. Today, a small volcanic island, Wizard Island, appears on the west side of the lake. This cinder cone rises 760 feet above the lake and is surrounded by black volcanic lava blocks. A small crater, 300 feet across and 90 feet deep, rests on the summit. The crater is filled by snow during the winter months, but remains dry during the summer. The lake level fluctuates slightly from year to year. The highest level was reached in 1975 when the water level rose to 6,179.34 feet above sea level. The lowest level was recorded in 1942 when it dropped to 6,163.20 feet. For such a deep lake, the maximum observed variation of 16 feet is minor (less than 1 percent).

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Mount Garibaldi, British Columbia

From: Geological Survey of Canada Website, Terrain Sciences Division, Natural Resources Canada, March 2001
The alpine meadows, glaciers, and striking blue lakes of Garibaldi Provincial Park are set in a volcanic landscape of lava flows and cinder cone volcanoes. These landforms record the interaction of volcanic eruptions with glacial ice. The most recent volcanic activity occurred during the last Ice Age that ended 10,000 years ago. Mount Garibaldi, an eroded volcano, towers two and a half kilometers above downtown Squamish. Mount Garibaldi was built by violent volcanic eruptions 15 to 20 thousand years ago when the Squamish Valley was filled with a large glacier. Volcanic debris that formed the western flank of the volcano spread across the surface of the glacier. When the glacier later melted, the western side of the volcano collapsed into the Squamish Valley.

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Glacier Peak, Washington

From: Mastin and Waitt, 2000, Glacier Peak -- History and Hazards of a Cascade Volcano: USGS Fact Sheet 058-00
Glacier Peak is the most remote of the five active volcanoes in Washington State. It is not prominently visible from any major population center, and so its attractions, as well as its hazards, tend to be over-looked. Yet since the end of the last ice age, Glacier Peak has produced some of the largest and most explosive eruptions in the state. During this time period, Glacier Peak has erupted multiple times during at least six separate episodes, most recently about 300 years ago.

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Mount Hood, Oregon

From: Scott, et.al., 1997, Volcano Hazards in the Mount Hood Region, Oregon: USGS Open-File Report 97-89
Snow-clad Mount Hood dominates the Cascade skyline from the Portland metropolitan area to the wheat fields of Wasco and Sherman Counties. The mountain contributes valuable water, scenic, and recreational resources that help sustain the agricultural and tourist segments of the economies of surrounding cities and counties. Mount Hood is also one of the major volcanoes of the Cascade Range, having erupted repeatedly for hundreds of thousands of years, most recently during two episodes in the past 1,500 years. The last episode ended shortly before the arrival of Lewis and Clark in 1805. When Mount Hood erupts again, it will severely affect areas on its flanks and far downstream in the major river valleys that head on the volcano. Volcanic ash may fall on areas up to several hundred kilometers downwind. Eruptive activity at Mount Hood during the past 30,000 years has been dominated by growth and collapse of lava domes. The last two episodes of eruptive activity occurred 1,500 and 200 years ago. Repeated collapse of lava domes extruded near the site of Crater Rock, Mount Hood's youngest lava dome, generated pyroclastic flows and lahars and built much of the broad smooth fan on the south and southwest flank of the volcano.

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Mount Jefferson, Oregon

From: Walder, et.al., 1999, Volcano Hazards in the Mount Jefferson Region, Oregon: USGS Open-File Report 99-24
Mount Jefferson is a prominent feature of the landscape seen from highways east and west of the Cascades. Mount Jefferson (one of thirteen major volcanic centers in the Cascade Range) has erupted repeatedly for hundreds of thousands of years, with its last eruptive episode during the last major glaciation which culminated about 15,000 years ago. Geologic evidence shows that Mount Jefferson is capable of large explosive eruptions. The largest such eruption occurred between 35,000 and 100,000 years ago, and caused ash to fall as far away as the present-day town of Arco in southeast Idaho. Although there has not been an eruption at Mount Jefferson for some time, experience at explosive volcanoes elsewhere suggests that Mount Jefferson cannot be regarded as extinct. If Mount Jefferson erupts again, areas close to the eruptive vent will be severely affected, and even areas tens of kilometers (tens of miles) downstream along river valleys or hundreds of kilometers (hundreds of miles) downwind may be at risk.

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Lassen Peak, California

From: U. S. National Park Service Website, Geology Fieldnotes - Lassen Volcanic National Park, California, April 2000
Before the 1980 eruption of Mount St. Helens in Washington, Lassen Peak was the most recent volcanic outburst in the contiguous 48 states. The peak is the southernmost volcano in the Cascade Range which extends from here into Canada. The western part of the park features great lava pinnacles (huge mountains created by lava flows), jagged craters, and steaming sulphur vents. It is cut by spectacular glaciated canyons and is dotted and threaded by lakes and rushing clear streams. Snowbanks persist year-round and beautiful meadows are spread with wildflowers in spring. The eastern part of the park is a vast lava plateau more than one mile above sea level. Here are found small cinder cones (Fairfield Peak, Hat Mountain, and Crater Butte). Forested with pine and fir, this area is studded with small lakes, but it boasts few streams. Warner Valley, marking the southern edge of the Lassen Plateau, features hot spring areas (Boiling Springs Lake, Devils Kitchen, and Terminal Geyser). This forested, steep valley also has gorgeous large meadows. ... The Lassen geothermal area -- Sulphur Works, Bumpass Hell (largest), Little Hot Springs Valley, Boining Springs Lake, Devils Kitchen, and Terminal Geyser -- offer bubbling mud pots, steaming fumaroles, and boiling water. Some of these thermal features are getting hotter.

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Meager Mountain, British Columbia

From: Geological Survey of Canada Website, Terrain Sciences Division, Natural Resources Canada, March 2001
Mount Meager is a dormant volcano. However, about 2,400 years ago it erupted a great volcanic cloud that deposited ash as far east as Alberta. The eruption was similar in size to the 1980 eruption of Mount St. Helens. The earth beneath Mount Meager is hot. Surface waters seep under the volcano and become heated, then rise along fractures to reach the surface as hot springs. Holes have been drilled to 3,000 meters below the mountain to test this hot water plumbing system as a geothermal energy source. When hot water rises quickly in a drill hole it changes to steam; the force of this expanding steam can be used to generate electricity.

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Medicine Lake, California

From: U. S. National Park Service Website, Lava Beds National Monument, 2001
The Medicine Lake shield volcano, a sleeping giant, is the largest volcano in the Cascade Range. Filling up the entire southern skyline, it has been erupting off and on for half a million years. The eruptions were gentle rather than explosive like Mount St. Helens, coating the volcano's sides with flow after flow of basaltic lava. This created a shield-shaped mountain approximately 150 miles around the base and 7900 feet high. Medicine Lake is part of the old caldera, a bowl-shaped depression in the mountain. It is believed that the Medicine Lake volcano is unique, having many small magma chambers rather than one large one.

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Newberry Caldera, Oregon

From: MacLeod, et.al., 1981, Newberry Volcano, Oregon: IN: Guides to Some Volcanic Terranes in Washington, Idaho, Oregon, and Northern California: USGS Circular 838
Newberry Volcano, centered about 20 miles southeast of Bend, Oregon, is among the largest Quaternary volcanoes in thee conterminous United States. It covers and area in excess of 500 square miles, and lavas from it extend northward many tens of miles beyond the volcano. The highest point on the volcano, Paulina Peak with an elevation of 7,984 feet, is about 4,000 feet higher than the terrain surrounding the volcano. The gently sloping flanks, embellished by more than 400 cinder cones, consist of basalt and basaltic andesite flows, andesitic to rhyolitic ash-flow and air-fall tuffs and other types of pyroclastic deposits, dacite to rhyolite domes and flows, and alluvial sediments produced during periods of erosion of the volcano. At Newberry's summit is a 4- to 5-mile-wide caldera that contains scenic Paulina and East Lakes. The caldera has been the site of numerous Holocene eruptions, mostly of rhyolitic composition, that occurred as recently as 1,400 years ago. ... Newberry lies 40 miles east of the crest of the Cascade Range in a setting similar to Medicine Lake Volcano in California. Both volcanoes have the same shape, are marked by summit calderas, contain abundant rhyolitic domes and flows, have widespread ash flows in addition to the more areally extensive basalt and basaltic-andesite flows and their related cinder cones, have similar petrochemistry, and have been the sites of eruptions of pumiceous tephra and obsidian flows during the last few thousand years. Newberry lies at the west end of the High Lava Plains, a terrain formed of Miocene to Quaternary basalt flows and vents punctuated by rhyolitic domes and vent complexes.

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Mount Rainier, Washington

From: Thomas W. Sisson, 1995, History and Hazards of Mount Rainier, Washington: USGS Open-File Report 95-642
Mount Rainier is an active volcano that first erupted about half a million years ago. Because of Rainier's great height (14,410 feet above sea level) and northerly location, glaciers have cut deeply into its lavas, making it appear deceptively older than it actually is. Mount Rainier is known to have erupted as recently as in the 1840s, and large eruptions took place as recently as about 1,000 and 2,300 years ago. Mount Rainier and other similar volcanoes in the Cascade Range, such as Mount Adams and Mount Baker, erupt much less frequently than the more familiar Hawaiian volcanoes, but their eruptions are vastly more destructive. Hot lava and rock debris from Rainier's eruptions have melted snow and glacier ice and triggered debris flows (mudflows) - with a consistency of churning wet concrete - that have swept down all of the river valleys that head on the volcano. Debris flows have also formed by collapse of unstable parts of the volcano without accompanying eruptions. Some debris flows have traveled as far as the present margin of Puget Sound, and much of the lowland to the east of Tacoma and the south of Seattle is formed of pre-historic debris from Mount Rainier

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Mount Shasta, California

From: Miller, 1980, Potential Hazards from Future Eruptions in the Vicinity of Mount Shasta Volcano, Northern California: USGS Bulletin 1503
Mount Shasta is located in the Cascade Range in northern California about 65 kilometers (40 miles) south of the Oregon-California border and about midway between the Pacific Coast and the Nevada border. One of the largest and highest of the Cascade volcanoes, snowclad Mount Shasta is near the southern end of the range that terminates near Lassen Peak. Mount Shasta is a massive compound stratovolcano composed of overlapping cones centered at four or more main vents; it was constructed during a period of more than 100,000 years. Each of the cone-building periods produced pyroxene-andesite lava flows, block-and-ash flows, and mudflows originating mainly at the central vents. ... Construction of each cone was followed by eruption of domes and pyroclastic flows of more silicic rock at central vents, and of domes, cinder cones, and lava flows at vents on the flanks of the cones. Two of the main eruptive centers at Mount Shasta, the Shastina and Hotlum cones were constructed during Holocene time, which includes about the last 10,000 years. Holocene eruptions also occurred at Black Butte, a group of overlapping dacite domes about 13 kilometers (8 miles) west of Mount Shasta. ... The communities of Weed, Mount Shasta, and McCloud ... are situated on the broad apron at the base of the volcano. A fourth nearby community Dunsmuir ..., is located in the canyon of the Sacramento River about 23 kilometers (15 miles) south of Mount Shasta.

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Mount St. Helens, Washington

From: Simon, 1999, Channel and Drainage-Basin Response of the Toutle River System in the Aftermath of the 1980 Eruption of Mount St. Helens, Washington: USGS Open-File Report 96-633
The 1980 eruptions of Mount St. Helens in southwestern Washington marked the re-awakening of a relatively young (40,000 years) volcano that had been dormant since 1857. Frequent dacitic eruptions during the previous 2,500 years had produced pyroclastic flows, ash falls, debris flows, lava domes, and lava flows of andesite and basalt. Pyroclastic flows and lahars accompanied most eruptive periods and were largely responsible for forming fans around the base of the volcano, some of which dammed the North Fork Toutle River to form Spirit Lake between 3,300 and 4,000 years ago. The magnitudes of the 1980 eruptions were not exceptional by worldwide historical standards; however, they were the first volcanic eruptions in the conterminous United States since 1914 (Lassen Peak) and focused national attention on events leading up to the climactic eruption of May 18, 1980. That eruption led to exceptional opportunities for scientific observations, data collection, and the study of infrequent and often inaccessible geologic events and processes.

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Three Sisters, Oregon

From: Scott, et.al., 2001, Volcano Hazards in the Three Sisters Region, Oregon: USGS Open-File Report 99-437
Three Sisters is one of three potentially active volcanic centers that lie close to rapidly growing communities and resort areas in Central Oregon. Two types of volcanoes exist in the Three Sisters region and each poses distinct hazards to people and property. South Sister, Middle Sister, and Broken Top, major composite volcanoes clustered near the center of the region, have erupted repeatedly over tens of thousands of years and may erupt explosively in the future. In contrast, mafic volcanoes, which range from small cinder cones to large shield volcanoes like North Sister and Belknap Crater, are typically short-lived (weeks to centuries) and erupt less explosively than do composite volcanoes. Hundreds of mafic volcanoes scattered through the Three Sisters region are part of a much longer zone along the High Cascades of Oregon in which birth of new mafic volcanoes is possible.

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03/27/02, Lyn Topinka