Geology of the Lassen volcanic area

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The geology of the Lassen volcanic area started in the late Cenozoic by the westward tilting of the Sierra Nevada along with extensive volcanism. Huge lahars (volcanic-derived mud flows) from the volcanoes, especially from the Pliocene-aged Mount Yana and Mount Maidu, became the Tuscan Formation. This formation is not exposed anywhere in Lassen Volcanic National Park but it is just below the surface in many areas. Basaltic flows erupted from vents and fissures in the southern part of the park. These and later flows of andesite covered increasingly large areas and built a lava plateau.

Mount Tehama (also known as Brokeoff Volcano) rose as a stratovolcano in the southeastern corner of the park during the Pleistocene. At its height, Tehama was probably about 11,000 feet high. Approximately 350,000 years ago its cone collapsed into itself and formed a two-mile wide caldera in a series of eruptions. One of these eruptions occurred where Lassen Peak now stands, and consisted of fluid, black, glassy dacite.

Roughly 27,000 years ago (older data gave an age of 18,000 years), Lassen Peak started to form as a dacite lava dome quickly pushed its way through Tehama's destroyed north-eastern flank. Lassen rose and reached its present height in a relatively short time, probably in as little as a few years. Since then, smaller dacite domes formed around Lassen. The largest of these, Chaos Crags, is just north of Lassen Peak. Phreatic (steam explosion) eruptions, dacite and andesite lava flows and cinder cone formation have persisted into modern times. Most notable of these is the 1914 to 1921 eruption of Lassen Peak.

Contents 1 Regional geologic setting 1.1 Current setting 1.2 Geologic history of the region 2 Formation of basement rocks 3 Volcanoes rise and fall in the park area 3.1 Growth of Mount Tehama and pre-Lassen volcanics 3.2 Development of Lassen Peak 3.3 Glacial action 3.4 Demise of Tehama and pre-20th century activity 3.5 1914 to 1921 activity at Lassen Peak 4 Volcanic hazards 5 References 5.1 Works cited 5.2 Notes

[edit] Regional geologic setting

[edit] Current setting

Lassen Volcanic National Park lies at the southern extremity of the Cascade Range, which extends northward some 500 miles through Oregon and Washington and into British Columbia. Lassen Peak and the 16 other major volcanoes of the Cascades are a segment of a ring of volcanoes that circle the Pacific Ocean, known collectively as the 'Pacific Ring of Fire.' The Cascade Volcanoes are fed by heat generated as the Gorda and Juan de Fuca Plates are being subducted below the North American Plate. Lying some 300 miles offshore, the spreading center (divergent plate boundary) of the Gorda Plate pushes out about an inch (2.5 cm) of new crust toward the coast of Northernmost California and southern Oregon every year.^[1]

Northwest of the park lies the Klamath Mountains (a collective term for the Siskiyou, Trinity, Salmon and Marble Mountains). To the west lies the Sacramento Valley. Just south of the park begins the Sierra Nevada Mountains and to the east lie the Modoc Plateau and then the Great Basin.

[edit] Geologic history of the region

All rock now exposed in the park is volcanic, but this has not always been the case. For hundreds of millions of years, the Lassen region had undergone repeated uplifting to form mountains, only to have them worn down and submerged under encroaching seas. During the periods of submersion, sands, muds and limestones would be deposited and occasionally volcanic activity was associated with the mountain building.

About 70 million years ago, the area where the Cascade Range is now situated was under the most recent encroachment by the Pacific Ocean.^[2] The Sierra Nevadas and the Klamath Mountains were already in existence. Some 70 million years before (140 million years before present), the Klamaths broke away from the Sierras and moved 60 miles (100 km) west,^[3] leaving the flooded 'Lassen Strait'. This broad depression was a seaway that connected the marine basin in California with that in east central Oregon.^[2]

At this time, the entire western portion of North America became subject to profound earth movements in the Laramide orogeny. Gradually during millions of years, the rocks of the crusts were folded and fractured and the seas were driven away. This same bending and breaking of rocks relieved pressure on the hot material beneath the earth's crust and permitted magma to rise to toward the surface. From Washington southward along the Cascades and the Sierra Nevada volcanoes burst into activity starting 30 million years ago.^[4] This activity continued until approximately 11 or 12 million years ago, piling lavas and ashes on each other until in places their thickness approached 10,000 feet, forming what is now known as the western Cascades.^[2] These have been eroded until they are now rolling hills.

Meanwhile, toward the end of this activity, eruptions of a different kind took place on an unprecedented scale in eastern Oregon and Washington. From innumerable cracks, floods of highly fluid basaltic lava spread to cover an area of over 200,000 square miles.^[2] Known as the Columbian Plateau, this great lava bed of flood basalt covers much of Oregon, Washington and even parts of Idaho. California's Modoc Plateau is a thinner basaltic flow which some associate with the Columbia Plateau, but there are technical objections to this. The High Cascades took shape as a distinct mountain belt as a result of this upheaval and the bending of the thick blanket of volcanic rocks. As a result of this upheaval, many features opened near the crust, and during the next 10 million years, a series of new basaltic volcanic cones similar to those now found in Hawaii were built.

[edit] Formation of basement rocks

Between two and three million years ago, during the Pliocene, the Sierra Nevada was uplifted and tilted westward. For one million years, a series of volcanic mudflows (lahars) from three major source areas contributed debris that covered almost 2,000 square miles to form the oldest distinctive formation of the High Cascades.^[2] The resulting Tuscan Formation is not exposed anywhere in the national park but it is just below the surface in many areas.

Probably the oldest major source of the formation was Mount Yana (centered a few miles southwest of Butt Mountain and south of the park). Mount Yana had probably reached its full size, 10,000 feet in elevation and 15 miles in diameter, before Mount Maidu, the second source, had acquired half its growth. Mount Maidu, which eventually surpassed Mount Yana in size, was centered over the town of Mineral, California but has been extinct for hundreds of thousands of years (the grassy plain around the town is Maidu's caldera).^[5] A third source situated north of Latour Butte made a lesser contribution to the formation. Other minor sources included an area near Hatchet Mountain Pass (northwest of Burney Mountain), dikes south and southwest of Inskip Hill and possibly Campbell Mound (north of Chico, California).^[2]

Meanwhile, within the park boundary other volcanic events were taking place. In the southwestern portion of the park, basaltic lavas poured forth in the vicinity of Willow Lake. These were followed by a very thick sequence of very fluid andesitic lavas which erupted near Juniper Lake and flowed westward about four miles.^[2] At about the same time, other andesitic lavas poured from several vents on the central plateau to cover an area of at least 30 square miles. Included among these flows were the Twin Lake lavas of black porphyritic andesite which are notable in that they contain xenocrysts of quartz.^[6] The Flatiron andesites spread over the southwestern part of the park area around this time. Apparently, the vents of these lavas renewed activity at a much later date to form three cinder cones: Hat Mountain, Crater Butte and Fairfield Peak.

Somewhat later, andesitic lavas poured out from what is now Reading Peak, flowing chiefly to the south and east, reaching the head of Warner Valley. By this time, the park's eastern portion had been transformed into a relatively flat plain.^[2] The activity was followed by an eruption of the Eastern basalts from volcanoes east of the park.^[6] These thick flows have subsequently eroded to produce rugged hills that limit the park on the east. Taken together, these various flows built the lava plateau the Lassen volcanic area is located on.^[7]

[edit] Volcanoes rise and fall in the park area

[edit] Growth of Mount Tehama and pre-Lassen volcanics

The greater Lassen volcanic area has been volcanically active for about 3 million years.^[9] From 600,000 to 400,000 years ago, eruptions built a large conical stratovolcano in what is now the southwest corner of the park, Mount Tehama (also called Brokeoff Volcano).^[10] It was made of roughly alternating layers of andesitic lavas and tephra (volcanic ash, breccia, and pumice) with increasing amounts of tephra with elevation.

Starting its activity after Mount Yana but before Mount Maidu started its, Tehama reached an elevation of 11,000 feet and covered an area of 100 square miles.^[5] Its principal vent lay in the neighborhood of what is now the Sulphur Works, but a second vent from which no lavas issued lay on the eastern flank of Little Hot Springs Valley. Contrary to popular opinion, Bumpass Hell is not one of Tehama's main vents since it is located outside the caldera.^[2]

During Tehama's later history, four shield volcanoes, Raker and Prospect Peaks, Sifford Mountain and Mount Harkness, grew to elevations of between 7000 to 8400 feet at the corners of the central plateau.^[11] Raker Peak erupted andesite lavas while basalt issued from the others. During their last stages of eruption, each of these volcanoes developed a cinder cone on its summit. Later, a mass of rhyolite was forced through the north flank of Sifford Mountain and a plug of dacite was pushed up through the west flank of Raker Peak.^[2]

In the past 50,000 years, at least seven major episodes of dacitic volcanism produced lava domes and pyroclastic deposits in the Lassen volcanic area, and another five episodes produced basaltic and andesitic (silica content between basalt and dacite) lava flows.^[10] In addition, about 30 smaller volcanoes erupted basaltic lavas in the larger region surrounding the Lassen volcanic center.^[10]

[edit] Development of Lassen Peak

Radiometric dating indicates that around 31,000 years ago, a new vent opened up on the northeastern slope of Tehama, probably close to where Lassen Peak now stands.^[12] From this new crater streams of fluid dacite flowed radically, but chiefly toward the north, piling up lava to a thickness of 1,500 feet and covering perhaps 20 square miles.^[2][13] These pre-Lassen dacites are the black, glassy, columnar lavas that now encircle Lassen Peak. They are called the Loomis Sequence ^[13].

Sometime between 25,000 and 31,000 years ago, Lassen Peak, a Pelean type plug lava dome volcano, was pushed up through the pre-Lassen dacites.^[12] Lassen grew past the normal maximum size of plug dome volconoes, 1000 feet (300 m), and reached a height of 1800 feet (550 m)^[12] in a relatively short time, probably in as little as a few years. As its partly solid and viscous dacite lavas rose, its margins abraded and polished against the vent walls, and the surface of the growing pile crumbled continually, forming enormous banks of talus. When Lassen Peak formed, it looked much like the nearby Chaos Crags domes do today, with steep sides covered by angular rock talus. However, from 25,000 to 18,000 years ago, during the last glacial period of the current ice age, Lassen Peak’s shape was significantly altered by glacial erosion ^[10] with at least one of its glaciers extending as much as 7 miles (11 km) from the volcano itself.

Later, but not precisely dated, eruptions from the Lassen volcanic center have formed over 30 smaller steep-sided, mound-shaped accumulations of volcanic rock, called lava domes.^[10] Crescent Crater, which at first glance appears as a parasite on Lassen's northeast flank, has been more heavily glaciated and thus is older. Other dacite domes which rose on Tehama's flanks are Bumpass Mountain, Helen Ridge, Eagle Peak, Vulcan's Castle and Reading Peak.^[14] An upper limit of 10,000 years has been set for the domes next to Lost Creek (north domes).^[2] All of these domes must have risen with great rapidity.

[edit] Glacial action

Glaciation has played an important but incompletely understood role in the park. Glaciers existed throughout the park area during most of the Pleistocene with smaller ones persisting at higher elevations until comparatively recent times. Lassen Peak is situated at a center from which many of these glaciers originated. Ice that glaciated the valley of Mill Creek (whose canyon is mostly post-glacial), Blue Lake Canyon, Kings Creek Meadows, Flatiron Ridge, Warner Valley and the valley of Manzanita, Hat and Lost Creeks originated from there. Indeed, Lassen Peak appears to be sitting in the depression carved by the Lost Creek Glacier.^[2]

Reading Peak is situated at a second center from which ice moved north into Hat Creek and Summit Creek. Ice moving southward united with some of the above glaciers and emptied into Warner Valley. On the central plateau, the ridge connecting Hat Mountain with Crater Butte served as a divide between ice flowing northward to Badger Flat and Hat Creek and that moving southward to Corral Meadows, Kings Creek and Warner Valley. Ice from Mt. Harkness and Sifford Mountain also wound up in Warner Valley.

The crest of Saddle Mountain served as a divide with ice north of it moving into the depression containing Snag and Butte Lakes, while those to the south entered Warner Valley. The ice varied from a thickness of 1,600 feet in Warner Valley to much thinner sheets in the higher mountains.^[2] By carving beautiful cirques, canyons and most of Lassen's lakes, glaciers have contributed much to the beauty of Lassen's landscape.

[edit] Demise of Tehama and pre-20th century activity

The destruction of the Tehama volcano probably began about this time. It is possible that Tehama collapsed along a series of fault lines which criss-crossed it--a collapse which may have been brought about by the extrusion of extensive amounts of lava necessary to form the dacite domes on its flank. More likely, the volcano weathered away; intense glacial erosion combined with hot gases and steam turning hard rock into soft clay.^[13] Either way, the largest remnants of Tehama include Brokeoff Mountain, Mount Conard, Mount Diller, and Diamond Peak.

Subsequent to the rise of Lassen Peak, several dacitic pumice cones developed in a rift extending northwest from the base of Lassen Peak. Then about 1,100 years ago, several dacitic domes, the Chaos Crags, protruded through these cones, obliterating all but half of the southernmost cone. A series of large avalanches possibly triggered by steam explosions have occurred on the north side of the Crags at least 300 years ago.^[15] These avalanches created their own 'air cushions' that helped accelerate them to speeds exceeding 100 miles per hour (160 km/ph) and push them partway up Table Mountain.^[15] The resulting wilderness of debris, the Chaos Jumbles, covers an area of 2 1/2 square miles.^[2] Manzanita Lake was formed as a result of Manzanita Creek being dammed by the debris. Reflection Lake and Lily Pond are depressions in the debris.

Around the mid 18th century a series of eruptions produced Cinder Cone in the northeast corner of the park, mantling an area of 30 square miles with ejecta in the process.^[2] In the meantime, ashes falling on the streams of lava pouring from the cone's east flank formed the Painted Dunes. At the same time a flow of quartz-studded basalt lava (the Fantastic Lava Beds) poured from the Cinder Cone and entered Butte Lake and damned the drainage into Butte Lake to form a new lake--Snag Lake. In the late 18th century Cinder Cone had its most recent eruption and lava flow. Steam rose from the domes of Chaos Crags until 1857,^[15] but no important eruptions occurred again until Lassen Peak burst into activity in 1914.

[edit] 1914 to 1921 activity at Lassen Peak

Explosions recurred at irregular intervals on Lassen Peak for most of 1914. Then, on May 19, 1915, a mass of lava rose in the summit crater and spilled over the southwestern and northeastern sides of the volcano. On the southwestern slope glowing lava descended 1,000 feet toward the Sacramento Valley, then cooled and hardened.^[2] Extensive lahars (mudflows) were created on the northeastern side as snowbanks were melted. The resulting debris swept down the slope. Divided by Raker Peak, part of this mudflow raced down Lost Creek; the remaining flow passed over the 100 foot rise east of the park road and rushed down Hat Creek.^[2] A wide barren swath was torn through the forest.

Three days later, May 22, 1915, a great explosion blasted out a new crater. A volcanic cloud rose 40,000 feet, but a portion of the explosive force was deflected downward.^[2] The resulting pyroclastic flow (nuee ardente) of super-heated gas, rocks and ash roared down the same path taken by the mudflow, resulting in further damage along the headwaters of Hat and Lost Creeks. Ash from the eruption blew eastward with some fine ash falling at least as far as 200 miles from the volcano.^[10] Thereafter, activity declined, finally ending in 1921.^[2] Since then, the volcano has lain dormant, although a little steam still rises from small vents in its summit and on its flanks. Pumice ejected during the 1915 eruption of Lassen Peak is conspicuously banded with light streaks of dacite and dark ones of andesite, which appears to represent two distinct magmas imperfectly mixed during the eruption.

[edit] Volcanic hazards

Volcano hazards are generally evaluated on the basis of an area’s record of eruptions over the past 10,000 years, because future eruptions are most likely to occur near areas that have most recently had volcanic activity. However, in the Lassen region eruptions occur infrequently, so the record of activity in the past 50,000 years was used to provide an adequate basis for defining hazard zones.^[10] During this period, eruptions in the Lassen region have occurred at sites including Lassen Peak, Chaos Crags, and Sunflower Flat (explosive dacite eruptions followed by dome growth) and Tumble Buttes, Hat Mountain, and Prospect Peak (basalt eruptions). The areas of highest hazard are those that could be affected by pyroclastic flows and larhars (see map). These areas, including Hat Creek Valley, are those in the immediate vicinity and downhill from likely eruption sites. Fallout of ash will affect areas downwind at the time of an eruption. Within the hazard zones, relative hazard is gradational, decreasing away from the location of potential vents.

The composition of the molten rock (magma) that feeds volcanism in the Lassen area ranges widely in its content of silica (SiO₂). When high-silica (dacite) magma rises to the Earth's surface, it can erupt explosively to produce ash clouds and pyroclastic flows. Dacite magma extruded nonexplosively forms lava domes, because it is too viscous to flow far away from its source. Low-silica (basalt) magma is more fluid and usually erupts less explosively than dacite magma. Eruptions of basalt magma typically produce elongate lava flows, as well as build cinder cones (piles of small frothy lava fragments or 'cinders') around volcanic vents.^[10]

The most common volcanic activity in the Lassen volcanic area consists of small to moderate-sized eruptions that produce basaltic lava flows and localized ash falls.^[10] These eruptions typically last a few months to a year, but may continue for several years. They can cover more than a square mile with lava flows, build cinder cones as high as 1,000 feet, and blanket many square miles with ash a few inches to several feet deep. Because these eruptions are relatively nonviolent, they rarely cause human fatalities.

Basaltic volcanism in the Lassen area occurs mainly along chains of vents aligned in a north or northwest direction, parallel to regional faults.^[10] Examples include Poison Buttes, Subglacial Buttes, Tumble Buttes, the Prospect Peak-Red Cinder area, the east side of the Hat Creek Valley and Potato Buttes-Sugarloaf area, and the Red Lake Mountain area. Prolonged basaltic volcanism at a single site can produce a sizeable edifice, like the broad, relatively flat shield volcanoes of Prospect Peak and Sifford Mountain.

Dacite eruptions in the Lassen area typically begin with steam explosions caused by the interaction of rising magma with ground water.^[10] When dacite magma charged with volcanic gases reaches the surface, it erupts explosively, usually as a vertical column of gas and ash that can rise several miles into the atmosphere. Heavy fallback of hot ash and rock fragments from eruption columns may generate highly mobile pyroclastic flows that can rush several miles down a volcano's slopes and adjacent valleys. Fallout from the eruption column can blanket areas within a few miles of the vent with a thick layer of pumice, and high-altitude winds may carry finer ash tens to hundreds of miles from the volcano, posing a hazard to flying aircraft, particularly those with jet engines.

After an initial explosive eruption, extrusion of gas-depleted dacite magma commonly forms lava domes. Growing lava domes are inherently unstable, and collapse of their steep sides often generates pyroclastic flows of lava blocks and ash that can travel several miles. Such a sequence of events is recorded by the deposits related to the emplacement of Chaos Crags domes between 1,100 and 1,000 years ago.^[10]

Interaction of hot pyroclastic flows with snow and ice can generate highly mobile flows of mud and debris (called lahars) that may rush down valleys leading away from a volcano. Because of this, active volcanoes that have a significant snow and ice cover can be particularly dangerous. The lahars that threatened residents of the Lassen area in May 1915 were generated by relatively small eruptions of Lassen Peak. Nonetheless, they traveled down creek beds as far as 12 miles and released floods that affected valleys for 30 miles downstream.^[10]

Additional volcano hazards at Lassen are rockfalls and landslides not directly related to eruptions. Recently erupted volcanic domes are unstable and can collapse, generating small to large rockfalls. Approximately 350 years ago, collapse of one of the Chaos Crags domes generated huge rockfalls, creating an area now called the Chaos Jumbles.^[10] The first and largest of these traveled 4 miles downslope and was able to climb 400 feet up the side of Table Mountain. The trigger for the rockfall is unknown, but it was most likely a large earthquake. Normal weathering also weakens fractured volcanic rock and contributes to small rockfalls. In the summer of 1994, a rockfall of 13,000 cubic yards (the volume of about 500 minivans) occurred on the northeastern flank of Lassen Peak.^[10] During periods of extreme rainfall or snow melt, mudflows are sometimes generated by mobilization of loose volcanic debris and soil on the slopes of volcanoes.

[edit] References

[edit] Works cited

• National Park Service: Lassen Volcanic National Park, Nature & science, Volcanoes / Lava Flows (adapted public domain text; accessed 22 September 2006)
• Volcano Hazards of the Lassen Volcanic National Park Area, California, U.S. Geological Survey Fact Sheet 022-00, Online version 1.0 (adapted public domain text; accessed 25 September 2006)
• Alloway, B.V., Westgate, J.A., Sandhu, A.S. and Bright, R.C. (1992). Isothermal plateau fission-track age and revised distribution of the widespread mid-Pleistocene Rockland tephra in west-central United States., Geophysical Research Letters 19(6): doi: 10.1029/92GL00358. ISSN 0094-8276.
• Geology of National Parks: Fifth Edition, Ann G. Harris, Esther Tuttle, Sherwood D., Tuttle (Iowa, Kendall/Hunt Publishing; 1997) ISBN 0-7872-5353-7
• Geology of U.S. Parklands: Fifth Edition, Eugene P. Kiver and David V. Harris (Jonh Wiley & Sons; New York; 1999) ISBN 0-471-33218-6
• Roadside Geology of Northern California, David D. Alt, Donald W. Hyndman (Mountain Press Publishing Company; Missoula, Montana; 1986) ISBN 0-87842-055-X
• Roadside Geology of Northern and Central California, David D. Alt, Donald W. Hyndman (Mountain Press Publishing Company; Missoula, Montana; 2000) ISBN 0-87842-409-1
• Fire Mountains of the West: The Cascade and Mono Lake Volcanoes, Stephen L. Harris (Mountain Press Publishing Company; Missoula, Montana; 2001) ISBN 0-87842-220-X

[edit] Notes

See above for full reference information

1. ^ Geology of National Parks, page 467
2. ^ ^{a b c d e f g h i j k l m n o p q r s t} National Park Service, Lassen Volcanoes / Lava Flows
3. ^ Roadside Geology of Northern California, page 193
4. ^ Roadside Geology of Northern California, page 194
5. ^ ^{a b} Fire Mountains of the West, page 73
6. ^ ^{a b} Geology of National Parks, page 474
7. ^ Geology of National Parks, page 473
8. ^ Alloway et al. 1992
9. ^ Geology of U.S. Parklands, page 156
10. ^ ^{a b c d e f g h i j k l m n o} USGS: Volcano Hazards of the Lassen Volcanic National Park Area, California
11. ^ Geology of National Parks, page 470
12. ^ ^{a b c} Geology of U.S. Parklands, page 159
13. ^ ^{a b c} Fire Mountains of the West, page 75
14. ^ Geology of National Parks, page 466
15. ^ ^{a b c} Geology of U.S. Parklands, page 160