DESCRIPTION:
Debris Avalanches and Volcanic Landslides

Shasta82_hummocks_09-22-82_med.jpg
USGS Photograph taken September 22, 1982, by Harry Glicken.
[medium size]

MSH83_debris_avalanche_north_fork_toutle_11-30-83.jpg
Downstream view of the North Fork Toutle River valley, north and west of St. Helens, shows part of the nearly 2/3 cubic miles (2.3 cubic kilometers) of debris avalanche that slid from the volcano on May 18, 1980. This is enough material to cover Washington, D.C. to a depth of 14 feet (4 meters). The avalanche traveled approximately 15 miles (24 kilometers) downstream at a velocity exceeding 150 miles per hour (240 km/hr). It left behind a hummocky deposit with an average thickness of 150 feet (45 meters) and a maximum thicknes of 600 feet (180 meters).
USGS Photograph taken on November 30, 1983, by Lyn Topinka.
[medium size] ... [large size]

Volcanic Landslides (Debris Avalanches): A landslide is a rapid downslope movement of rock, snow, and ice. Landslides range in size from small movements of loose debris on the surface of a volcano to massive failures of the entire summit or flanks of a volcano. Volcanic landslides are not always associated with eruptions; heavy rainfall or a large regional earthquake can trigger a landslide on steep slopes. Volcanoes are susceptible to landslides because they are composed of layers of weak, fragmented, volcanic rocks that tower above the surrounding terrane. Furthermore, some of these rocks have been altered to soft, slippery, clay minerals by hot, acidic ground water inside the volcano. At least five large landslides swept down the slopes of Mount Rainier during the past 6,000 years. The largest volcanic landslide in historical time occurred at Mount St. Helens on May 18, 1980.

The term debris avalanche is used to refer to the sudden and very rapid movement of an incoherent, unsorted mass of rock and soil mobilized by gravity ... Movement is characterized by flowage in a dry or wet state, or both. Debris avalanches commonly originate in massive rockslides which, during their movement, disintegrate into fragments ranging in size from small particles to blocks hundreds of meters across. If the avalanche has a large water content, its matrix may continue to flow downslope as a lahar after its coarser parts have come to rest. ...

Volcanic landslides: Gravity-driven slides, often rapid, of a mass of rock and soil that can occur in all sizes, from those involving a small amount of loose debris on the surface of a volcano to massive failures of the entire summit and (or) the flanks of a volcano. Volcanic landslides need not be associated with eruptions; heavy rainfall or a large earthquake can trigger landslides on steep volcanic slopes. Landslides that have evolved into a chaotic tumbling flow are termed debris avalanches.

More than 150 Quaternary debris-avalanche deposits have been identified in recent studies by Lee Siebert of the Smithsonian Institution in Washington, D.C., on the basis of geologic literature, topographic maps, and aerial photographs. Siebert's compilation shows that 17 volcanic debris avalanches are known of inferred to have formed in the last 400 years, about 4 per century. This rate is several times the historic rate for eruptions producing Krakatau-type calderas, one of the most hazardous types of explosive eruptions. Siebert also notes that debris avalanches occurring on volcanoes near the ocean may produce tsunamis by entering the sea; in historic times three debris avalanches and associated tsunamis have claimed about 17,000 lives. These observations underscore the importance of including debris avalanches in volcanic hazard studies.

Debris avalanches are flowing or sliding, incoherent and chaotic, wet or dry mixtures of soil and rock debris that move away from their source at high speed. Volcanic-debris avalanches occur occasionally at large central-vent volcanoes and are among the most hazardous of volcanic events. Such avalanches form when part of a volcanic edifice fails catastrophically and moves downslope away from the volcano. Disruption of a volcanic cone may be the result of intrusion of magma and earthquake shaking, as at Mount St. Helens in 1980, or of a volcanic blast as apparently occurred at Bezymianny in Kamchatka, USSR, in 1956. Steep-sided volcanic cones may also fail due to the influence of gravity after gradual weakening by hydrothermal alteration.

Debris avalanches produce thick, hummocky deposits that can extend as far as several tens of kilometers from a volcano and cover an area of a few tens of square kilometers, as occurred at Mount St. Helens in 1980, to a few hundred square kilometers, as occurred at Mount Shasta between 300,000 and 360,000 years ago. Debris avalanches are most likely at steep-sided volcanoes and, thus, are a potential hazard primarily at Mount Shasta, Lassen Peak, and possibly Mammoth Mountain in Long Valley.

The avalanche from Mount Shasta flowed northwesterly a distance of about 45 kilometers and the deposit covers an area of at least 450 square kilometers -- [webnote: use updated figures from Crandell, 1989, USGS Bulletin 1861] . A late-Pleistocene avalanche at Mammoth Mountain was considerably smaller and traveled less than 6 kilometers from its source. About 300 years ago, three rockfall-debris avalanches occurred from domes at the Chaos Crags eruptive center near Lassen Peak. The Chaos Crags avalanches traveled as far as 4.3 kilometers from their source areas.

Debris avalanches destroy everything in their paths by impact or burial beneath tens of meters of debris. Because debris avalanches occur with little warning and can travel at high speeds, areas that might be affected should be evacuated before such avalanches occur. Therefore, local government officials might decide to evacuate some areas in advance of a threatened eruption.

Nearly a century ago, the north flank of Bandai Volcano in Japan collapsed during an eruption ... S. Sekiya and Y. Kikuchi from the Imperial University of Tokyo visited Bandai Volcano within days of the eruption. ... They described a "deluge of rock and earth" (debris avalanche) that descended the north side of the mountain and covered several villages and killed 461 people. They described conical hills and small cones up to 15m in height "standing out from the debris like so many miniature Fujiyamas." ...

Japan Volcanoes and Volcanics Menu

The Colima volcanic complex is the most prominent volcanic center of the western Trans-Mexican Volcanic Belt. It consists of two southward-younging volcanoes, Nevado de Colima (the 4,320-meter-high point of the complex) on the north and the historically active Volcan de Colima at the south. A group of cinder cones of probable late-Pleistocene age is located on the floor of the Colima graben west and east of the Colima Complex. Volcan de Colima (also known as Volcan Fuego) is a youthful stratovolcano constructed within a 5-kilometer-wide caldera, breached to the south, that has been the source of large debris avalanches. Slope failure has occurred repetitively from both the Nevado and Fuego cones, and has produced a thick apron of debris-avalanche deposits on three sides of the complex. Frequent historical eruptions from Colima's summit crater have produced vertical pyroclastic columns, pyroclastic flows, and lava flows.

Mexico Volcanoes and Volcanics Menu

On the southeast slope of Galunggung Volcano on the densely populated island of Java, a hummocky deposit called the "Ten Thousand Hills of Tasik Malaja" ... Since 1980, geologists from the Volcanological Survey of Indonesia and the U. S. Geological Survey have reinterpreted the deposit as a debris-avalanche deposit. ...

Indonesia Volcanoes and Volcanics Menu

Debris avalanches produce thick, hummocky deposits that can extend as far as several tens of kilometers from a volcano and cover an area of a few tens of square kilometers, as occurred at Mount St. Helens in 1980, to a few hundred square kilometers, as occurred at Mount Shasta between 300,000 and 360,000 years ago. Debris avalanches are most likely at steep-sided volcanoes and, thus, are a potential hazard primarily at Mount Shasta, Lassen Peak, and possibly Mammoth Mountain in Long Valley. ...

About 300 years ago, three rockfall-debris avalanches occurred from domes at the Chaos Crags eruptive center near Lassen Peak. The Chaos Crags avalanches traveled as far as 4.3 kilometers from their source areas.

Lassen Peak Menu

Debris avalanches are flowing or sliding, incoherent and chaotic, wet or dry mixtures of soil and rock debris that move away from their source at high speed. Volcanic-debris avalanches occur occasionally at large central-vent volcanoes and are among the most hazardous of volcanic events. Such avalanches form when part of a volcanic edifice fails catastrophically and moves downslope away from the volcano. ...

Debris avalanches produce thick, hummocky deposits that can extend as far as several tens of kilometers from a volcano and cover an area of a few tens of square kilometers, as occurred at Mount St. Helens in 1980, to a few hundred square kilometers, as occurred at Mount Shasta between 300,000 and 360,000 years ago. Debris avalanches are most likely at steep-sided volcanoes and, thus, are a potential hazard primarily at Mount Shasta, Lassen Peak, and possibly Mammoth Mountain in Long Valley. ...

The avalanche from Mount Shasta flowed northwesterly a distance of about 45 kilometers and the deposit covers an area of at least 450 square kilometers -- [webnote: use updated figures from Crandell, 1989, USGS Bulletin 1861] . A late-Pleistocene avalanche at Mammoth Mountain was considerably smaller and traveled less than 6 kilometers from its source.

Link to: Long Valley Observatory Website for MORE Information

That large landslides may have played a major role in the history of the Hawaiian Islands is suggested by Molokai Island, on of the small islands in the Hawaiian chain. The island of Molokai is composed of three shield volcanoes, East Molokai, the Kalaupapa Peninsula, and West Molokai. The windward north shore of East Molokai Volcano is characterized by huge cliffs ascending 1100 meters from the sea at an angle of more than 55 degrees. The southern flank slopes gently to the sea, typical of shield volcanoes. Early researchers hypothesized that the unusually high sea cliffs might be a fault scarp or the result of marine erosion, but these interpretations were made with only a sketchy view of the volcano's structure and without any knowledge of the seafloor topography north of the cliffs. Recent studies by James Moore and Robin Holcomb of the U.S. Geological Survey suggest that a tremendous landslide bisected East Molokai Volcano, forming the sea cliffs and a lumpy seafloor deposit adjacent to the cliffs.

The geological and magnetic-polarity stratigraphy of rocks in the north sea cliffs, recently mapped by Holcomb and his colleagues, shows that East Molokai's summit contains a caldera much larger than was previously estimated -- the caldera is bisected and the northern half is removed. Holcomb's field party identified vertical polarity boundaries in rocks exposed at Haupu Bay and Papaluaua Valley. This type of boundary, which separates lava flows of normal polarity in the central section and lava flows of reversed polarity on the flanks, indicates an age difference between the two lava-flow sequences. Vertical polarity boundaries can occur where younger caldera-filling flows abut against older outward-sloping lava flows along near-vertical caldera walls.

At Haupu Bay, a vertical contact separates thin-bedded lavas to the west and lavas ponded atop lithified talus breccia of the caldera to the east. This exposure was previously interpreted to be a small pit crater on the flank of East Molokai Volcano. The age difference between the lavas on either side of the exposure, revealed by their magnetic polarities, however, indicates this new interpretation of the structure of East Molokai volcano, Holcomb estimates that about 500 cubic kilometers is missing from the northern half of the volcano.

Holcomb contends that marine erosion would be too slow to account for the removal. The shield volcano comprising the Kalaupapa Peninsula is almost unaffected by marine erosion, yet recent K-Ar dates show that it is nearly 0.5 million years old. East Molokai Volcano stopped growing only about 1.5 million years ago, so unless the rates of erosion were much greater before Kalaupapa grew, half of East Molokai could not have been eroded in only 1 million years. Instead, the northern half of East Molokai must have sunk beneath the sea, probably in a giant landslide that James Moore proposed in 1964.

An irregular, "lumpy" seafloor extending at least 80 kilometers north of Molokai and 160 kilometers northeast of Oahu was reported by Moore when techniques of seafloor mapping were new. Bathymetric maps of the area revealed massive structures commonly from 8-25 kilometers long and 5-15 kilometers wide. They are commonly bounded by steep slopes as high as 2,000 meters and may have relatively flat tops of which most are tilted toward the Hawaiian Ridge, a volcanic ridge which extends 2,600 kilometers west-northwest from the island of Hawaii. The upper end of these features is marked by a concave escarpment. Moore interpreted these features as deposits from submarine landslides originating on the Hawaiian Ridge, one of the Earth's steepest and youngest major topographic features. The lumpy terrain is likely analogous to the hummocky subaerial debris-avalanche deposits.

Link to: Hawaiian Volcano Observatory Website for MORE Information

Volcanic Landslides (Debris Avalanches): A landslide is a rapid downslope movement of rock, snow, and ice. Landslides range in size from small movements of loose debris on the surface of a volcano to massive failures of the entire summit or flanks of a volcano. Volcanic landslides are not always associated with eruptions; heavy rainfall or a large regional earthquake can trigger a landslide on steep slopes. Volcanoes are susceptible to landslides because they are composed of layers of weak, fragmented, volcanic rocks that tower above the surrounding terrane. Furthermore, some of these rocks have been altered to soft, slippery, clay minerals by hot, acidic ground water inside the volcano. At least five large landslides swept down the slopes of Mount Rainier during the past 6,000 years. The largest volcanic landslide in historical time occurred at Mount St. Helens on May 18, 1980.

Mount Rainier Volcano Menu

Shasta82_hummocks_09-22-82_med.jpg
USGS Photograph taken September 22, 1982, by Harry Glicken.
[medium size]

The deposits of an exceptionally large debris avalanche extend from the base of Mount Shasta volcano northward across the floor of Shasta Valley in northern California. The debris-avalanche deposits underlie an area of about 675 square kilometers, and their estimated volume is at least 45 cubic kilometers. Radiometric limiting dates suggest that the debris avalanche occurred between about 300,000 and 380,000 years ago. Hundreds of mounds, hills, and ridges formed by the avalanche deposits are separated by flat areas that slope generally northward at about 5 meters per kilometer. ...

Mount Shasta Debris Avalanche Menu

MSH83_debris_avalanche_north_fork_toutle_11-30-83.jpg
Downstream view of the North Fork Toutle River valley, north and west of St. Helens, shows part of the nearly 2/3 cubic miles (2.3 cubic kilometers) of debris avalanche that slid from the volcano on May 18, 1980. This is enough material to cover Washington, D.C. to a depth of 14 feet (4 meters). The avalanche traveled approximately 15 miles (24 kilometers) downstream at a velocity exceeding 150 miles per hour (240 km/hr). It left behind a hummocky deposit with an average thickness of 150 feet (45 meters) and a maximum thicknes of 600 feet (180 meters).
USGS Photograph taken on November 30, 1983, by Lyn Topinka.
[medium size] ... [large size]

As Mount St. Helens illustrated so dramatically on May 18, 1980, debris avalanches from volcanoes pose significant hazards to people and property. Debris avalanches may occur without warning, move great distances at high speed, cover large areas, initiate later blasts, and, if they enter the sea, cause tsunamis. The May 18 eruption was the first time eye-witness accounts and photographs documented the emplacement of a large volcanic debris avalanche. The debris-avalanche deposit at Mount St. Helens has provided a basis for interpretation of similar deposits elsewhere and has led to the realization that large-scale gravitational slope failures of volcanoes are more common than previously thought. Since 1980, volcanic hazard assessments have included consideration of hazards posed by debris avalanches in addition to other, more common products of eruptions, such as pyroclastic flows, lahars, lava flows, and tephra. ...

The debris-avalanche deposit at Mount St. Helens covers about 60 square kilometers of the North Fork Toutle River valley with about 2.5 cubic kilometers of unconsolidated rock debris. For a distance of 25 kilometers the deposit fills the valley to an average depth of 45 meters but is locally as deep as 180 meters. The most conspicuous feature of the deposit is its hummocky chaotic surface morphology. Levees as high as 30 meters occur along the margins of the deposit against valley walls. Individual hummocks and ridges as high as 70 meters are separated by low-lying areas and closed depressions, many of which form ponds. ...

Mount St. Helens Debris Avalanche Menu

Santa Ana, El Salvador's highest volcano (2,365 meters), is a massive stratovolcano immediately west of Coatepeque caldera. Collapse of the volcano during the late Pleistocene or early Holocene produced a massive debris avalanche that swept into the Pacific, forming the Acajutla Peninsula. Reconstruction of the volcano rapidly filled the collapse scarp. The broad summit of the volcano is cut by several crescentic craters, and a series of parasitic vents and cones have formed along a 20-kilometer-long fissure system that extends from near the town of Chalchuapa NNW of the volcano to the San Marcelino and Cerro Chino cinder cones on the SE flank. Historical activity, largely consisting of small-to-moderate explosive eruptions from both summit and flank vents, has been documented since the 16th century. The San Marcelino cinder cone on the SE flank produced a lava flow in 1722 that traveled 11 kilometers to the east.

El Salvador Volcanoes and Volcanics Menu

Unzen is well know for Japan's greatest volcanic disaster. In 1792, about a month after lava stopped erupting from the volcano, a landslide from nearby Mount Mayuyama swept through ancient Shimabara City, entered the sea, and generated a tsunami that struck nearby areas. More than 15,000 people were killed by the landslide and tsunami. The amphitheater-shaped scar of the landslide is still clearly visible on Mount Mayuyama just above the city.

Japan Volcanoes and Volcanics Menu

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