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.
The debris-avalanche deposit at Mount St. Helens covers about 60 square km of the North Fork Toutle River valley with about 2.5 cubic km of unconsolidated rock debris. For a distance of 25km the deposit fills the valley to an average depth of 45m but is locally as deep as 180m. The most conspicuous feature of the deposit is its hummocky chaotic surface morphology. Levees as high as 30m occur along the margins of the deposit against valley walls. Individual hummocks and ridges as high as 70m are separated by low-lying areas and closed depressions, many of which form ponds.
The deposit contains a wide range of debris reflecting the variation in volcanic material of the cone from which it was derived. The debris includes pieces of lava flows and domes and the deposits of lahars and pyroclastic flows. Individual rock fragments range from clay-sized particles to boulders several meters in diameter. The deposit is divided into block facies (unmixed) and matrix facies (mixed). The block facies, which make up most of the deposit, consist of pieces of the old Mount St. Helens that were shattered during the initial rockslide and blast, but were not disaggregated. Some of these debris-avalanche blocks are several thousand cubic meters in volume. They were transported gently in the debris avalanche and deposited relatively intact. Some of the original stratigraphy of lava flows and volcanic dikes in the old mountain is preserved in debris-avalanche blocks, yet most of the large clasts in the lava flows and dikes were thoroughly shattered. The matrix facies consist of all rock types from the old Mount St. Helens, as well as juvenile material from the May 18, 1980, eruption, blended together. Then matrix facies was produced by mixing of material during the explosions of the blast as well as from disaggregation and mining of debris-avalanche blocks during transport.
In addition to similarities in surface morphology, the internal structure of both deposits is similar -- the Mount Shasta deposit also consists of block facies and matrix facies. The block facies form hummocks and include individual debris-avalanche blocks ranging in size from tens to hundreds of meters in maximum dimension. Such blocks consist of a single rock type of layers of airfall tephra, pyroclastic flows, lahars, and pieces of soil. These blocks formed part of the former volcanic cone. The debris-avalanche blocks are generally not as shattered as in the Mount St. helens deposit. Surrounding these debris-avalanche blocks, sometimes forming dikes that penetrate narrow cracks and joints in the blocks, is a mixture of clasts that range in size from microns to meters in diameter -- the matrix facies. Most of the largest clasts in the matrix facies are andesitic rocks, similar to rocks that make up the present Mount Shasta volcano. Other clasts include sedimentary and metamorphic rocks, and rocks derived from alluvial and lake sediments incorporated by the avalanche as it traversed the floor of Shasta Valley. Fossils of lacustrine organisms are also found in the matrix facies at several localities.
The Mount Shasta debris-avalanche deposit covers and area of at least 450 square km with about 26 cubic km of debris -- [webnote: use updated figures from Crandell, 1989, USGS Bulletin 1861] -- -- roughly 10 times the volume of the 1980 Mount St. Helens avalanche deposit. Radiometric ages of rocks in the deposit and of a post-avalanche basalt flow indicate that the avalanche occurred between about 300,000 and 360,000 years ago.
The horseshoe shape of Galunggung's crater and the nature of the hummocks, however, suggest a different cause for the formation of the Ten Thousand Hills. Since 1980, geologists from the Volcanological Survey of Indonesia and the U.S. Geological Survey have reinterpreted the deposit as a debris-avalanche deposit. Quarry exposures show pieces of the old volcano -- the block facies -- shattered but intact, that are similar to the deposits at Mount St. Helens and Mount Shasta. Radiocarbon dates of a lava flow within the deposit show that the debris avalanche is less than 23,000 years old.
S. Sekiya and Y. Kikuchi from the Imperial University of Tokyo visited Bandai Volcano within days of the eruption. After spending several months studying the new crater and area of devastation, they published a report (in English) that is a classic of volcanology. 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." They also described "heated blasts of steam and air (lateral blasts) thickly mixed with dust and rock fragments fierce enough to crush the trees and strip them of not only of the branches but even of their bark." In a drainage basin on the east side of the mountain these blasts had felled a forest so that "trees with a diameter of more than a meter had been laid prostrate on the ground in thousands."
Although the Bandai eruption was similar in many ways to the recent eruption of Mount St. Helens, there were some significant differences. Seikiya and Kikuchi noted that the destructive agency was merely the "sudden expansion of imprisoned steam, unaccompanied by lava flows or pumice ejection." A study of the eruption by Yoichi Nakamura of Utsonomiya University in Japan in the 1970's confirmed the absence of juvenile material (new magma). The lateral blast and debris avalanche deposits at Mount St. Helens, however, contain about 100 million cubic meters of juvenile material. The debris-avalanche and blast-affected areas at Bandai are both considerably smaller than those areas at Mount St. Helens. Blast deposits were not documented on the north side of the volcano by Seikiya and Kikuchi, even though the lateral blast was observed to be directed primarily to the north; likely most of the blast deposits were lost amid the hummocks of the debris-avalanche deposit. They were found only where the blast explosions spilled over the south flank of the volcano.
Today Bandai Volcano and the area around it is a ski and vacation resort. The area is heavily vegetated, and the only signs of the catastrophic eruption are the horseshoe-shaped crater and the debris-avalanche hummocks. The trees "laid prostate on the ground in thousands" are nowhere to be found, and the blast deposit is not easily recognized.
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 km 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 80km north of Molokai and 160km northeast of Oahu was reported by Moore when techniques of seafloor mapping were new. Bathymetric maps of the area revealed massive structures commonly from 8-25km long and 5-15km wide. They are commonly bounded by steep slopes as high as 2,000m and may have relatively flat tops of which most are tilted toward the Hawaiian Ridge, a volcanic ridge which extends 2,600km 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.
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