DESCRIPTION:
Mount St. Helens 1980 Debris Avalanche Deposit

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.
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(On May 18, 1980) ... The collapse of the north flank ... (of Mount St. Helens)... produced the largest landslide-debris avalanche recorded in historic time. Detailed analysis of photographs and other data shows that an estimated 7-20 seconds (about 10 seconds seems most reasonable) elapsed between the triggering earthquake and the onset of the flank collapse. During the next 15 seconds, first one large block slid away, then another large block began to move, only to be followed by still another block. The series of slide blocks merged downslope into a gigantic debris avalanche, which moved northward at speeds of 110 to 155 miles an hour. Part of the avalanche surged into and across Spirit Lake, but most of it flowed westward into the upper reaches of the North Fork of the Toutle River. At one location, about 4 miles north of the summit, the advancing front of the avalanche still had sufficient momentum to flow over a ridge more than 1,150 feet high. The resulting hummocky avalanche deposit consisted of intermixed volcanic debris, glacial ice, and, possibly, water displaced from Spirit Lake. Covering an area of about 24 square miles, the debris avalanche advanced more than 13 miles down the North Fork of the Toutle River and filled the valley to an average depth of about 150 feet; the total volume of the deposit was about 0.7 cubic mile. The dumping of avalanche debris into Spirit Lake raised its bottom by about 295 feet and its water level by about 200 feet.

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.

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.

Details of the rockslide event, as revealed by eyewitness photographs, are correlated with features of the deposit. The photographs show three slide blocks in the rockslide movement. Slide block I was triggered by a magnitude 5.1 earthquake at 8:32 a.m. Pacific Daylight Time. An exploding cryptodome burst through slide block II to produce the "blast surge." Slide block III consisted of many discrete failures that were carried out in continuing pyroclastic currents generated from the exploding cryptodome. The cryptodome continued to depressurize after slide block III, producing a blast deposit that rests on top of the debris-avalanche deposit.

The hummocky 2.5 cubic kilometer debris-avalanche deposit consists of block facies (pieces of the pre-eruption Mount St. Helens transported relatively intact) and matrix facies (a mixture of rocks from the old mountain and cryptodome dacite). Block facies is divided into five lithologic units. Matrix facies was derived from the explosively generated current of slide block III as well as from disaggregation and mixing of debris-avalanche blocks.

The mean density of the old cone was measured to be abut 20 percent greater than the mean density of the avalanche deposit. Density in the deposit does not decrease with distance which suggests that debris-avalanche blocks were dilated at the mountain, rather than during transport. Various grain-size parameters that show that clast size converges about a mean with distance suggest mixing during transport.

The debris-avalanche flow can be considered a grain flow, where particles -- either debris-avalanche blocks or the clasts within the blocks -- collided and created dispersive stress normal to the movement of material. The dispersive stress preserved the dilation of the material and allowed it to flow.

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