Collapse of the summit and flank of a volcano can produce massive volcanic landslides. The resulting highly mobile debris avalanches rapidly sweep down the volcano and far beyond its flanks. These small hills in the foreground of Alaska's Augustine volcano show a morphology common to debris-avalanche deposits. The hummocks consist of relatively intact segments of the volcanic edifice that were transported long distances without disaggregating. This debris avalanche traveled roughly 11 km from the summit about 450 years ago.
This wave-cut section exposes the interior of the Burr Point debris-avalanche deposit from the 1883 eruption of Augustine volcano. The color mottling, which results from the transport of discrete segments of the volcano that are not thoroughly mixed, is a characteristic texture of debris-avalanche deposits. The scale bar is 2-m high.
The small hills in this photo were produced during a 1956 eruption of Bezymianny volcano that closely resembled the 1980 eruption of Mount St. Helens. The hummocky terrain is reminiscent of the debris-avalanche deposit filling the Toutle River at Mount St. Helens, and likewise was produced by a massive volcanic landslide when Bezymianny collapsed on March 30, 1956. The hummocks consist of material formerly composing the Bezymianny edifice that swept up to 18 km to the east in a highly mobile debris avalanche.
Collapse of Mexico's Colima volcanic complex during the late Pleistocene produced a major debris avalanche and debris flow that traveled 120 km from Nevado de Colima volcano to the Pacific Ocean. The quarried hummock in the foreground and the two brown hills in the middle distance are formed both of debris transported more than 100 km from Colima volcano (seen in the far distance above the lefthand hummock) and material from valley floors incorporated during transport.
Textures associated with volcanic landslides are preserved in this quarry wall more than 100 km from the source of a major avalanche from México's Colima volcano. Large fractured blocks appear at the top of the exposure, and a color mottling is produced by adjacent segments of the volcano or valley floor material incorporated during transport that are fractured, but not thoroughly mixed. A 2-m-high measuring rule provides scale.
Grass-covered conical hills dot the plains around Mount Egmont, on New Zealand's North Island. Small hills such as these, often located in lowland areas well beyond the flanks of a volcano, were once thought to be cinder cones or small secondary vents produced by explosions when a lava flow passed over a body of water. They now are known to be hummocks of massive debris avalanches produced by volcanic landslides. Debris-avalanche deposits originating from repetitive collapse of Egmont surround the volcano to distances of about 40 km.
This roadcut provides an outstanding section through a debris-avalanche deposit from Fuego volcano in Guatemala. The pronounced color mottling is a common texture of debris-avalanche deposits. It distinguishes them from deposits produced by other volcanic processes such as mudflows or pyroclastic flows. This texture results from the transport of coherent segments of the volcano over long distances with only partial disaggregation. Note that individual lava flow segments and pyroclastic units can be traced across small offsetting faults.
The hummocky surface in the foreground in front of the twin volcanoes of Fuego and Acatenango in Guatemala is a massive debris-avalanche deposit produced by collapse of ancestral Fuego volcano during the early Holocene. The avalanche, the largest known in Guatemala, has an estimated volume of about 15 cu km and traveled 50 km from its source at Fuego. The avalanche traveled for its last 30 km over flat Pacific coastal plain slopes of less than 1 degree, illustrating the extremely high mobility of volcanic debris avalanches.
This outcrop along the Oshira-kawa river east of Haku-san volcano in Japan shows textures that are common at debris-avalanche deposits. Large fractured clasts are carried in a finer matrix that shows variations in color. This results from the transport of small discrete segments of the volcano for long distances without being totally disaggregated and mixed together. This debris avalanche was produced by a volcanic landslide from the summit and eastern flank of Haku-san about 4200 years ago.
Collapse of the summit and flanks of volcanoes during major volcanic landslides creates large horseshoe-shaped depressions that open in the direction of the landslide. This 2.1 x 3.5 km horseshoe-shaped caldera at Iriga volcano in the Philippines was produced by a massive landslide that most likely occurred in 1628 AD. The resulting debris avalanche traveled more than 10 km to the SE and flowed into Lake Buhi at the upper right. This view is from the south, with the summit of Iriga at the extreme left.
Las Isletas, a group of islands NE of Mombacho volcano, were created by a large Holocene debris avalanche from Mombacho that swept into Lake Nicaragua. The avalanche traveled at least 12 km from Mombacho. The hummocky debris from the avalanche created an arcuate peninsula that extends into the lake as well as hundreds of small islands. This morphology is common where debris avalanches enter shallow bodies of water.
MacKenney cone, the historically active vent of Pacaya volcano in Guatemala, was constructed within a horseshoe-shaped caldera produced by collapse of the summit of an ancestral volcano about 1100 years ago. The SW caldera rim forms the steep-sided scarp at the right, and the small knob halfway down the left-hand skyline is a remnant of the partially buried opposite rim. The blocky hill in the foreground is a hummock from the debris avalanche produced by the collapse. The avalanche traveled 25 km down to the Pacific coastal plain.
A quarry displays a cross section of a debris-avalanche hummock, showing bedded layers of tephra on the right and a segment of a lava flow on the left. Both the the massive lava and the unconsolidated tephra layers were transported relatively intact for about 30 km within a massive debris avalanche from Raung volcano in eastern Java. The preservation of original stratigraphy from within the volcano is a common feature of debris-avalanche deposits.
The foreground hills are part of the Shasta Valley debris-avalanche deposit produced by one of the largest known Quaternary volcanic landslides. Roughly 46 cu km of an ancestral Mount Shasta collapsed about 350,000 year ago, producing a massive debris avalanche that swept some 50 km to the north, filling the broad Shasta Valley with hummocky debris.
The hilly topography in the foreground is part of the massive debris-avalanche deposit produced by collapse of Mount Shasta. The roughly 46 cu km, highly mobile debris avalanche swept some 50 km to the north. The hummocky area represents relatively intact segments of the volcano that were carried within a more fluid matrix facies of the avalanche. Individual hummocks range up to a few hundred m in height and roughly 1 km in length.
This photo, one of a series of dramatic photos taken from Bear Meadow NE of Mount St. Helens, shows the initial seconds of the May 18, 1980 eruption. A vertical eruption plume rises from the summit crater, while immediately below it the onset of the lateral blast can be seen. Both explosions were triggered by collapse of the north flank in a giant landslide, whose crest appears at the lower right as a gigantic wave mantled by fragments of white glacial ice.
The hill in the background is one of many hummocks forming the surface of the massive debris-avalanche deposit produced by collapse of the summit of Mount St. Helens on May 18, 1980. The different-colored rocks represent portions of the former volcano that were transported relatively intact far from the volcano. The avalanche traveled 25 km, filling the upper North Fork Toutle River to a maximum depth of nearly 200 m. The lighter-colored rocks in the foreground are pyroclastic-flow deposits.
The shear headwall of the landslide scarp created by collapse of Mount St. Helens on May 18, 1980, towers 550 m above the crater floor. The white areas on the crater rim are glaciers that were truncated by the collapse. Steam rises at the right from the new crater of St. Helens in this August 1980 view. Mount Hood is visible in the distance to the south across the Columbia River.
The horseshoe-shaped crater at Mount St. Helens is typical of scarps formed by massive landslides. On May 18, 1980 the upper 400 m of the summit was removed, leaving a 2 x 3.5 km horseshoe-shaped crater open to the north. Explosions played only an incidental role in formation of the crater, which resulted primarily from a massive landslide, the world's largest during historical time. The volume of the missing portion of the volcano closely matches the volume of the debris-avalanche deposit filling the North Fork Toutle River below the volcano.
This 18th century water-color map of Unzen volcano on the Shimabara Peninsula shows the extent of the catastrophic landslide from Mayu-yama (lower center) in 1792 that swept into the Ariake Sea at the bottom of the map. The irregular, orange-colored area along the coast delineates the extent of runup of the tsunami that was created when the avalanche entered the sea. The tsunami swept a 77-km length of the peninsula and caused nearly 15,000 fatalities here and along coastlines across the Ariake Sea. Map from Shimabara City Honko temple (published in Miyachi et al., 1987).