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Landslides are common on tall, steep, and weak volcanic cones
Landslides are large masses of wet or dry rock and soil that fall, slide or sink very rapidly under the influence of gravity. Landslides usually take the form of massive rockfalls or avalanches, which disintegrate during their movement into fragments ranging from small particles to blocks several hundred metres in diameter. If the landslide is large enough and contains a sufficiently high percentage of water and fine material (typically >3-5 per cent clay-sized particles), it can develop into a lahar that can travel up to 200 km downstream.
Landslides are common on volcanic cones because they are high, steep and weakened by the rising and eruption of molten rock. The magma releases volcanic gases that partially dissolve into the groundwater, creating a hot, acidic hydrothermal system that weakens the rock by turning the minerals into clay. In addition, the mass of thousands of layers of lava and fragmented rock debris can lead to frequently shifting fault zones.
Volcano landslides (debris avalanches) range in size from less than 1 km3 to more than 100 km3 . The high speed and momentum of landslides allow them to cross watersheds and travel up slopes of several hundred metres. For example, the landslide that occurred on 18 May 1980 at Mount St Helens had a volume of 2.5 km3 , reached speeds of 50-80 m/s and surged up slopes of several hundred metres.
After the May 18, 1980 eruption, Mount St. Helens’ elevation was only 8,364 feet (2,550 meters) and the volcano had a one-mile-wide (1.5 kilometers) and approximately 600 m (2000 ft) deep horseshoe-shaped crater. View here is from the northwest.
Hummocks on West Island, Alaska, 8 km (5 mi) WNW from Augustine Volcano summit. These small hills were deposited as part of the 1883 debris avalanche (landslide) that triggered a Cook Inlet tsunami.
Landslide scarp on Mt. Mayuyama, part of the Unzen Volcano complex – Kyushu Island, Japan. East flank collapsed May 21, 1792 and a landslide swept through Shimabara City to the sea forming a tsunami.
Several conditions can trigger landslides:
- the intrusion of magma into a volcano.
- explosive eruptions.
- large earthquake directly beneath a volcano or nearby (typically >M5).
- heavy or long-lived rainfall that saturates the ground.
Early on the morning of May 18, 1980, the landslide swept into the upper North Fork Toutle River valley and came to rest within about 22 km of the volcano. The landslide deposit, however, was saturated with water and contained snow and ice blocks from the volcano’s former glaciers. As soon as the landslide stopped moving, water percolated to the top of the deposit and poured across its irregular surface, forming many lahars that merged as they rushed down the valley. The peak flow swept from the deposit about 5 hours after the landslide was emplaced!
The lahar flowed down the Toutle River throughout the afternoon and evening, reaching its peak at midnight about 60 km downstream from the volcano. The lahar destroyed roads, bridges, and homes
Large landslides typically destroy everything in their path and may cause additional hazards
By removing a large part of a volcano’s cone, a landslide can abruptly decrease the pressure on shallow magma and hydrothermal systems, which can generate explosions ranging from small steam explosions to large steam- and magma-driven explosions. These lead to the risk of tephra and ash falling into the surrounding areas.
Large landslides often bury valleys with tens to hundreds of metres of rock debris, forming a chaotic landscape marked by dozens of small hills (hummocks) and closed depressions. If the deposit is thick enough, it can dam up tributary streams to form lakes; these can eventually drain catastrophically, forming lahars and floods downstream. Landslides also generate some of the largest and most deadly lahars, either by becoming a lahar directly or, after ceasing to move, by breaking away from the deposit.
Historically, the deadliest volcanic landslide occurred in 1792, when sliding debris from Mount Mayuyama, near Unzen volcano in Japan, crashed into the Ariaka Sea and generated a tsunami that reached the opposite shore and killed nearly 15,000 people.
Large horseshoe-shaped craters, open at one end, have long been observed in many volcanic regions of the world. The origin of these open craters is controversial, but since the debris avalanche and eruption of Mount St Helens in 1980, scientists believe that many of them formed as a result of landslides. If the primary eruptive vent is located within these deep craters, it is likely to direct subsequent volcanic activity (lava flows, pyroclastic flows or lahars) towards its broken opening.
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