Volcano Landslides and their Effects

Landslides are large masses of rock and soil that fall, slide, or flow very rapidly under the force of gravity. These mixtures of debris move in a wet or dry state, or both. Landslides commonly originate as massive rockslides or avalanches which disintegrate during movement into fragments ranging in size from small particles to enormous blocks hundreds of meters across. If the moving rock debris is large enough and contains a large content of water and fine material (typically, >3-5 percent of clay-sized particles), the landslide may transform into a lahar and flow downvalley more than 100 km from a volcano!

Volcano landslides range in size from less than 1 km3 to more than 100 km3. The high velocity (>100 km/hr) and great momentum of landslides allows them to run up slopes and to cross valley divides up to several hundred meters high. For example, the landslide at Mount St. Helens on May 18, 1980, had a volume of 2.5 km3, reached speeds of 50-80 m/s (180-288 km/hr), and surged up and over a 400 m tall ridge located about 5 km from the volcano!

Landslides are common on volcanoes because their massive cones (1) typically rise hundreds to thousands of meters above the surrounding terrain; and (2) are often weakened by the very process that created them--the rise and eruption of molten rock. Each time magma moves toward the surface, overlying rocks are shouldered aside as the molten rock makes room for itself, often creating internal shear zones or oversteepening one or more sides of the cone. Magma that remains within the cone releases volcanic gases that partially dissolve in groundwater, resulting in a hot acidic hydrothermal system that weakens rock by altering rock minerals to clay. Furthermore, the tremendous mass of thousands of layers lava and loose fragmented rock debris can lead to internal faults and fault zones that move frequently as the cone "settles" under the downward pull of gravity.

These conditions permit a number of factors to trigger a landslide or to allow part of a volcano's cone to simply collapse under the influence of gravity:

• intrusion of magma into a volcano
• explosive eruptions (magmatic or phreatic--steam-driven explosions)
• large earthquake directly beneath a volcano or nearby (typically >M5)
• intense rainfall that saturates a volcano or adjacent tephra-covered hillslopes with water, especially before or during a large earthquake.

A landslide typically destroys everything in its path and may generate a variety of related activity. Historically, landslides have caused explosive eruptions, buried river valleys with tens of meters of rock debris, generated lahars, triggered waves and tsunami, and created deep horseshoe-shaped craters.

By removing a large part of a volcano's cone, a landslide may abruptly decrease pressure on the shallow magmatic and hydrothermal systems, which can generate explosions ranging from a small steam explosion to large steam- and magma-driven directed blasts. A large landslide often buries valleys with tens to hundreds of meters of rock debris, forming a chaotic landscape marked by dozens of small hills and closed depressions. If the deposit is thick enough, it may dam tributary streams to form lakes in the subsequent days to months; the lakes may eventually drain catastrophically and generate lahars and floods downstream.

Landslides also generate some of the largest and most deadly lahars, either by transforming directly into a lahar or, after it stops moving, from dewatering of the deposit. Historically, however, the most deadly volcano landslide occurred in 1792 when sliding debris from Mt. Mayuyama near Unzen Volcano in Japan slammed into the Ariaka Sea and generated a wave on the opposite side that killed nearly 15,000 people.

On a volcano, landslides typically carve deep gashes into its cone or create large horseshoe-shaped craters hundreds of meters deep and more than a kilometer in width.

Volcanic landslides can...

The illustration shows the landslide (green) and directed blast (red) that occurred during the first few minutes of the eruption of Mount St. Helens in 1980.

Before the eruption, an estimated 0.11 km³ of dacite magma had intruded into the volcano (equivalent to sphere about 600 m in diameter!). The rising magma forced the volcano's north flank (right side of illustration) outward about 150 m and heated the volcano's ground water system, causing many steam-driven explosions (phreatic eruptions).

The hot magma and surrounding hydrothermal system were unroofed by the landslide (green), and the resulting rapid depressurization caused a series of steam- and volcanic-gas-driven explosions. The explosions burst through part of the landslide, blasting rock debris northward. The resulting pyroclastic surge quickly overran the landslide and spread over ridges and valleys across an area of 550 km².

This house is partially buried in a lahar deposit that was formed by the dewatering of a large volcano landslide from Mount St. Helens, Washington. 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.

Many volcano landslides do not stop so close to their source, but instead keep moving by transforming directly into a lahar. These lahars can be extremely hazardous because of their size and mobility (they may travel more than 100 km). Photograph by L. Topinka in 1981

View of Mt. Mayuyama is toward the north (Photograph by T. Casadevall in 1991). Mt. Mayuyama is one of the dacite lava domes that form the Unzen Volcano complex on Kyushu Island, Japan. The east flank of Mt. Mayuyama collapsed without warning on May 21, 1792, and generated a landslide that swept through Shimabara City and slammed into the Ariaka Sea. The displacement of water triggered a tsunami along the adjacent shoreline of Shimabara Peninsula (visible above and right of Mayuyama) and also 17-23 km across the Ariaka Sea in neighboring provinces. The landslide and tsunami killed nearly 15,000 people, Japan's worst historical volcanic disaster.

Scientists have interpreted the conspicuous hummocks along the shore as part of a landslide deposit that occurred before 1792. Maps submitted to the Tokugawa Shogunate in 1792 as the official documents of the Shimabara Catastrophe clearly show the existence of small islands before the disaster struck.

Other historical volcano landslides are known to have generated tsunami, including:

• landslide from Kamagatake volcano on Hokkaido Island, Japan, in 1640 killed 700 people
• landslide from Oshima-Oshima Volcano on Hokkaido Island, Japan, in 1741-42 killed 1,474 people on Hokkaido and northern Honshu
• landslide from Augustine Volcano, Alaska, in 1883 triggered a tsunami that swept across Cook Inlet onto the Kenai Peninsula but caused no damage

A scientist stands on one of the many small hills called hummocks that form the chaotic surface of a massive landslide deposit in the upper North Fork Toutle River valley below Mount St. Helens volcano (10 km in distance). Before the landslide and eruption on May 18, 1980, a forest grew on this part of the valley floor, and a highway followed the meandering river to Spirit Lake, a popular recreation area.

The landslide deposit extends about 22 km from the volcano and buries the river valley to an average depth of about 45 m. In places, the deposit is nearly 200 m thick! The landslide covers an area of about 60 km².

An exceptionally large landslide deposit was discovered at Mount Shasta shortly after the eruption of Mount St. Helens. This landslide has a volume of about 45 km³--nearly 20 times larger than the one that buries the North Fork Toutle River valley (above)--and it covers an area of 675 km². Photograph by L. Topinka in 1981.

View is looking northwest up the valley of former Coldwater Creek, now filled with a lake. When the landslide from Mount St. Helens slid into the North Fork Toutle River valley (foreground), it blocked the flow of Coldwater Creek. Water backed up behind the landslide deposit, gradually forming a lake about 8 km long and 55 m deep. The landslide was rushed down the Toutle valley from right to left.

Concern about the sudden breakout of water from Coldwater Lake from failure of the drebris dam or overtopping and subsequent erosion of the dam, led the Corps of Engineers in 1981 to control the lake level by excavating an outlet channel that delivers water to the Toutle River.

The fan-shaped delta on the southeast shore of Coldwater Lake forms where a stream from South Fork Coldwater Creek pours into the lake. The delta began to grow quickly when water from Spirit Lake was diverted into Coldwater Creek beginning in 1985. A long tunnel was drilled through a ridge to deliver water from Spirit Lake into South Fork Coldwater Creek in order to stabilize the level of Spirit Lake. Photograph by L. Topinka on January 13, 1984.

View is looking south into the crater of Mount St. Helens formed by an enormous landslide on May 18, 1980. The newly-formed crater is about 2 km wide (east-west), 3 km long (north-south), and about 600 m deep. The landslide removed about 2.3 km³ from the volcano's cone, which towered 1,035 m above the crater floor!

Large horseshoe-shaped craters, open at one end, have long been noted in many volcanic regions around the world. The origin of these breached craters has been controversial, but since the landslide and eruption of Mount St. Helens in 1980, many have been interpreted by scientists as the result of a landslide.

If a large landslide creates a horseshoe-shaped crater that exposes a volcano's eruptive vent, the deep crater will likely direct subsequent volcanic activity (lava flows, pyroclastic flows, or lahars) toward its breached opening. A new hazard assessment may be necessary to determine the way in which volcano-hazard areas downslope from the crater may have changed. Photograph by C.D. Miller in 1980.

All cases can be found on our old site

Historical landslides

• Mount St. Helens, Washington, 1980
• Otake volcano, Japan, 1984
• Huila Volcano, Colombia, 1994
• Casita Volcano, Nicaragua, 1999

Pre-historical landslides

• Landslides at Mount Rainier volcano, Washington
• Examples of volcanic landslide deposits from the United States and around the world