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Primary reference(s)

Varnes, D.J., 1978. Slope movement types and processes. In: Schuster, R.L. and R.J. Krizek (eds.), Landslides, Analysis and Control. Special report 176: Transportation Research Board. National Academy of Sciences, pp. 11-33.

Additional scientific description

The term ‘landslide’ encompasses five modes of slope movement: falls, topples, slides, spreads, and flows. These are subdivided according to the type of geological material (bedrock, debris, or earth). Slope movement occurs when forces acting down-slope (mainly due to gravity) exceed the strength of the earth materials that compose the slope.

Landslides are common on volcanic cones because they are tall, steep, and weakened by the rise and eruption of molten rock. Magma releases volcanic gases that partially dissolve in groundwater, resulting in a hot acidic hydrothermal system that weakens rock by altering minerals to clay (USGS, no date).

Volcano landslides (debris avalanches) range in size from less than 1 km3 to more than 100 km3 (USGS, no date). They comprise masses of rock, soil and snow that are mobilised when the flank of a volcano collapses and slides downslope. The mobilised sediment can be very destructive and entrain more sediment (as well as vegetation or structures) along its path. The high velocity and momentum allows them to cross valleys and run up slopes several hundred metres high. The larger landslides are generally more deep-seated, involving weak hydrothermal and magmatic systems in the volcano.

The landslides leave a hummocky terrain that reflects the initial structure of the edifice (de Vries and Davies, 2015). The sediment largely comprises unsorted and unstratified angular-to-subangular debris (Siebert, 1996). Runout lengths are commonly many times the height of the volcano. Many landslides contain or incorporate water that leads to secondary debris flow and lahar generation. Runout varies with the extent of air or fluid entrainment; however, the physical basis of the long runouts is not fully understood. Most are the result of several factors, including volcanic flank failures. Landslides on volcanic islands such as Hawaii, Reunion and Tristan da Cunha are characterised by long runout distances and volumes exceeding 1000 km3 (Hürlimann et al., 2000).

Metrics and numeric limits

Landslide movement is likely to be moderate in velocity (1.5 metres per day) to extremely rapid. With increased velocity, the landslide mass of translational failures may disintegrate and develop into a debris flow (Varnes, 1978). For example, the landslide at Mount St. Helens on 18 May 1980, with a volume of 2.5 km3, reached speeds of 50–80 m/s, with the energy to surge up and over a 400-m-tall ridge located about 5 km from the volcano (de Vries and Davies, 2015).

Examples of drivers, outcomes and risk management

Landslides can be extremely destructive, especially when failure is large, sudden and (or) the velocity is rapid. Rock avalanches pose some of the most dangerous and expensive geological hazards in mountainous terrain. The Mount St Helens eruption was triggered by landsliding as a consequence of structural instability of the volcano. The eruption caused the death of 57 people, 53 through direct impacts including asphyxiation, thermal injuries, and trauma. Snowmelt led to extensive river flooding (Oregon State University, 2020).

As well as the potential to trigger hydrothermal or magmatic eruptions and if the debris avalanches enter water bodies, tsunami may be generated (de Vries and Davies, 2015). As with other types of landslide, rock avalanche can cascade to form river dams with the potential for subsequent release and flooding.

The size of volcanos is such that remote sensing techniques can be used for monitoring, for example, GPS, aerial photography, and satellite imageries including InSAR (radar). At the local scale, ground-based techniques such as LiDAR and seismometers can be deployed (Moss et al., 1999; Highland and Bobrowsky, 2008) with varying degrees of success.

While the physical damage of landslides is well documented, health impacts are complex. The risk of an increase in infectious diseases is of concern during the response and recovery phase after any major disaster. Displacement of people due to the destruction of their homes and other infrastructure can place them in unfamiliar surroundings, which, if they conflict with traditional beliefs and practices with regard to water supply and hygiene, can result in unsafe behaviours. The medium- to long-term effects of changes to the environment caused by landslides, such as deforestation, and changes to river courses, can increase the risk of vector-borne diseases, and as a result, the health impacts can extend long after the initial disaster is over. Disruption of soil can also increase exposure to infectious organisms (Kennedy et al., 2015).

The psychosocial and mental health impacts on survivors and rescue personnel from landslides are increasingly recorded (e.g., Oregon State University, 2020). The prevalence of psychiatric disorders and wider support needed to reduce misuse of substances has been identified (Kennedy et al., 2015; Dell’Aringa et al., 2018).

Increasingly, the science of landslide physics is allowing the nature of these hazards to be understood, which is leading to better techniques through which they can be managed and mitigated.