Landslides are one of the most common geological hazards on Earth, capable of changing landscapes, destroying infrastructure, and endangering life with little or no notice. They occur in all parts of the world, from steep mountain ranges to moderate coastal slopes and affect millions of people each year. Between 1998 and 2017, landslides affected an estimated 4.8 million people and killed over 18,000 people worldwide.
Understanding landslides, why they occur and how communities might mitigate risk is critical for creating safer, more resilient settings.
In this Article
What Is a Landslide?
A landslide is the downward movement of rock, soil, earth or debris due to gravity. This movement can be slow and barely noticeable, or sudden and catastrophic, with speeds faster than a person can run. They are also a type of mass wasting, which is a natural process in which gravity propels materials downhill.
A landslip happens when shear pressures on a slope exceed the shear strength of the materials that keep it together. Shear strength is determined by particle friction, cohesive bonding and the sediment’s internal structure. When these opposing forces weaken or when pushing forces increase, the slope breaks and the material begins to move.
Why Do Landslides Occur?
Every slope is a delicate balance between gravity dragging material down and the ground’s intrinsic strength holding it in place. Landslides occur when anything upsets the balance.
Water is one of the most common triggers of landslides as heavy or prolonged rainfall saturates the ground, increasing weight and decreasing friction between soil particles. Water reduces interparticle friction in sandy soils, whereas in clays it dissolves bonding cements and eliminates capillary tension, greatly weakening the substance. This combination can turn a stable hillside into a fast-moving river of mud.
Apart from heavy and prolonged rainfall, earthquakes and volcanic eruptions can also lead to landslides as they instantly destabilise slopes by shaking free material that is already on the verge of failure. Moreover, volcanic eruptions can add further hazards as melting snow and ice can create massive amounts of water and debris, resulting in disastrous volcanic mudflows, known as lahars.
In addition to this, rivers, waves and wind gradually erode the base of slopes. Over years, this undercutting removes support until the overlying material suddenly collapses. This process is common along coastal cliffs and river valleys.
Human development often increases landslide risk. This can be the result of excavation for roads or buildings, which can remove supporting material from slopes. Poor drainage design, deforestation and land‑use changes all reduce slope stability and increase susceptibility to failure.
Types of Landslides
The term “landslide” covers several distinct types of movement, each defined by how material moves and what it is made of. Landslides fall into five main categories, such as:
- Falls: Material like rocks, detaches from a steep slope or cliff and falls freely through the air causing rockfalls. They are common along mountainous highways and coastal cliffs.
- Topples: A block of rock or soil rotates forward around a pivot point and often occur in fractured rock masses.
- Slides: Involve movement along a defined surface. They can be either rotational, which means that they move along curved surfaces or translational, which means they move along flat paths.
- Spreads: When a stronger upper layer fractures and moves over a weaker, often saturated lower layer. They are common in flat or gently sloping terrain and can be triggered by earthquakes.
- Flows: Behave like thick liquids. Debris flows, also called mudflows or mudslides, are among the most dangerous types because they travel rapidly and can carry boulders, trees and vehicles downslope
Many real‑world landslides are complex, involving combinations of these movements.
Where Landslides Occur
Landslides can happen anywhere with unstable slopes. Globally, they are prevalent in:
- Mountainous regions
- Volcanic landscapes
- Coastal cliffs
- River valleys
- Areas impacted by wildfires or deforestation.
- Regions receiving heavy rainfall or quick snowmelt.
Climate change is increasing landslides danger by strengthening rainfall, melting permafrost, and destabilising slopes in high-altitude areas.
Impacts of Landslides
Landslides can cause severe and long‑lasting impacts on communities, infrastructure and ecosystems. This includes:
- Infrastructure Damage: Landslides can bury homes, destroy roads and railways, rupture pipelines and disrupt utilities and communication networks. Even slow‑moving landslides can cause major structural damage over time.
- Environmental Impacts: Landslides reshape landscapes, alter river courses and transport large volumes of sediment. They can also trigger secondary hazards such as flooding or tsunamis in the case of submarine landslides.
- Human Impacts: Fast‑moving landslides can cause trauma, suffocation and fatalities. Survivors may face long‑term mental health effects due to loss of family, homes or livelihoods. Landslides also strain health systems and emergency services, especially in remote or vulnerable regions.
Warning Signs of Landslides
While some landslides strike without warning, others show clear signs of developing movement. Common indicators include:
- New cracks in the ground or walls
- Tilting trees, leaning poles or deformed fences
- Sudden changes in water flow or new springs
- Bulging ground at the base of slopes
- Small rockfalls or minor debris movement
These signs reflect early slope deformation and should prompt immediate assessment and monitoring.
Mitigation and Prevention
Although landslides cannot be completely prevented, their impacts can be significantly reduced through:
- Engineering Solutions:
- Drainage improvements reduce water saturation and pore pressure.
- Retaining walls, rock bolts and slope reinforcement stabilise vulnerable slopes.
- Terracing and grading help manage surface runoff.
- Vegetation and Land Management:
- Restoring vegetation increases root strength and soil cohesion.
- Avoiding deforestation and managing land‑use changes helps maintain slope stability.
- Hazard Mapping and Planning:
- Governments and geologists use landslide susceptibility maps to guide development.
- Avoiding construction in high‑risk zones is one of the most effective mitigation strategies.
- Community Preparedness:
- Early warning systems, emergency planning and public education reduce casualties and improve response during landslide events.
- WHO emphasises the importance of resilient health systems and coordinated emergency response to reduce disaster impacts
Conclusion
Landslides are powerful natural processes shaped by geology, climate and human activity. While they cannot be eliminated, understanding how they form and recognising early warning signs, can help communities reduce risk and build resilience. As climate change intensifies rainfall and destabilises slopes worldwide, landslide awareness and preparedness are more important than ever.
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