Why Do Landslides Happen? Exploring the Key Triggers

Every year, thousands of tons of earth suddenly break free from hillsides and rush downward, sweeping away homes, roads, and lives in their path. These powerful events, known as landslides, occur in nearly every corner of the globe, from the steep Himalayas to the gentle slopes of suburban neighborhoods. But what triggers these devastating movements of earth and rock? Understanding the causes of landslides is essential for predicting where and when they might strike, potentially saving countless lives and protecting communities from disaster.

Natural triggers that set slopes in motion 🔗

Nature has its own ways of destabilizing slopes, and among the most powerful triggers is water. When heavy rainfall saturates the soil, it acts like a lubricant between soil particles, dramatically reducing the friction that normally holds the slope in place. According to the U.S. Geological Survey, rainfall-induced landslides are among the most common types, as water adds weight to the slope while simultaneously weakening the bonds between soil particles. Imagine building a sandcastle at the beach-a little moisture helps hold the grains together, but too much water causes the entire structure to collapse and flow away.

The relationship between water and landslides becomes even more critical during periods of rapid snowmelt. In mountainous regions, thick snow accumulations can suddenly release vast amounts of water into the ground as temperatures rise, creating conditions similar to intense rainfall. This phenomenon transforms stable winter slopes into unstable masses ready to move at the first opportunity.

When the earth shakes 🔗

Earthquakes represent another formidable natural trigger for landslides. Ground shaking can destabilize slopes that were already on the verge of failure, or it can loosen material that had been stable for years. The Encyclopedia of the Environment notes that during the twentieth century, nearly 80 earthquakes caused between 100,000 and 1,000,000 ground movements worldwide, claiming tens of thousands of lives. The 2008 earthquake in China alone triggered more than 15,000 landslides, accounting for nearly one-third of the earthquake’s total fatalities.

What makes earthquake-triggered landslides particularly dangerous is their sudden onset and widespread distribution. A single seismic event can destabilize slopes across hundreds of square kilometers, creating multiple simultaneous hazards. Even earthquakes of moderate magnitude-as low as 4.0-can trigger landslides, especially in areas where slopes are already weakened by other factors.

Volcanic eruptions and their cascading effects 🔗

Volcanoes contribute to landslide hazards in multiple ways. During eruptions, volcanic activity can rapidly melt snow and ice on mountain peaks, creating massive flows of water, ash, rock, and debris known as lahars. These volcanic landslides are among the most devastating types, as they combine the destructive power of fast-moving material with extreme heat. The National Geographic Society points out that the largest landslide in recorded history occurred after the 1980 eruption of Mount St. Helens, with a volume of 2.9 cubic kilometers of material covering an area of 62 square kilometers.

Beyond the immediate eruption, volcanic deposits create unstable slopes that remain vulnerable to landslides for years afterward. Loose ash layers, when saturated with water, can flow downhill like liquid, creating ongoing hazards for communities near volcanic regions.

The slow work of erosion 🔗

Not all natural causes of landslides are dramatic. Erosion-the gradual wearing away of earth by water, wind, or ice-steadily undermines slopes over time. Rivers cutting through valley floors remove support from hillsides above, creating increasingly steep and unstable conditions. Coastal erosion from waves can similarly undercut cliffs, leading to eventual collapse. This process might take decades or even centuries, but it inexorably sets the stage for future slope failures.

Human activities that increase landslide risk 🔗

While nature plays a significant role in causing landslides, human activities have dramatically increased both the frequency and severity of these events. Research published in Nature Communications indicates that deforestation, infrastructure construction, and mining triggered approximately 16% of fatal landslides between 2004 and 2016.

Deforestation: removing nature’s anchor 🔗

Trees and vegetation act as natural stabilizers for slopes, with their root systems creating a network that binds soil particles together. When forests are cleared for agriculture, logging, or development, this crucial stabilizing mechanism disappears. Studies from the Kivu Rift region show that deforestation has considerably increased landslide occurrence, with the impact of drastic forest cover changes being far more significant than climate change effects.

The problem compounds over time as tree roots decay. Even after the initial clearing, slopes remain vulnerable for years as the underground root network gradually decomposes, leaving soil increasingly loose and susceptible to movement. This explains why landslides often occur in areas that were deforested years or even decades earlier.

Construction and excavation activities 🔗

Modern development frequently requires cutting into hillsides to create roads, build homes, or establish infrastructure. These activities fundamentally alter the natural geometry of slopes, often making them steeper and removing material that provided crucial support at the slope’s base. When construction workers excavate without proper engineering controls, they can inadvertently create conditions ripe for landslides.

The added weight of buildings and infrastructure on slopes introduces additional stress. Heavy structures increase downward force on the hillside, and if drainage systems are inadequately designed, water can accumulate in ways that nature never intended, creating new pathways for instability.

Mining operations and ground vibrations 🔗

Mining activities pose unique risks for landslide initiation. According to research on mining as a key human cause of landslides, approximately 50% of human activities that cause landslides involve mining operations. The vibrations from blasting techniques can reach hundreds of meters below the surface, destabilizing slopes that might otherwise remain stable. Open pit mining, underground mining, and quarrying all contribute to slope instability through a combination of excavation, vibration, and alteration of natural water drainage patterns.

Understanding slope instability factors 🔗

Beyond specific triggers, certain fundamental factors determine whether a slope is susceptible to landslides. These factors create the underlying conditions that allow triggers to transform stable hillsides into moving masses of earth.

The critical role of water content 🔗

Water content in soil and rock plays a paradoxical role in slope stability. Small amounts of moisture can actually increase stability by creating surface tension between particles. However, as water content increases, pore pressure builds up, pushing soil particles apart and dramatically reducing the friction and cohesion that hold the slope together. The U.S. Geological Survey explains that water cannot be compressed, so any pressure on water increases pressure on surrounding soil particles, potentially breaking the connections between them.

This explains why landslides so often follow heavy rainfall or rapid snowmelt-the sudden influx of water overwhelms the soil’s ability to maintain stability. In some cases, water can accumulate at the boundary between porous and impermeable soil layers, creating a slippery interface along which the upper material can slide.

Slope gradient and geometry 🔗

The steepness of a slope fundamentally affects its stability. Gravity constantly pulls material downward, and on steep slopes, this downward force becomes increasingly difficult for the earth’s internal strength to resist. When slopes exceed critical angles-which vary depending on the material involved-the driving forces that promote movement exceed the resisting forces that maintain stability.

The orientation of underlying rock layers also matters significantly. Slopes where rock layers dip in the same direction as the slope face are inherently less stable than those where layers run perpendicular to the slope. This geological structure can make the difference between a slope that remains stable for centuries and one that fails catastrophically.

Ground conditions and material strength 🔗

Not all earth materials are created equal when it comes to slope stability. Weak or fractured rock, clay-rich soils, and materials with varying strengths in different layers all create conditions favorable for landslides. Some geological formations are inherently unstable due to their composition or the way they were formed.

Temperature variations can also affect ground conditions. Repeated freezing and thawing cycles cause rocks to fracture and soil to expand and contract, gradually weakening the material over time. In volcanic regions, mineral composition and the way materials were deposited during past eruptions influence long-term stability.

What do you think? As human development continues to expand into mountainous and hilly terrain, how can communities balance the need for growth with the imperative to minimize landslide risks? What role should scientific understanding of landslide causes play in planning where and how we build?

References

1. https://www.usgs.gov/faqs/what-a-landslide-and-what-causes-one
2. https://www.encyclopedie-environnement.org/en/soil/why-earthquakes-trigger-catastrophic-landslides/
3. https://education.nationalgeographic.org/resource/landslide/
4. https://pmc.ncbi.nlm.nih.gov/articles/PMC8764696/
5. https://www.sciencedaily.com/releases/2021/08/210819113015.htm
6. https://www.irjet.net/archives/V7/i4/IRJET-V7I41241.pdf
7. https://pressbooks.cuny.edu/gorokhovich/chapter/landslide-causes-and-triggering-mechanisms/