Landslides are a form of mass wasting, defined as the movement of rock, earth, or debris down a slope under the influence of gravity. This movement occurs when the forces pushing the material down exceed the internal strength of the slope material. The term encompasses various types of movement, including falls, flows, slides, and spreads. Understanding where landslides occur and what causes them requires looking at static land conditions and dynamic external triggers.
Geological and Topographical Prerequisites
Landslides occur where the terrain possesses inherent instability for mass movement. The most significant topographical factor is the presence of a steep slope, which increases the downslope force exerted by gravity. While very high-angle slopes composed of solid rock are generally stable, slopes between approximately 20° and 50° are often highly susceptible to failure.
The stability of the slope is influenced by the composition of earth materials, particularly weak or highly weathered geology. Layers of unconsolidated sediment, such as fine-grained clays or silts, are inherently unstable, especially when saturated. Fractured bedrock or rock masses with joints and faults create low-strength surfaces along which movement can occur, particularly if these discontinuities are oriented parallel to the slope angle.
Tectonically active zones, such as mountainous regions or areas near fault lines, concentrate the geological prerequisites for landslides. The processes of folding and faulting create complex geological structures and highly stressed rock masses prone to failure. These areas, particularly those experiencing high rainfall, are constantly being pushed toward a state of instability.
Environmental and Climatic Triggers
Once the geological stage is set, dynamic natural events act as immediate triggers that push an unstable slope past its breaking point. Intense or prolonged heavy precipitation is the most common natural trigger for landslides globally. Water saturates the soil and rock debris, significantly increasing the material’s overall weight, which adds to the downslope force.
Water also reduces the shear strength, or friction, between soil particles by increasing the pore-water pressure. This process lubricates the potential failure surface, turning stable ground into a mobile slurry or rapidly moving flow. Rapid melting of snowpack can have a similar saturating effect on the underlying soil, leading to slope failure.
Seismic activity from earthquakes can trigger widespread landslides through rapid ground shaking and deformation. The sudden acceleration of the ground increases stress on the slope materials, causing existing weak planes to fail. Earthquakes can also induce liquefaction, where saturated, loose sediments temporarily lose strength and behave like a liquid, leading to a lateral spread.
Volcanic activity can generate unique types of landslides, most notably lahars, which are fast-moving mudflows. Eruptions melt snow and ice on the volcano’s slopes, mixing the water with loose ash, rock debris, and soil to create a dense, destructive flow. Hot, acidic fluids within a volcano can chemically alter and weaken the rock mass over time, making the slopes susceptible to failure even without an eruption.
Human Activity and Landslide Vulnerability
Human modifications to the landscape significantly increase the vulnerability of slopes to failure, often in marginally stable areas. Deforestation, particularly on steep hillsides, removes the root systems that bind the soil together, decreasing the soil’s cohesive strength. Without the canopy to intercept rainfall, water infiltration increases, raising the saturation level and pore-water pressure, accelerating destabilization.
Urbanization and infrastructure development directly alter the geometry and stress on slopes. Construction of roads, buildings, and mines often involves excavation at the base of a slope, removing supporting material and increasing the slope’s angle. Conversely, placing heavy fill material or buildings on the crest of a slope adds weight, which increases the downslope driving force, making the slope prone to collapse.
Altering natural drainage patterns is a significant human factor that can lead to localized failures. Poorly managed drainage systems, water leakage from utilities, or the diversion of rivers can concentrate water flow, leading to localized erosion or slope saturation. Even agricultural practices like irrigation introduce excessive water into the subsurface, weakening the soil structure and triggering landslides in areas not typically prone to heavy rainfall.