Landslides are a type of mass wasting, involving the downslope movement of rock, debris, or earth driven by gravity. Landslides occur when downslope forces overcome a material’s internal strength and resistance. They can happen suddenly or slowly, affecting environments from mountains to coastal cliffs. Multiple factors can contribute to their occurrence, ranging from inherent land characteristics to specific natural events and human activities.
Underlying Natural Factors
The inherent characteristics of a landscape significantly influence its susceptibility to landslides. Geological factors, such as the type of rock and soil present, determine how stable a slope is. Weak or fractured rock formations, like some sedimentary rocks such as shale and sandstone, are more prone to landslides due to their layered structure and potential for failure along bedding planes. Igneous and metamorphic rocks, while generally more resistant, can still be susceptible if they contain extensive jointing and fracturing.
The composition of soil also plays a significant role in slope stability. Clay-rich soils, for instance, can be particularly problematic because they absorb large amounts of water, becoming less cohesive and very slippery when wet. This reduces the friction between soil particles, making it easier for material to slide downslope. The material’s internal strength is crucial; when weakened, it can no longer withstand gravitational forces.
The steepness of a slope is another primary factor, as gravity exerts a stronger downslope force on steeper inclines. Slopes typically become unstable when their angle exceeds a certain threshold, often cited as above 15 degrees, though this varies based on material type. Pre-existing cracks, faults, or discontinuities within the rock and soil layers also create zones of weakness, providing pathways for water infiltration and reducing the overall stability of the slope. Weathering processes, such as freeze-thaw cycles, can further reduce the cohesion between rock grains over time, contributing to the weakening of the slope material.
Key Natural Triggers
Specific natural events often act as immediate catalysts for landslides, initiating movement in already susceptible slopes. Water saturation is a prominent trigger, typically resulting from heavy rainfall or rapid snowmelt. When soil and rock become saturated with water, the water adds significant weight to the slope, increasing the gravitational force pulling material downhill. Water also reduces friction between soil particles and within rock fractures, weakening the material’s internal strength. This process can transform solid material into a fluid-like slurry, leading to debris flows or mudflows.
Seismic activity, primarily earthquakes, can also trigger widespread landslides. The ground shaking and vibrations produced by earthquakes can destabilize slopes by applying cyclic shear loads to the soil and rock. This strong ground motion can cause a rapid decrease in the shear strength of the slope material. In some cases, seismic shaking can lead to soil liquefaction, where saturated granular soils temporarily lose their strength and behave like a liquid, making them highly susceptible to landslides.
Volcanic eruptions are another natural trigger for landslides. Volcanic cones are inherently unstable due to their steep slopes and the presence of weakened, fractured rock and loose pyroclastic deposits. Magma releases volcanic gases that can dissolve into groundwater, forming acidic hydrothermal systems that alter minerals into clay, further weakening the rock. Eruptions can directly trigger landslides (debris avalanches) through ground vibrations, or indirectly by melting snow and ice, which generates large volumes of water that mix with volcanic ash and debris to form destructive lahars.
Human Contributions
Human activities contribute to landslides, either by directly triggering them or indirectly increasing slope vulnerability. Improper land use practices, such as deforestation, significantly elevate landslide risk. Tree roots naturally bind soil particles together and help absorb rainwater, stabilizing slopes. When forests are cleared, this natural support is lost, leaving the soil exposed and unstable, particularly on steep slopes. The absence of vegetation leads to increased water runoff, which accelerates erosion and soil saturation, making the ground heavier and more prone to movement.
Excavation and cutting into slopes for construction, road building, or mining operations can directly destabilize the land. These activities alter the natural angle and support of a slope, potentially creating oversteepened sections that are unable to withstand gravitational forces. Blasting associated with mining and construction also generates vibrations that can weaken surrounding soil and rock, contributing to slope failure. These modifications can remove support from the base of a slope, increasing its susceptibility to collapse.
Altering natural drainage patterns is another human activity that increases landslide risk. Construction of buildings, roads, and other infrastructure can redirect surface water flow, concentrating it in areas that may not be able to handle the increased volume. This can lead to excessive soil saturation and erosion in new locations, weakening slopes. Additionally, adding excessive weight to slopes through the construction of buildings, waste disposal, or accumulation of fill material can increase the downslope forces, pushing already unstable ground past its breaking point. These modifications disrupt the balance of natural slopes, making them more susceptible to landslides.