A landslide is a mass movement of rock, debris, or earth down a slope under the influence of gravity. The velocity of a landslide can range dramatically from movements slower than human perception to those that travel at highway speeds. Understanding the rate of movement is fundamental, as speed determines the destructive potential and the possibility of warning or mitigation.
Classifying Landslide Speed: The Velocity Scale
Geologists use a standardized, logarithmic scale to classify landslide velocity, which provides a common language for describing the rate of movement. This system divides movement into seven distinct classes.
The slowest class, “Extremely Slow,” moves less than 0.6 inches (15 millimeters) per year, a rate only detectable through long-term instrument monitoring. These imperceptible movements often involve deep-seated creep in clay soils or bedrock.
The scale progresses through “Very Slow” (feet per year) and “Slow” (feet per month). Movement in these classes is noticeable but still allows for hazard mitigation efforts.
A landslide categorized as “Rapid” moves at speeds between 1.5 feet per minute and 1.5 feet per second. This speed is the threshold where movement becomes too fast for effective human intervention.
The fastest categories are designated “Very Rapid” and “Extremely Rapid.” An “Extremely Rapid” landslide exceeds 16 feet per second (5 meters per second), or over 11 miles per hour. At this velocity, the movement is sudden and swift, leaving no time for escape and presenting an immediate threat to life and infrastructure.
How Movement Type Dictates Velocity
The physical mechanism by which material moves down a slope is the primary determinant of its potential speed. Landslides are broadly categorized by their movement style: falls, slides, and flows, each with an inherent velocity range.
Falls are typically the fastest type, involving rock or debris detaching from a steep slope and descending mostly through the air by free-fall, bouncing, or rolling. This lack of friction against a failure surface allows rockfalls and debris avalanches to achieve “Extremely Rapid” speeds, sometimes exceeding 100 miles per hour.
Flows, which involve the internal deformation of material moving in a fluid-like manner, also frequently achieve high speeds. Debris flows are highly saturated mixtures of rock, soil, and water that travel rapidly down channels, often reaching “Rapid” or “Very Rapid” velocities. The high water content acts as a lubricant, reducing internal resistance and allowing the mass to move quickly and far.
In contrast, movements classified as slides, where a cohesive mass moves along a distinct surface of rupture, generally exhibit moderate speeds. Rotational slides, or slumps, often show “Slow” to “Moderate” movement because the curved failure surface tends to restore equilibrium, slowing the descent. Earthflows, which involve viscous movement of fine-grained, water-saturated material, are typically slower than debris flows, often moving at “Slow” or “Very Slow” rates. The slowest movements, such as soil creep, remain firmly in the “Extremely Slow” category.
Environmental and Material Factors Affecting Velocity
Even within a single movement type, local environmental and material conditions critically modify the final travel speed.
Water Content
Water content is a significant factor, as complete saturation of soil or debris reduces the effective strength and internal friction of the material. This increase in pore-water pressure can cause the soil to liquefy, transforming a stable slope into a highly mobile, high-velocity flow. The weight added by the water also increases the driving force, contributing to greater acceleration.
Slope Angle
The angle of the slope provides the gravitational force necessary to initiate and maintain movement. Steeper slopes naturally lead to greater gravitational acceleration, which directly increases the potential velocity of any landslide. This is why the fastest events, like rockfalls and debris avalanches, are almost exclusively found on very steep terrain. Conversely, gentler slopes can only sustain much slower forms of movement.
Material Composition and Triggers
Material composition plays a role by determining the internal resistance of the moving mass. Fine-grained materials like silt and clay, when saturated, can move faster than coarse, interlocking boulders because they offer less internal friction. The intensity of the initial trigger is also a modifier; a sudden, strong earthquake or an extremely intense burst of rainfall can initiate a failure at a higher initial speed compared to a gradual weakening over time. These variables combine to create the complex, dynamic range of velocities observed in natural landslides.