Geology Cafe

Landslide Hazards—An Awareness Guide

Table of Contents

Types of Landslides and Associated Processes

Landslide is a general term used for a variety of landscape processes including slumps, rockfalls, avalanches, debris flows, mudflows, and lahars.
Types of landslides
Selected landscape features associated with landslides.

A Rockfall is the relatively free falling or precipitous movement of a newly detached segment of bedrock of any size from a cliff or very steep slope; it is most frequent in mountainous areas during spring when there is repeated freezing and thawing of water in cracks of rock. Movement may be straight down or in a series of leaps and bounds down the slope; it is not guided by an underlying slip surface. Rockfalls are commonly associated with cliffs or escarpments. Over time, rockfalls will accumulate as talus (an accumulation of rock and soil that forms a rocky slope at the base of a cliff or escarpment).

An avalanche is like a landslide or a rockfall except the majority of the material is snow and ice (along with soil, rock, and other debris). Avalaches sometimes reach velocities over 300 miles/hour (500 km/hour). Avalanches occur in high-mountain terrains that receive winter precipitation as snow.

A slump is the downward slipping of a mass of rock or unconsolidated material, moving as a unit, usually with backward rotation on a more or less horizontal axis parallel to a slope or cliff from which it descends. Slumps typically have a fault-like escarpment (or scarp) and fissures at their upper end. In a slump, the earth moves as a great block or series of blocks and moves along a basal glide plane (or multiple glide planes in complex slumps). The landscape at the toe of a slump is typically chaotic, displaying a lumpy structure, sometimes with transverse ridges, ponded areas, and in forested areas the trees may be leaning or fallen over from slump movement (the name deranged forest or drunken forest is used to describe forested areas affected by landslides). Slumps typically move quite slowly, measurable in inches to many feet in a year. Slumps can also fail rapidly, potentially contributing to an debris flow, earthflow, mudflow, or debris flood.

Debris flows are a type of landslide where a moving mass of rock fragments, soil, and mud with 70 to 90 percent of the material consisting of sediment (the rest is water and trapped gasses). Slow debris flows may only move a few feet per year, whereas rapid ones can reach speeds greater than 100 miles per hour. Debris flows typically consist of a mix of mud, sand, and typically contain rock fragments ranging in size from fine gravel to massive boulders. The general term earthflow is a type of landslide where material detaches from a hillside and moves downslope to form lobe-shaped accumulations at the base of the slope. If it contains a high percentage of fine material and flows with a high degree of fluidity it is called a mudflow. A debris flood is a typically disastrous flood, intermediate between the turbid flood of a mountain stream and a debris flow, ranging in sediment load between 40 to 70 percent. Debris flows and debris floods may travel long distances down canyons before spreading out across a valley floor where, often unfortunately, the land may be occupied by people and property, sometime causing significant death and destruction.

A lahar is a landslide (mudflow or debris flow) consisting of volcanic (pyroclastic) material, water, and possibly ice and other debris that discharges down the flank of a volcano. Lahars occur as a result of volcanic eruptions, earthquakes, or the gravitational collapse of volcano or summit glaciers. The word lahar applies both to the process or deposits formed after the landslide event.

Creep is the slow, more or less continuous downslope movement of surface materials (rock and soil) under gravitational stresses. Creep occurs on many hillslope areas, but is particularly common in areas that experience seasonal freezing and thawing of the land's surface. Trees on a slow, creeping hillside tend to gradually realign themselves upward as the root stocks slowly rotate downhill over time. Although a perceptively slow landsliding process, creep is often responsible for the rupture of underground pipelines, the cracking of foundations, roads, and other infrastructure built on hillsides, particularly in regions with temperate climates (having warm summers and cold winters with highly fluctuating daily and seasonal temperature ranges).

A landslide complex is an extended area where multiple landslides are occurring and have occurred in the past. Parts of a landslide complex may be active whereas other parts may be dormant or stabilized (or extinct). Multiple landslide processes described above may be occurring throughout a landslide complex. Large landslide complexes develop over time in areas where landslide-prone conditions exist, typically on extended slopes, such as along a mountainous sea coast, along a mountain ridge flank, along a river vally, or along an extensive fault zone.