U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2004-3072

July 2004

Landslide Types and Processes

Printed on recycled paper

Landslides in the United States occur in all 50 States. The primary regions of landslide 

occurrence and potential are the coastal and mountainous areas of California, Oregon, 

and Washington, the States comprising the intermountain west, and the mountainous 

and hilly regions of the Eastern United States. Alaska and Hawaii also experience all types of 

landslides. 

Landslides in the United States cause approximately $3.5 billion (year 2001 dollars) in damage, and kill between 25 and 50 people annually. Casualties in the United States are primarily caused by rockfalls, rock slides, and debris flows. Worldwide, landslides occur and cause 

thousands of casualties and billions in monetary losses annually. 

The information in this publication provides an introductory primer on understanding basic 

scientific facts about landslides—the different types of landslides, how they are initiated, and 

some basic information about how they can begin to be managed as a hazard. 

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Figure 1. An idealized slump-earth flow showing commonly used nomenclature for labeling the 

parts of a landslide.

TYPES OF LANDSLIDES

The term “landslide” describes a wide variety 

of processes that result in the downward and 

outward movement of slope-forming materials 

including rock, soil, artificial fill, or a combination of these. The materials may move 

by falling, toppling, sliding, spreading, or 

flowing. Figure 1 shows a graphic illustration 

of a landslide, with the commonly accepted 

terminology describing its features. 

The various types of landslides can be differentiated by the kinds of material involved 

and the mode of movement. A classification 

system based on these parameters is shown in 

figure 2. Other classification systems incorporate additional variables, such as the rate of 

movement and the water, air, or ice content of 

the landslide material.

Although landslides are primarily associated with mountainous regions, they can 

also occur in areas of generally low relief. In 

low-relief areas, landslides occur as cut-andfill failures (roadway and building excavations), river bluff failures, lateral spreading 

landslides, collapse of mine-waste piles 

(especially coal), and a wide variety of slope 

failures associated with quarries and open-pit 

mines. The most common types of landslides 

are described as follows and are illustrated in 

figure 3. SLIDES: Although many types of mass 

movements are included in the general term 

“landslide,” the more restrictive use of the 

term refers only to mass movements, where 

there is a distinct zone of weakness that 

separates the slide material from more stable 

underlying material. The two major types of 

slides are rotational slides and translational 

slides.

Rotational slide: This is a slide in which the 

surface of rupture is curved concavely upward 

and the slide movement is roughly rotational 

about an axis that is parallel to the ground surface and transverse across the slide (fig. 3A). 

Translational slide: In this type of slide, the 

landslide mass moves along a roughly planar 

surface with little rotation or backward tilting 

(fig. 3B). A block slide is a translational slide 

in which the moving mass consists of a single 

unit or a few closely related units that move 

downslope as a relatively coherent mass (fig. 

3C).

FALLS: Falls are abrupt movements of 

masses of geologic materials, such as 

rocks and boulders, that become detached 

from steep slopes or cliffs (fig. 3D). 

La Conchita, coastal area of southern California. This landslide and earthflow occurred in 

the spring of 1995. People were evacuated and 

the houses nearest the slide were completely 

destroyed. This is a typical type of landslide.

Photo by R.L. Schuster, U.S. Geological Survey.
Separation occurs along discontinuities such 

as fractures, joints, and bedding planes, and 

movement occurs by free-fall, bouncing, and 

rolling. Falls are strongly influenced by gravity, mechanical weathering, and the presence 

of interstitial water. 

TOPPLES: Toppling failures are distinguished by the forward rotation of a unit or 

units about some pivotal point, below or low 

in the unit, under the actions of gravity and 

forces exerted by adjacent units or by fluids in 

cracks (fig. 3E).

FLOWS: There are five basic categories of 

flows that differ from one another in fundamental ways.

a. Debris flow: A debris flow is a form of 

rapid mass movement in which a combination of loose soil, rock, organic matter, air, 

and water mobilize as a slurry that flows 

downslope (fig. 3F). Debris flows include 

<50% fines. Debris flows are commonly 

caused by intense surface-water flow, due to 

heavy precipitation or rapid snowmelt, that 

erodes and mobilizes loose soil or rock on 

steep slopes. Debris flows also commonly 

mobilize from other types of landslides that 

occur on steep slopes, are nearly saturated, 

and consist of a large proportion of silt- and 

sand-sized material. Debris-flow source areas 

are often associated with steep gullies, and 

debris-flow deposits are usually indicated by 

the presence of debris fans at the mouths of 

gullies. Fires that denude slopes of vegetation 

intensify the susceptibility of slopes to debris 

flows.

b. Debris avalanche: This is a variety of very 

rapid to extremely rapid debris flow (fig. 3G). 

c. Earthflow: Earthflows have a characteristic 

“hourglass” shape (fig. 3H). The slope material liquefies and runs out, forming a bowl or 

depression at the head. The flow itself is elongate and usually occurs in fine-grained materials or clay-bearing rocks on moderate slopes 

and under saturated conditions. However, dry 

flows of granular material are also possible.

d. Mudflow: A mudflow is an earthflow consisting of material that is wet enough to flow 

rapidly and that contains at least 50 percent 

sand-, silt-, and clay-sized particles. In some 

instances, for example in many newspaper 

reports, mudflows and debris flows are commonly referred to as “mudslides.” 

e. Creep: Creep is the imperceptibly slow, 

steady, downward movement of slope-forming soil or rock. Movement is caused by 

shear stress sufficient to produce permanent 

deformation, but too small to produce shear 

failure. There are generally three types of 

creep: (1) seasonal, where movement is within 

the depth of soil affected by seasonal changes 

in soil moisture and soil temperature; (2) 

continuous, where shear stress continuously 

exceeds the strength of the material; and 

(3) progressive, where slopes are reaching 

the point of failure as other types of mass 

movements. Creep is indicated by curved 

tree trunks, bent fences or retaining walls, 

tilted poles or fences, and small soil ripples or 

ridges (fig. 3I). 

LATERAL SPREADS: Lateral spreads are 

distinctive because they usually occur on 

very gentle slopes or flat terrain (fig. 3J). 

The dominant mode of movement is lateral 

extension accompanied by shear or tensile 

fractures. The failure is caused by liquefaction, the process whereby saturated, loose, 

cohesionless sediments (usually sands and 

silts) are transformed from a solid into a 

liquefied state. Failure is usually triggered by 

rapid ground motion, such as that experienced during an earthquake, but can also be 

artificially induced. When coherent material, 

either bedrock or soil, rests on materials that 

liquefy, the upper units may undergo fracturing and extension and may then subside, 

translate, rotate, disintegrate, or liquefy and 

flow. Lateral spreading in fine-grained materials on shallow slopes is usually progressive. 

The failure starts suddenly in a small area 

and spreads rapidly. Often the initial failure 

is a slump, but in some materials movement 

occurs for no apparent reason. Combination 

of two or more of the above types is known as 

a complex landslide.

LANDSLIDE CAUSES

1. Geological causes

a. Weak or sensitive materials

b. Weathered materials

c. Sheared, jointed, or fissured materials

d. Adversely oriented discontinuity 

(bedding, schistosity, fault, 

unconformity, contact, and so forth)

e. Contrast in permeability and/or 

stiffness of materials

2. Morphological causes

a. Tectonic or volcanic uplift

b. Glacial rebound

c. Fluvial, wave, or glacial erosion of 

slope toe or lateral margins

d. Subterranean erosion (solution, 

piping)

e. Deposition loading slope or its crest

f. Vegetation removal (by fire, drought)

g. Thawing

h. Freeze-and-thaw weathering

i. Shrink-and-swell weathering

3. Human causes

a. Excavation of slope or its toe

b. Loading of slope or its crest

c. Drawdown (of reservoirs)

d. Deforestation

e. Irrigation

f. Mining

g. Artificial vibration

h. Water leakage from utilities

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Figure 2. Types of landslides. Abbreviated version of Varnes’ classification of slope movements (Varnes, 1978).
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Figure 3. These sche

these processes and where to find photos, please see “Where to Go For More Information” at the end of this fact sheet.
Although there are multiple types of 

causes of landslides, the three that cause most 

of the damaging landslides around the world 

are these:

Landslides and Water

Slope saturation by water is a primary 

cause of landslides. This effect can occur 

in the form of intense rainfall, snowmelt, 

changes in ground-water levels, and waterlevel changes along coastlines, earth dams, 

and the banks of lakes, reservoirs, canals, and 

rivers.

Landsliding and flooding are closely allied 

because both are related to precipitation, 

runoff, and the saturation of ground by water. 

In addition, debris flows and mudflows usually occur in small, steep stream channels and 

often are mistaken for floods; in fact, these 

two events often occur simultaneously in the 

same area.

Landslides can cause flooding by forming 

landslide dams that block valleys and stream 

channels, allowing large amounts of water 

to back up. This causes backwater flooding 

and, if the dam fails, subsequent downstream 

flooding. Also, solid landslide debris can 

“bulk” or add volume and density to otherwise 

normal streamflow or cause channel blockages and diversions creating flood conditions 

or localized erosion. Landslides can also cause 

overtopping of reservoirs and/or reduced 

capacity of reservoirs to store water.

Landslides and Seismic Activity

Many mountainous areas that are vulnerable to landslides have also experienced at 

least moderate rates of earthquake occurrence 

in recorded times. The occurrence of earthquakes in steep landslide-prone areas greatly 

increases the likelihood that landslides will 

occur, due to ground shaking alone or shaking-caused dilation of soil materials, which 

allows rapid infiltration of water. The 1964 

Great Alaska Earthquake caused widespread 

landsliding and other ground failure, which 

caused most of the monetary loss due to the 

earthquake. Other areas of the United States, 

such as California and the Puget Sound region 

in Washington, have experienced slides, 

lateral spreading, and other types of ground 

failure due to moderate to large earthquakes. 

Widespread rockfalls also are caused by loosening of rocks as a result of ground shaking. 

Worldwide, landslides caused by earthquakes 

kill people and damage structures at higher 

rates than in the United States.

Landslides and Volcanic Activity

Landslides due to volcanic activity are 

some of the most devastating types. Volcanic 

lava may melt snow at a rapid rate, causing 

a deluge of rock, soil, ash, and water that 

accelerates rapidly on the steep slopes of 

volcanoes, devastating anything in its path. 

These volcanic debris flows (also known as 

lahars) reach great distances, once they leave 

the flanks of the volcano, and can damage 

structures in flat areas surrounding the volcanoes. The 1980 eruption of Mount St. Helens, 

in Washington triggered a massive landslide 

on the north flank of the volcano, the largest 

landslide in recorded times. 

Landslide Mitigation—How to Reduce 

the Effects of Landslides

Vulnerability to landslide hazards is a 

function of location, type of human activity, 

use, and frequency of landslide events. The 

effects of landslides on people and structures 

can be lessened by total avoidance of landslide hazard areas or by restricting, prohibiting, or imposing conditions on hazard-zone 

activity. Local governments can reduce 

landslide effects through land-use policies 

and regulations. Individuals can reduce their 

exposure to hazards by educating themselves 

on the past hazard history of a site and by 

making inquiries to planning and engineering departments of local governments. They 

can also obtain the professional services of 

an engineering geologist, a geotechnical 

engineer, or a civil engineer, who can properly 

evaluate the hazard potential of a site, built or 

unbuilt.

The hazard from landslides can be reduced 

by avoiding construction on steep slopes 

and existing landslides, or by stabilizing the 

slopes. Stability increases when ground water 

is prevented from rising in the landslide mass 

by (1) covering the landslide with an impermeable membrane, (2) directing surface water 

away from the landslide, (3) draining ground 

water away from the landslide, and (4) minimizing surface irrigation. Slope stability is 

also increased when a retaining structure and/

or the weight of a soil/rock berm are placed 

at the toe of the landslide or when mass is 

removed from the top of the slope.

Where to go for more information

1. The U.S. Geological Survey Landslide 

Program has information, publications, and 

educational information on its Web site. 

Please see:

http://landslides.usgs.gov

or phone toll-free:

1-800-654-4966

2. For general information about slides, 

debris flows, rock falls, or other types of 

landslides in your area, contact your city 

or county geology or planning office. In 

addition, all 50 States have State Geological Surveys that can be accessed through a 

link at the USGS Web site, 

http://landslides.usgs.gov

3. For an assessment of the landslide risk 

to an individual property or homesite, 

obtain the services of a State-licensed geotechnical engineer or engineering geologist. 

These professionals can be found through 

the membership listings of two professional 

societies, the American Society of Civil 

Engineers (ASCE), http://www.asce.org

and the Association of Engineering 

Geologists http://www.aegweb.org. Often, 

personnel in State or county planning or 

engineering departments can refer competent geotechnical engineers or engineering 

geologists. 

4. For more information about the design 

and construction of debris-flow mitigation measures which may include debris 

basins, debris fences, deflection walls, or 

other protective works, consult your city 

or county engineer, local flood-control 

agency, or the U.S. Department of Agriculture, Natural Resources Conservation 

Service: 

http://www.ncgc.nrcs.usda.gov/

5. For photos of landslide types please see:

http://landslides.usgs.gov/html_files/nlic/

nlicmisc.html

6. For more detailed information: two 

excellent publications that very clearly 

describe the processes of landslides were 

consulted for this fact sheet:

Varnes, D.J., 1978, Slope movement types 

and processes, in Schuster, R.L., and 

Krizek, R.J., eds., Landslides—Analysis and control: National Research 

Council, Washington, D.C., Transportation Research Board, Special Report 

176, p. 11–33.

Turner, Keith A., and Schuster, Robert L., 

1996, Landslides—Investigation and 

mitigation: Transportation Research 

Board, National Research Council, 

National Academy Press.

Compiled by Lynn Highland

Graphics and layout design by Margo 

Johnson

This fact sheet is available online at 

http://pubs.usgs.gov/fs/2002/3072/