Landslides in 60 Seconds

• A landslide is the downslope movement of rock, soil or debris under gravity.
• Mass movement or mass wasting is the broader family that includes
  slides, falls, topples, flows, spreads, creep and avalanches.
• A mudslide is a popular term for a wet, fast-moving slope failure. Many events described
  as mudslides are technically debris flows, earthflows or shallow landslides.
• Common triggers include intense rain, prolonged rainfall, snowmelt, earthquakes, erosion, wildfires,
  volcanic activity and human modification of slopes.
• Water is particularly important because it adds weight, weakens material and raises pore-water pressure.
• Landslides may move from millimeters per year to tens of meters per second.
• Large failures can block rivers, generate floods, enter lakes or oceans and produce destructive waves.
• Warning signs may include new cracks, bulging ground, tilted trees, sticking doors, unusual seepage,
  muddy streams and rumbling sounds.
• Rockfalls and snow avalanches are mass movements but are explored in dedicated guides because their
  materials, mechanics and hazards are distinct.
• Sinkholes and widespread land subsidence are not normally classified as landslides because the dominant
  movement is downward into a subsurface void or compacting layer rather than downslope.

What Is a Landslide?

A landslide occurs when a mass of rock, earth or debris moves downslope because the
forces pulling it downward exceed the strength holding the slope together.

Gravity acts on every hillside, cliff, embankment and mountain. Stable slopes resist gravity through
friction, cohesion, intact rock bridges, root reinforcement and support from material at the bottom of
the slope. A landslide begins when those resisting forces weaken or the driving forces become stronger.

Landslides can involve:

• dry rock or loose boulders;
• water-saturated soil;
• clay-rich earth;
• gravel, mud and trees;
• volcanic ash and debris;
• snow, glacier ice and rock;
• artificial fill, mining waste or landfill material.

The word landslide refers both to the process and to the resulting deposit or landform.
A landslide may involve a few cubic meters of roadside soil or billions of cubic meters of rock
collapsing from an entire mountain.

Are all landslides sudden?

No. Some landslides occur in seconds, while others deform over months, decades or centuries. A slowly
moving slope may accelerate after heavy rain, excavation or earthquake shaking. Conversely, a rapid
failure may stop after traveling only a short distance.

Are landslides always natural?

No. Many landslides occur naturally, but road construction, mining, leaking pipes, deforestation,
excavation and poor drainage can destabilize slopes or increase the probability of failure.

What Is Mass Movement?

Mass movement, also known as mass wasting, is the broad geological term
for the downslope movement of rock, sediment, soil, snow or debris under gravity.

Landslides are one part of this larger family, which includes:

• rockfalls;
• topples;
• rotational slides;
• translational slides;
• block slides;
• earthflows;
• mudflows;
• debris flows;
• debris avalanches;
• lateral spreads;
• soil creep;
• solifluction;
• snow avalanches;
• ice avalanches;
• complex or compound slope failures.

Gravity is the ultimate driving force in every case, but water, ice, earthquakes, erosion, weathering and
human activity frequently determine when and how the movement occurs.

Mass movement versus erosion

Mass movement and erosion are related but not identical. Erosion involves the removal and transport of
material by water, wind, waves or ice. Mass movement occurs when gravity causes material to move downslope.

A river may erode the base of a slope and trigger a landslide. The landslide may then dump sediment back
into the river, where flowing water transports it farther downstream.

Why Do Slopes Remain Stable—or Fail?

A slope remains stable when its resisting strength is greater than the forces attempting to move it.
Geologists and engineers often describe this balance using the concept of a factor of safety.

When resistance comfortably exceeds driving stress, the slope is stable. When both become nearly equal,
the slope is marginally stable. When driving stress exceeds resistance, movement begins.

Driving forces

Driving forces increase when:

• the slope becomes steeper;
• water, buildings, fill or debris add weight;
• earthquake shaking temporarily increases stress;
• erosion removes support from the slope base;
• excavation undercuts the hillside;
• a reservoir or flood changes groundwater conditions.

Resisting forces

Resisting forces depend on:

• friction between grains or rock surfaces;
• cohesion within soil and clay;
• the strength of intact rock;
• root reinforcement;
• support at the foot of the slope;
• drainage and groundwater pressure;
• the orientation of bedding, faults and joints.

The angle of repose

Loose granular material tends to form a characteristic maximum stable angle called the
angle of repose. The precise angle depends on grain size, shape, moisture and friction.

Rock slopes may stand much steeper than loose sediment because intact rock and fractures provide additional
strength. However, a steep rock wall can fail catastrophically when joints, faults or weak layers become
unfavorably oriented.

Why apparently stable slopes collapse

Slopes can weaken invisibly through:

• deep weathering;
• slow crack growth;
• repeated wetting and drying;
• freeze–thaw expansion;
• root decay;
• gradual erosion;
• groundwater rise;
• small episodes of creep;
• previous landslide movement.

The final trigger may be dramatic, but the long-term preparation for failure is often quiet.

Anatomy of a Landslide

Landslides often leave recognizable features that help geologists determine where the slope detached,
how the material moved and where it came to rest.

Crown

The relatively undisturbed ground above the uppermost part of the landslide.

Main scarp

The steep exposed surface created when the moving mass separated from stable ground.

Head

The upper portion of displaced material immediately below the main scarp.

Minor scarps

Smaller internal breaks formed as the moving mass deforms.

Flanks

The lateral boundaries of the moving material.

Surface of rupture

The boundary along which the landslide detached and moved.

Body

The principal mass of displaced rock, soil or debris.

Toe

The lower and often bulging edge of the landslide deposit.

Runout zone

The area crossed or covered after the initial slope failure.

Fresh landslides may expose bare soil or rock, while older slides can become covered by forests,
farmland or buildings. Old landslide deposits often remain weaker than surrounding ground and may
reactivate during storms or earthquakes.

How Are Landslides Classified?

Landslides are classified primarily by:

1. the material involved;
2. the type of movement;
3. the speed and moisture content;
4. whether the movement is shallow, deep or complex.

Classification by material

Classification by movement

• Fall: material detaches and descends through the air.
• Topple: material rotates forward out of a slope.
• Slide: material moves along one or more defined rupture surfaces.
• Spread: coherent ground extends and breaks apart above weaker material.
• Flow: internal deformation occurs throughout the moving mass.
• Complex: one type of movement transforms into another.

Main Types of Landslides

Rotational landslides

A rotational landslide, often called a slump, moves along a curved rupture surface.
The upper part of the displaced block commonly rotates backward toward the slope, while the lower portion
bulges outward.

Rotational slides frequently occur in:

• clay-rich soils;
• riverbanks;
• coastal bluffs;
• artificial embankments;
• weathered sedimentary layers.

They may produce a steep head scarp, tilted blocks, internal ponds and a hummocky toe.

Translational landslides

A translational landslide moves along a relatively planar surface such as a bedding plane,
fault, joint, soil boundary or contact between strong and weak layers.

Translational slides may involve thin sheets of soil or enormous slabs of bedrock. Because their rupture
surfaces are often less curved than those of rotational slides, the moving mass may remain relatively intact.

Block slides

A block slide occurs when one or more coherent masses move downslope as comparatively intact units.
Buildings, roads and vegetation may be transported with the block before it fragments.

Shallow landslides

Shallow landslides involve near-surface soil or weathered material. They commonly develop during intense
rainfall when water saturates a thin soil layer above less permeable bedrock.

These slides may transform into debris flows after entering a gully or stream channel.

Deep-seated landslides

Deep-seated landslides extend far below the surface and may involve large portions of hillsides or mountains.
Their rupture surfaces can penetrate bedrock and remain active for decades.

Deep-seated movement may be slow and intermittent, but acceleration can damage roads, pipelines, houses and
entire communities.

Compound landslides

Compound landslides contain more than one style of movement. A failure may begin rotationally near the head,
become translational farther downslope and transform into a debris flow in the valley.

What Is a Mudslide?

Mudslide is a widely used popular term for fast-moving, water-rich earth or debris moving
downslope. It is understandable to the public but is not always the most precise geological classification.

Events described in news reports as mudslides may actually be:

• mudflows;
• debris flows;
• earthflows;
• shallow landslides;
• post-fire debris flows;
• lahars.

Mudslide versus landslide

For SEO and public understanding, the word mudslide belongs in this cornerstone guide. Within
technical explanations, however, it is useful to identify whether the event was a debris flow, earthflow
or another specific process.

Debris Flows and Mudflows

A debris flow is a rapidly moving mixture of water, mud, gravel, boulders, vegetation
and sometimes human-made debris.

Debris flows often begin when intense rainfall, snowmelt or sudden water release mobilizes loose material
on a steep slope. Once the moving mixture enters a channel, it can accelerate, erode the channel bed and
incorporate more sediment.

What does a debris flow look like?

Debris flows may resemble wet concrete filled with rocks, logs and wreckage. Their fronts can contain
the largest boulders and trees, while a more fluid mixture follows behind.

They may:

• surge in pulses;
• produce loud roaring sounds;
• carry multi-ton boulders;
• destroy bridges and culverts;
• overflow channels;
• spread across alluvial fans;
• travel far beyond the initial slope failure.

Where do debris flows occur?

They are especially common in:

• steep mountain gullies;
• recent wildfire burn scars;
• volcanic terrain;
• areas with loose glacial sediment;
• desert mountains during intense storms;
• deeply weathered tropical slopes.

Mudflow versus debris flow

A mudflow generally contains a larger proportion of fine sediment, while a debris flow carries more coarse
material such as gravel, boulders and trees. In reality, natural events form a continuum, and terminology
may vary among agencies and regions.

Hyperconcentrated flows

A hyperconcentrated flow contains more sediment than a normal flood but less than a typical debris flow.
It behaves partly like water and partly like a dense sediment mixture.

Transitions between floods, hyperconcentrated flows and debris flows can occur during the same event.

Alluvial fans

When debris flows leave a confined mountain channel and reach flatter terrain, they spread outward and
deposit sediment across an alluvial fan.

Alluvial fans can appear attractive for development because they are relatively flat, but their shape is
often the product of repeated floods and debris flows.

Debris Avalanches

A debris avalanche is an extremely rapid and turbulent movement of fragmented rock,
soil or volcanic debris.

Debris avalanches differ from water-rich debris flows because they do not necessarily require abundant
water. They may originate when a large landslide or volcanic flank collapses and breaks apart during movement.

Characteristics of debris avalanches

• very high speed;
• intense fragmentation;
• long runout;
• hummocky deposits;
• large intact blocks carried within crushed debris;
• possible transformation into debris flows or lahars.

Volcanic debris avalanches

Volcanoes are particularly susceptible because they are steep, fractured and often weakened by hydrothermal
alteration. Collapse of a volcanic flank can generate a debris avalanche covering hundreds of square kilometers.

The 1980 collapse of Mount St. Helens began as an enormous debris avalanche and rapidly depressurized the
volcano, contributing to the devastating lateral blast.

Should debris avalanches have their own pillar?

Within the final StrangeSounds architecture, debris avalanches remain a substantial section of this
cornerstone rather than a separate pillar. They are closely connected to landslides, debris flows and
volcanic flank collapse, and separating them would create unnecessary overlap.

Earthflows, Creep and Slow Slope Movement

Some of the most damaging slope movements are not dramatic collapses. They deform gradually, damaging
roads, foundations and utilities over years.

Earthflows

An earthflow occurs when fine-grained, often clay-rich material moves downslope through internal deformation.
Earthflows may form elongated lobes with a head scarp and bulging toe.

Movement can be rapid, but many earthflows remain active intermittently for decades, accelerating during wet periods.

Soil creep

Soil creep is extremely slow downslope movement caused by repeated expansion and contraction,
wetting and drying, freezing and thawing, or disturbance by roots and animals.

Signs include:

• curved tree trunks;
• tilted fence posts;
• leaning utility poles;
• small terraces on grassy slopes;
• gradually displaced walls and roads.

Solifluction

Solifluction occurs when water-saturated soil slowly flows over frozen, impermeable or poorly drained ground.
It is common in Arctic, sub-Arctic and high-mountain environments.

Deep-seated gravitational slope deformation

Entire mountain slopes may slowly deform under their own weight. Features can include:

• ridge-top trenches;
• double ridges;
• uphill-facing scarps;
• bulging valley walls;
• large open cracks;
• slowly displaced bedrock blocks.

These slopes do not always collapse catastrophically, but smaller failures may develop within them.

Why this remains a section

Slow slope movements are scientifically important but currently fit best as a major section of the
cornerstone. Your archive contains fewer dedicated creep and deep-seated deformation stories than general
landslides, rockfalls or avalanches.

Rockfalls, Rockslides and Rock Avalanches

Rock-slope failures involve bedrock rather than primarily soil or mud. They range from isolated boulders
falling onto roads to entire mountain ridges collapsing in seconds.

Rockfall

A rockfall begins when blocks detach from a steep cliff. They descend by falling, bouncing, rolling or
fragmenting on impact.

Rockslide

A rockslide occurs when a coherent rock mass moves along a geological surface such as a bedding plane,
fault or joint.

Rock avalanche

A rock avalanche is a very large and rapid flow-like movement of fragmented rock. It may cross valleys,
climb opposing slopes and travel much farther than expected.

Coastal cliff collapse

Coastal failures occur when waves remove support from the foot of a cliff while rainfall, groundwater and
weathering weaken it from above. Because the dominant process is often rockfall, toppling or sliding, coastal
cliff collapse belongs inside the rockfall pillar rather than requiring a separate evergreen pillar.

Continue reading:

Rockfalls, Rockslides & Rock Avalanches Explained
.

Snow and Ice Avalanches

Snow and ice avalanches are gravity-driven mass movements, but their materials, forecasting methods and
safety systems differ enough from soil and rock landslides to justify a dedicated pillar.

Snow avalanches

A snow avalanche occurs when snow loses stability and rapidly descends a slope. Main types include:

• slab avalanches;
• loose-snow avalanches;
• wet-snow avalanches;
• dry powder avalanches;
• cornice-triggered avalanches;
• glide avalanches.

Ice avalanches

Ice avalanches occur when glacier ice, seracs or hanging glaciers collapse and descend steep terrain.
They may entrain snow, rock and sediment.

Rock–ice avalanches

Rock–ice avalanches combine collapsing bedrock with snow or glacier ice. Melting and fragmentation can
make these failures extremely mobile.

Continue reading:

Snow & Ice Avalanches Explained
.

What Causes Landslides?

Landslides rarely have a single cause. Most result from long-term weakening combined with a short-term trigger.

Long-term preconditions

• steep topography;
• weak or weathered rock;
• clay-rich layers;
• faults, joints and fractures;
• bedding planes dipping toward a valley;
• ancient landslide deposits;
• poor drainage;
• erosion at the base of a slope;
• loss of glacier or permafrost support.

Short-term triggers

• intense rainfall;
• prolonged rainfall;
• rapid snowmelt;
• earthquake shaking;
• volcanic eruptions;
• river flooding;
• coastal storms;
• wildfires;
• freeze–thaw cycles;
• construction or excavation;
• leaking water infrastructure;
• mining or blasting.

Cause versus trigger

A storm may trigger a landslide, but the underlying cause may include weak geology, old slide deposits,
poor drainage and decades of erosion.

The trigger is often simply the final event that pushes an already unstable slope beyond its threshold.

Why Does Water Trigger So Many Landslides?

Water destabilizes slopes in several ways at once.

Water adds weight

Saturated soil is heavier than dry soil. The added weight increases the downslope force.

Water raises pore pressure

Water filling spaces between grains creates pore-water pressure. As pressure rises, grains
are pushed apart and effective friction decreases.

Water weakens clay and weathered rock

Some soils and altered rocks lose strength when wet. Clay-rich layers may become natural sliding surfaces.

Water enters fractures

Cracks allow rainfall and snowmelt to penetrate deep into a slope. Water pressure may then act directly
along joints or rupture surfaces.

Water erodes support

Rivers, waves and runoff can remove material from the base of a slope, making it steeper and less supported.

Why landslides happen after rain stops

Water may require hours or days to infiltrate deep enough to raise groundwater pressure along a weak layer.
Slopes can therefore fail after the storm appears to have ended.

Rainfall intensity versus duration

Short, intense storms often trigger shallow slides and debris flows. Long periods of moderate rain can
saturate deeper layers and activate large, deep-seated landslides.

Earthquake-Triggered Landslides

Strong earthquakes can trigger thousands of landslides across entire mountain regions within minutes.

Shaking may:

• increase stress on steep slopes;
• open existing fractures;
• dislodge blocks from cliffs;
• weaken water-saturated sediment;
• cause liquefaction and lateral spreading;
• reactivate ancient landslides;
• collapse road cuts and embankments.

Why earthquake landslides are so destructive

Earthquake-triggered landslides can:

• bury settlements already damaged by shaking;
• block emergency roads;
• isolate mountain communities;
• dam rivers;
• destroy pipelines and hydropower systems;
• continue during aftershocks.

Post-earthquake landslide risk

Slopes fractured by an earthquake may remain more susceptible to rainfall-induced failure for years.
Strong aftershocks can also trigger new rockfalls and landslides.

Related guide:
Earthquake Hazards Explained.

Volcanic Landslides, Flank Collapse and Lahars

Volcanoes are particularly vulnerable to mass movement because they are steep, fractured and built from
alternating layers of lava, ash and loose debris.

Volcanic flank collapse

A volcano’s flank may collapse because of:

• magma intrusion;
• earthquake shaking;
• hydrothermal alteration;
• erosion;
• oversteepening;
• rapid eruption-related deformation.

Large flank collapses can generate debris avalanches and, on volcanic islands, tsunamis.

Lahars

A lahar is a volcanic debris flow composed of water, ash, rock fragments and other volcanic
material.

Lahars may be triggered by:

• heavy rain on loose ash;
• rapid melting of snow and ice;
• crater-lake breakout;
• collapse of eruption deposits;
• failure of volcanic debris dams.

Lahars can follow valleys for tens of kilometers and remain dangerous long after an eruption ends.

Related guides:
Volcanic Hazards Explained
and
Lahars Explained.

Wildfires and Post-Fire Debris Flows

Wildfires can sharply increase the risk of shallow landslides and debris flows.

Fire may:

• remove vegetation that intercepts rainfall;
• damage roots that reinforce soil;
• leave loose ash and sediment;
• reduce surface roughness;
• create water-repellent soil layers;
• increase rapid runoff into channels.

Why burn scars are dangerous

Burned slopes may generate debris flows during short, intense downpours even when total rainfall is not extreme.
Multiple flows can begin at once and converge into gullies leading directly toward roads or communities.

How long does the danger last?

Elevated danger may persist for several rainy seasons, depending on burn severity, vegetation recovery,
soil properties and storm intensity.

Snowmelt, Freeze–Thaw and Permafrost Degradation

Rapid snowmelt

Warm weather or rain-on-snow events can release large volumes of water into slopes. If frozen or saturated
ground cannot absorb the water, pore pressure rises rapidly.

Freeze–thaw weathering

Water entering rock fractures expands when it freezes. Repeated freezing and thawing can gradually widen
cracks and loosen blocks.

Mountain permafrost

Ice within fractured mountain rock can help bind unstable blocks together. As mountain permafrost warms and
thaws, rock walls may lose part of that support.

Glacier retreat

Glaciers support valley walls and remove loose debris. When they retreat, steep rock slopes may become
exposed and mechanically unsupported.

Rockfalls and rock–ice avalanches linked to these processes are explored in:

Rockfalls, Rockslides & Rock Avalanches Explained
.

River Erosion, Coastal Erosion and Cliff Collapse

River undercutting

Rivers erode the base of valley walls and riverbanks. During floods, rapid erosion may remove enough support
to trigger slides or slumps.

Coastal erosion

Waves attack the foot of cliffs while rainfall and groundwater weaken them from above. Storms may accelerate
both processes simultaneously.

Reservoir and lake shorelines

Changes in water level can saturate slopes, remove support or alter groundwater pressure. Rapid drawdown is
particularly dangerous because water pressure inside the slope may remain high after external support drops.

Why coastal collapse remains within the rockfall pillar

Coastal cliff collapse is a setting rather than one single movement type. Failures may occur as rockfalls,
topples, rotational landslides or block slides. The dedicated rockfall pillar can cover this environment
without creating another overlapping cornerstone.

How Human Activity Destabilizes Slopes

Human development may disturb a slope’s balance by increasing weight, removing support or altering drainage.

Road cuts and excavation

Cutting into a hillside can steepen the slope and remove support from material above.

Overloading

Buildings, embankments, waste piles and artificial fill add weight near the top of a slope.

Poor drainage

Blocked culverts, leaking pipes, irrigation and poorly designed stormwater systems may concentrate water
inside unstable ground.

Deforestation

Removing vegetation reduces root reinforcement and can increase runoff and erosion.

Mining and quarrying

Excavation, blasting, vibration, waste piles and groundwater changes can destabilize mine walls and natural slopes.

Landfills

Landfill waste may become unstable when poorly compacted, saturated, oversteepened or affected by gas and
leachate pressure.

Reservoir construction

Filling a reservoir changes groundwater pressure and saturates valley slopes. Rapid changes in water level
can also affect stability.

Urban expansion

Development may place homes directly on old landslide deposits or debris-flow fans. In many cases, exposure
increases faster than the natural hazard itself.

Climate Change and Landslide Risk

Climate change does not affect every slope in the same way, but it can alter several important landslide triggers.

More intense rainfall

In some regions, heavier rainfall events can increase shallow landslides, debris flows and erosion.

Changing snowmelt

Earlier or more rapid snowmelt can alter the timing of slope saturation.

Wildfire expansion

Larger or more severe fires may create more extensive burn scars exposed to post-fire debris flows.

Permafrost thaw

Warming mountain permafrost can weaken fractured rock and increase high-altitude rockfall activity.

Glacier retreat

Retreating glaciers expose steep valley walls and remove support from unstable slopes.

Sea-level rise and coastal erosion

Higher water levels and changing storm patterns can increase erosion at the base of vulnerable coastal cliffs.

Attribution requires care. A single landslide cannot automatically be blamed on climate change, but changing
rainfall, wildfire, snow and ice conditions may alter regional landslide probability.

How Fast Can a Landslide Move?

Landslide velocity ranges from nearly imperceptible creep to catastrophic avalanches moving faster than vehicles.

Speed may change during an event. A slowly moving block can fragment and accelerate, while a debris flow may
slow when it leaves a confined channel.

Why Do Some Landslides Travel So Far?

The distance between a landslide’s source and the end of its deposit is called its runout.

Runout depends on:

• volume;
• initial height;
• slope angle;
• material type;
• water and ice content;
• degree of fragmentation;
• valley geometry;
• material entrainment.

Fragmentation

A coherent rock mass may break into millions of fragments, allowing it to behave more like a granular flow.

Water and ice

Water or melting ice can reduce resistance and help mobilize fine sediment.

Channel confinement

Narrow valleys prevent material from spreading sideways and may direct it far downstream.

Entrainment

A moving landslide can pick up soil, trees, snow, water and loose sediment, increasing its volume and momentum.

Air and fluid pressure

Rapid movement may trap air or generate high pressure within saturated debris, temporarily reducing friction.

How Landslides Transform During Movement

Many destructive events cannot be described by a single movement type.

A typical sequence might be:

1. a block begins sliding along a rupture surface;
2. the block fragments into debris;
3. the debris entrains water or snow;
4. the movement becomes a debris avalanche;
5. it enters a channel and transforms into a debris flow;
6. the flow deposits sediment across an alluvial fan.

Why transformation matters

The final hazard may be much more mobile than the initial slope failure. A landslide expected to stop near
the hill can transform into a flow capable of reaching communities kilometers away.

Rock–ice–debris cascades

High-mountain failures may involve rockfall, glacier ice collapse, snow entrainment, melting and debris flow
in one cascading event.

Volcanic cascades

A volcanic flank collapse can generate a debris avalanche, trigger an eruption, melt snow and produce lahars.

Major Landslide Hazards

Burial and impact

Moving rock, mud and debris can crush or bury buildings, vehicles and people.

Infrastructure destruction

Landslides can sever:

• roads;
• railways;
• bridges;
• pipelines;
• power lines;
• water systems;
• communications networks.

River blockage

Large landslides can dam rivers and create unstable lakes.

Flooding

Upstream flooding may begin immediately, while downstream areas face danger if the natural dam fails.

Displacement waves

Landslides entering lakes, reservoirs or fjords can generate destructive waves.

Isolation

Mountain communities may be cut off when multiple roads and bridges are destroyed.

Secondary landslides

The initial event often leaves unstable scarps and debris that may fail during later rainfall or aftershocks.

Water contamination

Landslides can increase sediment, damage sewage systems, rupture pipelines and contaminate drinking water.

Economic disruption

Even relatively small landslides can close major transport corridors for weeks or months.

Landslide Dams and Outburst Floods

When a landslide blocks a river, water accumulates behind the debris and forms a
landslide-dammed lake.

Some natural dams remain stable for centuries. Others fail within hours or days because they are made of
loose, unsorted debris.

How landslide dams fail

• overtopping and rapid erosion;
• internal seepage and piping;
• continued movement of the landslide;
• aftershocks;
• new landslides entering the lake;
• heavy rain or rapid snowmelt.

Upstream hazards

Rising water may inundate settlements, farms and roads upstream.

Downstream hazards

Sudden failure can release a powerful flood containing water, sediment, trees and boulders.

Emergency response

Authorities may construct spillways, pump water, stabilize the debris or evacuate downstream areas.

Can Landslides Cause Tsunamis?

Yes. Landslides can generate waves when large volumes of material rapidly enter or move beneath water.

Lake and reservoir landslides

A landslide entering a confined lake or reservoir can displace water almost instantly. Waves may run far
above the normal shoreline.

Fjord landslides

Narrow fjords can focus landslide-generated waves, producing extreme local run-up.

Volcanic island collapse

Collapse of a volcanic island flank can displace ocean water and potentially generate regional waves.

Submarine landslides

Underwater slopes may fail along continental margins, deltas, lake basins and volcanic islands.
Rapid movement can generate tsunamis without visible surface collapse.

Reservoir overtopping

A displacement wave can overtop a dam even when the dam itself remains structurally intact.

Related guide:

Ocean & Coastal Phenomena
.

Where Do Landslides Occur?

Landslides occur on every continent and beneath oceans and lakes. Risk is highest where steep relief,
weak geology, water and active erosion occur together.

Mountain belts

The Himalaya, Andes, Alps, Rockies, Caucasus and other mountain ranges experience frequent landslides because
of steep terrain, fractured rock, earthquakes and intense erosion.

Tropical mountains

Heavy rainfall, deep weathering and steep slopes make many tropical regions highly susceptible to shallow
landslides and debris flows.

Volcanic regions

Volcanoes contain steep slopes, loose ash, fractured lava and hydrothermally altered rock.

Earthquake zones

Active tectonic regions frequently experience widespread landsliding during strong earthquakes.

River valleys

Rivers continually undercut slopes and may trigger failures during floods.

Coastal cliffs

Waves remove support while rainfall and groundwater weaken cliffs from above.

Cold and high-mountain regions

Freeze–thaw cycles, glacier retreat and permafrost degradation can destabilize rock walls.

Urban and engineered slopes

Excavation, fill, leaking infrastructure and development on old landslide deposits can create serious risk.

Landslides in the United States

Landslide susceptibility is especially important in:

• the Pacific Coast ranges;
• the Cascade Range;
• the Rocky Mountains;
• the Appalachian Mountains;
• Alaska;
• Hawaii;
• Puerto Rico;
• steep river valleys and coastal bluffs.

What Are the Warning Signs of a Landslide?

Some rapid landslides provide almost no warning, but many unstable slopes show changes before major movement.

Ground warning signs

• new cracks in soil, roads or paths;
• cracks widening rapidly;
• fresh scarps or step-like breaks;
• bulging ground near the base of a hill;
• sunken or uplifted road sections;
• hummocky or newly uneven terrain;
• soil pulling away from foundations.

Structural warning signs

• doors and windows suddenly sticking;
• new cracks in foundations or walls;
• retaining walls leaning or cracking;
• fences, trees or utility poles tilting;
• decks or steps separating from buildings;
• underground pipes breaking.

Water warning signs

• new seepage or springs;
• previously wet areas becoming dry;
• streams suddenly turning muddy;
• rapid changes in stream flow;
• water ponding in new locations;
• unexpected water emerging from cracks.

Sound warning signs

• cracking or snapping trees;
• falling rocks;
• low rumbling that becomes louder;
• booming from a cliff;
• roaring water and debris in a channel.

Warning signs after wildfire

During heavy rain near a burn scar, watch for:

• rapidly rising or muddy streams;
• unusual roaring from gullies;
• floating branches and debris;
• sudden pulses of water or sediment.

What Should You Do Before, During and After a Landslide?

Before periods of elevated danger

• Check whether your property lies within a mapped landslide or debris-flow zone.
• Know evacuation routes that avoid gullies, channels and unstable slopes.
• Follow weather warnings and burn-scar alerts.
• Maintain drains, culverts and stormwater systems.
• Avoid directing roof or irrigation water onto steep ground.
• Have suspicious movement assessed by qualified local professionals.

During intense rainfall

• Stay alert to unusual sounds and changes in water flow.
• Avoid steep slopes, ravines and recent burn scars.
• Do not stop beneath cliffs or road cuts.
• Leave immediately when authorities issue an evacuation order.

If a landslide begins

• Move away from the path toward higher, stable ground when safe.
• Do not cross an active debris flow.
• Never drive across moving mud, fresh cracks or a sagging road.
• Call emergency services if lives or infrastructure are in immediate danger.
• Warn others only when doing so does not increase your own risk.

After a landslide

• Stay away because secondary failures may occur.
• Watch for flooding from blocked streams.
• Avoid damaged utilities and fallen power lines.
• Do not enter damaged structures until they are assessed.
• Expect further movement during aftershocks or renewed rainfall.

This guide provides general educational information. Local geological agencies, emergency authorities and
evacuation instructions always take priority during an active event.

How Are Landslides Monitored?

Monitoring aims to detect movement, identify acceleration and understand the conditions that could lead to failure.

Field mapping

Geologists map scarps, cracks, displaced drainage, tilted trees, springs and old landslide deposits.

Survey markers

Repeated measurements reveal whether different parts of a slope are moving.

GPS and GNSS

Receivers measure three-dimensional displacement with high precision.

Inclinometers

Borehole instruments detect deformation at depth and help locate the active rupture surface.

Piezometers

Piezometers measure groundwater levels and pore-water pressure.

Extensometers

Crack meters and extensometers track widening fractures.

Rain gauges

Rainfall intensity and duration can be compared with thresholds associated with previous failures.

LiDAR

Laser-based topographic surveys reveal subtle scarps, old landslide deposits and changes in surface shape.

Satellite radar and InSAR

Repeated radar observations can detect slow ground deformation across large areas.

Drones

Drones provide detailed images and three-dimensional models without exposing teams to unstable slopes.

Seismic and acoustic sensors

Rapid rockfalls, avalanches and debris flows generate vibrations or low-frequency sound.

Time-lapse cameras

Cameras can document crack growth, rockfall frequency and visible acceleration.

Artificial intelligence and automated detection

Automated systems can combine rainfall, satellite data, ground sensors and terrain models to identify
abnormal movement. These systems improve situational awareness but do not eliminate uncertainty.

Rainfall thresholds

Some warning systems issue alerts when rainfall intensity and duration exceed levels associated with
previous landslides or debris flows.

Can Landslides Be Prevented?

Not every landslide can be prevented, especially during extreme earthquakes or storms. However, risk can
often be reduced through drainage, engineering, land-use planning and early warning.

Improving drainage

• surface channels;
• subsurface drains;
• horizontal drainage holes;
• maintained culverts;
• controlled stormwater discharge.

Changing slope geometry

Flattening, benching or removing material from the upper slope can reduce driving stress.

Retaining structures

Retaining walls, piles, anchors, soil nails and reinforced-earth systems can increase resistance.

Rockfall protection

Nets, barriers, catch ditches, scaling and rock bolts reduce danger below unstable cliffs.

Erosion control

Riverbank protection, coastal defenses and vegetation measures may reduce undercutting and surface erosion.

Debris-flow basins

Barriers and catchment basins can trap part of a debris flow before it reaches developed areas.

Land-use planning

Avoiding active landslides, runout zones and debris-flow channels is often more effective than attempting
to engineer away the hazard after development.

Early-warning systems

Sensors, rainfall thresholds and automatic alarms may provide evacuation time where recurring processes are
well understood.

Engineering must match local geology. A retaining wall or drain installed without understanding the full
failure mechanism can worsen instability or transfer it elsewhere.

Historic and Giant Landslides

Some landslides rank among the largest and deadliest natural disasters in recorded history. Others occurred
thousands or millions of years ago but still shape modern landscapes.

Saidmarreh Landslide, Iran

The prehistoric Saidmarreh landslide in the Zagros Mountains is frequently cited among the largest known
subaerial landslides by volume. It transported an enormous limestone mass across the valley.

Flims Rockslide, Switzerland

The Flims event was one of the largest known Alpine rockslides. Its deposits reshaped the Rhine Valley and
contributed to the landscape surrounding the Rhine Gorge.

Frank Slide, Canada, 1903

A massive rockslide from Turtle Mountain buried part of the mining town of Frank in Alberta.

Vajont, Italy, 1963

A large rockslide entered the Vajont reservoir and displaced a wave over the dam. The dam largely survived,
but downstream communities were devastated.

Huascarán and Yungay, Peru, 1970

An earthquake triggered a catastrophic rock-and-ice avalanche from Huascarán. The mass transformed during
descent and buried the town of Yungay.

Mount St. Helens, United States, 1980

Collapse of the volcano’s north flank produced the largest landslide directly observed in modern history
and contributed to the lateral blast.

Kolka–Karmadon, Russia, 2002

A high-speed rock-and-ice avalanche swept through the Karmadon Gorge, demonstrating the extreme mobility
of mixed rock, glacier ice and meltwater.

Oso, United States, 2014

A large landslide in Washington State crossed a river valley and destroyed part of the Steelhead Haven community.

Attabad, Pakistan, 2010

A major landslide blocked the Hunza River, created Attabad Lake and submerged roads and settlements.

Montecito, United States, 2018

Intense rainfall over wildfire-burned slopes generated destructive debris flows through communities in
Southern California.

Enga Province, Papua New Guinea, 2024

A devastating slope failure buried a broad area beneath rock and debris, illustrating the challenges of
emergency response in remote mountainous terrain.

Landslide Records and Extremes

Largest known landslides

Estimates depend on how volume, area and movement type are defined. Prehistoric landslides such as Saidmarreh,
Flims and Heart Mountain rank among the largest known terrestrial mass movements.

Largest directly observed landslide

The 1980 Mount St. Helens debris avalanche is widely recognized as the largest landslide directly observed
in modern history.

Deadliest landslides

Some of the deadliest disasters have involved earthquakes, volcanic lahars, debris flows and landslide-generated
waves rather than simple sliding alone.

Fastest landslides

Large rock avalanches and debris avalanches can reach tens of meters per second, especially when falling
from great height or entraining ice and water.

Longest runout

Exceptional long-runout events may travel tens of kilometers. Exact comparisons depend on whether volcanic
debris avalanches, rock–ice avalanches and extraterrestrial landslides are included.

Landslides on other worlds

Giant landslides have been identified on Mars, the Moon and icy planetary bodies. Lower gravity, impact
craters, volcanic terrain and ice can produce mass movements much larger than typical terrestrial failures.

Landslide Event Archive

This permanent archive preserves the essential facts of notable landslides, mudslides, earthflows and debris
flows after shorter incident reports are consolidated into the evergreen mass-movement cluster.

2026

• YYYY-MM-DD — Location:
  Movement type, likely trigger, principal impacts and authoritative source.

Frequently Asked Questions About Landslides and Mudslides

What is the main cause of a landslide?

Gravity is the driving force behind every landslide. Failure occurs when water, erosion, earthquakes,
weathering or human disturbance reduces slope strength or increases downslope stress.

What is the difference between a landslide and a mudslide?

Landslide is the broad term for rock, soil or debris moving downslope. Mudslide is a popular term for a
wet, flowing slope failure. Many mudslides are technically debris flows or earthflows.

What is mass movement?

Mass movement is the downslope movement of rock, soil, sediment, snow or debris under gravity. It includes
landslides, rockfalls, debris flows, creep and avalanches.

What is the difference between a landslide and a rockfall?

In a slide, material moves along a failure surface. In a rockfall, blocks detach from a steep face and
descend by falling, bouncing or rolling.

Is a debris flow a landslide?

Yes. A debris flow is a rapid, flow-like type of landslide containing water, mud, rocks, vegetation and debris.

What is a debris avalanche?

A debris avalanche is an extremely rapid and turbulent movement of fragmented rock, soil or volcanic material.
It does not necessarily require abundant water.

Can landslides happen without rain?

Yes. Earthquakes, volcanic activity, erosion, freeze–thaw weathering, glacier retreat, excavation and
progressive rock weakening can trigger landslides without rainfall.

Why do landslides happen after rain stops?

Water may take hours or days to infiltrate deeply enough to raise pore-water pressure along a weak layer.
Failure can therefore occur after the heaviest rain has ended.

Can landslides happen on gentle slopes?

Yes. Weak, saturated or liquefied material can spread or flow across relatively gentle terrain.

How fast can a landslide move?

Movement ranges from millimeters per year for creep to tens of meters per second for large debris flows
and rock avalanches.

Can earthquakes trigger landslides?

Yes. Strong shaking can trigger thousands of landslides, dislodge rockfalls and reactivate old slope failures.

Can volcanoes cause landslides?

Yes. Volcanic earthquakes, magma intrusion, hydrothermal weakening and steep unstable flanks can produce
landslides, debris avalanches and lahars.

Can wildfires cause mudslides?

Wildfires remove vegetation, weaken roots and may create water-repellent soils. Intense rainfall can then
generate rapid post-fire debris flows.

Can snowmelt trigger landslides?

Yes. Rapid snowmelt adds water to slopes, increases pore pressure and may trigger slides or debris flows.

Can landslides block rivers?

Yes. Large landslides can create natural dams and lakes. If the blockage fails, it may release a destructive
outburst flood.

Can landslides cause tsunamis?

Yes. Landslides entering lakes, reservoirs, fjords or oceans can generate displacement waves. Underwater
landslides may also produce tsunamis.

Are sinkholes a type of landslide?

No. Sinkholes form mainly through subsurface dissolution or loss of underground support. Landslides involve
material moving downslope.

What are the first signs of a landslide?

Signs may include new cracks, bulging ground, tilted trees, sticking doors, broken pipes, muddy streams,
new seepage and unusual rumbling or cracking sounds.

Can landslides be predicted?

Exact prediction remains difficult, but monitoring movement, rainfall and groundwater pressure can identify
elevated danger and support warnings.

Can landslides be prevented?

Not all landslides can be prevented. Drainage, slope reinforcement, erosion control, careful excavation and
avoiding development in high-risk areas can reduce risk.

What is the most dangerous type of landslide?

Extremely rapid debris flows, debris avalanches and rock avalanches are among the most dangerous because they
can travel quickly and provide little time to escape.

What is the largest landslide ever recorded?

The answer depends on whether prehistoric, submarine, volcanic or extraterrestrial events are included.
Saidmarreh, Flims and Heart Mountain are among the largest known terrestrial examples.

What was the largest landslide directly observed?

The 1980 Mount St. Helens debris avalanche is widely recognized as the largest landslide directly observed
in modern history.

Why are old landslide deposits dangerous?

Old landslide material is often fractured, poorly consolidated and already separated from stable bedrock.
It may reactivate during heavy rain or earthquake shaking.

Where are landslides most common?

Landslides are most common in steep mountain belts, tropical highlands, earthquake zones, volcanic regions,
river valleys, coastal cliffs and areas altered by construction.

Explore Landslides & Mass Movements

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