Landslide facts for kids

A landslide near Cusco, Peru, in 2018

Animation of a landslide in San Mateo County, California in the United States

An abandoned railway tunnel on the Taguchi Railroad [ja] in Japan

Noire River landslide in 1894, near Saint-Alban, Quebec, Canada

A landslide happens when a large mass of rock, dirt, or debris suddenly rushes down a slope. Scientists also call this event a mass movement of earth. Landslides can happen very quickly like a sudden crash, or they can move slowly over many years. They occur on high mountain peaks, along steep coastal cliffs, and even deep underwater. When they slide beneath the sea, geologists call them submarine landslides.

The biggest force behind every slide is gravity. Gravity pulls everything down toward the center of the Earth. Slopes stay in place when the ground is strong enough to resist this downward pull. This balance is known as slope stability. However, when water, tremors, or human digging weaken the ground, gravity wins. That causes a sudden collapse called a slope failure.

Big slides can move giant boulders, tons of mud, and many trees in a few seconds. They can block roads, knock down buildings, and change how valleys look forever. Studying landslides helps communities understand the natural world, protect towns, and stay safe from sudden earth movements.

What Causes Landslides to Happen?

The Mameyes Landslide in Ponce, Puerto Rico, buried homes after heavy rain.

A slide in Surte, Sweden, in 1950 involving soft quick clay.

A hillside stays safe and solid when the ground holds tightly together. Scientists measure how well soil resists sliding by checking its shear strength. At the same time, gravity creates a pulling force called shear stress. When the stress becomes greater than the strength, the hill slides. This can happen due to natural forces or human choices.

Natural Triggers and Forces

Water is the most common natural trigger for downhill slides. When heavy rain falls or when thick winter snow melts quickly, water sinks deep into the ground. A melting glacier can also pour extra water into mountain slopes. This water fills empty spaces between soil particles, raising the level of groundwater.

The underground water pressure pushes dirt particles apart, making the ground loose and slick. An underground rock layer that holds water is an aquifer. When aquifers fill up completely, the water pushes against the rocks above.

Moving rivers and pounding ocean waves can cause erosion at the bottom of a hill. When waves wash away the bottom support, the top of the cliff collapses. Over many years, weathering from freezing ice, hot sun, and wind breaks hard stone into weak dirt.

Wildfires can burn away plants, destroying essential soil nutrients and natural soil structure. Without plant matter, the soil cannot hold together during the next heavy storm.

Violent ground movements also trigger huge slides. An earthquake shakes mountain sides and can quickly trigger soil liquefaction, turning firm ground into soupy slush. Exploding volcanic eruptions shake the ground, melt summit glaciers, and push huge piles of loose ash down mountain peaks.

Human Activities That Weaken Slopes

People can accidentally make hillsides much less stable through construction and land development. Cutting flat benches into hills to build roads or houses removes essential base support. This type of earthmoving work is called an earthwork project.

Cutting down forests, also known as deforestation, causes severe problems. Trees have large root networks that hold loose topsoil tightly against solid bedrock. When people clear land for cultivation or timber logging, those roots rot away, leaving loose colluvium dirt behind.

Cities expanding into natural hills through urban sprawl can also cause landslides. Heavy machinery, railway systems, and heavy highway traffic create constant ground vibrations. Mining companies that use blasting can fracture nearby rock layers.

Changes in land use and land cover also affect how water drains across the landscape. When farmland is abandoned or covered in concrete, rainwater cannot soak into the ground gently. Instead, it rushes across the surface, causing severe soil erosion and triggering mudslides.

Different Types of Landslides

Different types of landslide movements.

Scientists classify landslides based on the materials involved and how those materials travel downhill. Long ago, people used the word landslide to describe any mass movement of loose regolith or rock. Over time, geologists developed detailed classification systems to study these dangerous events more carefully.

The Classification System

In 1978, a geologist named David Varnes created a clear system to organize different kinds of ground movement. Later researchers updated this system to include modern discoveries about soil physics and subsidence.

Under this modern system, geologists divide slope movements into six primary categories:

• Falls happen when individual chunks of rock or ice drop freely through the empty air.
• Topples occur when tall blocks of stone tilt forward and tip over like falling dominoes.
• Slides take place when a solid block of soil or rock glides along a slick underground surface.
• Spreads occur when soft underground layers turn to liquid, causing the harder ground on top to crack open sideways.
• Flows happen when dirt, rock, and water mix together, moving fluidly like a rushing river.
• Slope deformations are very slow, deep-seated bendings of rock that gently change the shape of whole mountainsides.

Rapid Flows and Rushing Mud

The Costa della Gaveta earthflow in Potenza, Italy.

A rock slide in Guerrero, Mexico

A landslide in Peru following flash floods.

When dirt and rock become soaked with water, they often turn into a debris flow or a mudflow. These flows behave like wet concrete flowing down a chute. They can move at amazing speeds, sweeping away boulders, cars, and whole buildings.

These flows are especially dangerous in steep alpine valleys. Narrow mountain canyons squeeze the rushing mud, forcing it to move faster. Sometimes, rushing debris jams a river canyon, creating a natural dam. When that temporary dam breaks, terrible flooding can rush into lower towns.

An earthflow is a slower flow of fine clay, silt, and sand. Some earthflows crawl forward just a few millimeters every year, while others surge downhill quite fast. They often form a rounded tongue shape, called a lobe, at the bottom of the slope.

The fastest and most terrifying flow is a rock avalanche, also called a sturzstrom. These rare events involve millions of tons of shattered rock roaring down a mountain. They travel great distances across flat ground, and some even rush up opposing hills. Scientists believe trapped underground steam and fine mineral powder act like slippery cushions, helping rock avalanches slide incredible distances.

Slides Along Weak Layers

A slide occurs when earth glides downhill along a slick boundary layer. In a planar slide, the ground slides over a straight, flat surface, like a tilted sheet of rock. In a rotational slide, the ground slips along a curved, bowl-shaped surface, causing the top to tilt backward.

A debris slide occurs when loose soil, fallen trees, and rocks slip down a steep slope. These slides often happen after storms soak thick forests. The wet top layer peels away from the firm rock beneath it, tumbling down into the valley below.

Clay slides happen when slippery clay layers absorb moisture. Water breaks the bonds between clay particles, turning the layer into a slick ramp. These slides can start slowly, but they often speed up after torrential rainstorms.

Shallow Versus Deep-Seated Slides

Rock sliding along an exposed bedrock plane near Lake Garda.

A deep landslide in Kihō, Japan, after Tropical Storm Talas.

Landslide debris covering a highway in Pakistan.

Geologists divide landslides into shallow slides and deep slides based on how deep the sliding surface goes. A shallow slide happens within the top layer of soil or slightly damaged bedrock. These slides are usually only a few inches to several feet deep.

Shallow slides often occur where sandy, loose soil sits on top of dense, hard clay. Water easily passes through the top soil but gets trapped by the dense clay below. The trapped water turns the top soil into a heavy, soggy sponge that quickly slips away.

Deep-seated landslides reach far beneath the surface, often extending much deeper than the roots of the tallest trees. They involve massive sections of fractured regolith and solid underground stone.

Deep slides often start along an ancient geological fault or a natural bedding plane between rock layers. At the top of a deep slide, you can often see a steep dirt cliff called a scarp. Over thousands of years, these massive deep slides carve rugged landscapes and dump debris into running fluvial streams.

Related Natural Events

Landslides are closely connected to several other powerful natural events that reshape mountains and valleys:

• A snow avalanche is a fast-moving rush of snow, ice, and broken rock tumbling down a steep peak.
• A glacier collapse happens when a massive chunk of solid ice breaks off a slowly moving glacier.
• A pyroclastic flow is a boiling cloud of hot volcanic ash, rock, and gas racing down an active volcano.
• Heavy desert rains can carve deep dirt trenches called gullies across flat plains through rapid water erosion.
• Powerful undersea slides or coastal rockfalls can push water aside suddenly, creating a giant ocean tsunami.
• Huge rockfalls dropping into narrow bays can create a megatsunami that washes hundreds of feet up surrounding hillsides, such as the famous 1958 Lituya Bay megatsunami in Alaska.

How Scientists Map and Predict Slides

An extensometer tracks moving cracks on a dangerous slope.

Scientists work hard to predict landslides before they cause damage. By understanding where hills are weak, cities can make smarter choices through land-use planning.

Geologists study many different subjects to evaluate slide hazards, including surface geomorphology, rock geology, and underground hydrogeology. Because there is so much information to look at, scientists rely on computer programs called a geographic information system (GIS).

Using a GIS, researchers can combine layers of data, including maps of rock types, slope angles, and rainfall patterns. This technology lets scientists see which hillsides are most likely to fail during the next big storm.

Scientists also use Remote sensing tools like airplanes and satellites to monitor changing landscapes. Satellites take high-resolution photos that reveal ground cracks, rock types, and changes in forest health. By comparing photos taken before and after a slide, scientists learn how slopes recover over time.

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  Global landslide risks

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  United States landslide hazard map

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  The Ferguson Slide along California State Route 140

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  A rock slide fence on the UPRR in Colfax, CA

Using computer maps helps emergency workers protect people from sudden natural disasters. When scientists identify dangerous slopes, planners can practice Sustainable land management to keep homes away from unsafe ground.

During the famous Val Pola landslide disaster in northern Italy, engineers linked GIS map tools with a decision support system. This smart setup gathered sensor data from the mountain and warned people down in the valley when the hillside began to move.

Giant Prehistoric Landslides

The Rhine river flowing through ancient Flims Rockslide debris.

The prehistoric Aussera slide in the Pyrenees mountains of France.

Landslides have shaped the surface of our planet for millions of years. Long before people built cities, giant collapses moved enormous mountains of rock:

• The Storegga Slide took place underwater off the coast of Norway about 8,000 years ago. It moved thousands of cubic kilometers of mud and triggered massive tsunamis across ancient Doggerland in the North Sea.
• The Heart Mountain slide in Wyoming was the largest continental slide known on Earth, moving a giant limestone peak across dozens of miles.
• The Flims Rockslide crashed down in the Swiss Alps about 10,000 years ago during the transition from the Pleistocene to the Holocene epoch. It was the largest dry-land rockslide in Europe.
• A massive landslide crashed into a gorge in New Zealand around 200 BC, damming the local river and creating Lake Waikaremoana on the North Island.
• The Cheekye Fan in British Columbia, Canada, was built up by huge debris flows during the Late Pleistocene age.
• In the Annapurna Region of Nepal, a massive mountain collapse formed the Marsyangdi Valley. An ancient peak taller than modern Annapurna completely crumbled, moving over 15 cubic kilometers of shattered stone.
• The Tsergo Ri landslide collapsed north of Kathmandu, Nepal, dropping a huge mountain peak into the valley below.

Famous Historical Landslides

Throughout history, landslides have changed communities and altered rivers around the world:

• The 1806 Goldau landslide destroyed several villages in Switzerland after weeks of heavy rain.
• The Québec rockslide fell from the cliffs of Cap Diamant in Canada in 1889.
• The Frank Slide in Alberta, Canada, happened in 1903 when the limestone peak of Turtle Mountain suddenly gave way.
• The Khait landslide was triggered by an earthquake in Tajikistan in 1949.
• A magnitude 7.5 earthquake near Yellowstone in 1959 created a slide that blocked a river, forming Quake Lake.
• The Monte Toc slide in 1963 fell straight into the water behind the Vajont Dam in Italy, creating a giant wave.
• The Hope Slide crashed down near Hope, British Columbia, in 1965, burying a highway under thick rock.
• The tragic 1966 Aberfan disaster in Wales happened when a coal spoil tip slipped down a hill after heavy rain.
• The Tuve landslide in Gothenburg, Sweden, occurred in 1977 due to unstable quick clay.
• The 1979 Abbotsford landslip slowly wrecked dozens of homes in Dunedin, New Zealand.
• The huge 1980 eruption of Mount St. Helens caused the largest landslide in recorded human history when the volcano's north face collapsed.
• The Val Pola landslide struck northern Italy in 1987 during the Valtellina disaster.
• The 1997 Thredbo landslide slid through an alpine ski resort village in Australia.
• The 1999 Vargas mudslides swept down mountain slopes in Vargas State, Venezuela, following days of extreme rain.
• The 2005 La Conchita landslide destroyed several seaside homes in southern California.
• The 2006 Southern Leyte mudslide buried an entire town in Saint Bernard, Southern Leyte, in the Philippines.
• The 2007 Chittagong mudslide hit port communities in Chittagong, Bangladesh, during monsoon season.
• The 2008 Cairo landslide dropped huge limestone boulders onto homes in Egypt.
• The 2010 Uganda landslide struck hilly farming communities in the Bududa District after weeks of rain.
• The 2010 Gansu mudslide poured tons of thick mud through Zhouqu County in Gansu, China.
• The Devil's Slide along the ocean coast of San Mateo County, California, has caused highway repairs for decades.
• The January 2011 Rio de Janeiro floods and mudslides caused widespread damage across mountain towns in Rio de Janeiro State, Brazil.
• The 2014 Pune landslide hit the village of Malin in Pune, India, after intense downpours.
• The 2014 Oso mudslide crossed a river and covered a community near Oso, Washington.
• The 2017 Mocoa landslide sent torrents of water and boulders into the town of Mocoa, Colombia.
• The 2022 Ischia landslide sent mud rushing through streets on an Italian island.
• The 2024 Gofa landslides struck mountain villages in the Gofa region of southern Ethiopia.
• The 2024 Wayanad landslides caused serious damage across hilly tea plantations in the Wayanad district of Kerala, India.
• In early 2025, a large rockfall caused damage near the mountain village of Blatten (Lötschen) in Switzerland.
• The 2025 Tarasin landslide struck hilly terrain in Sudan after intense seasonal rainfall.

Landslides on Other Planets

Earth is not the only place where landslides happen. Space probes have discovered signs of giant slides on other planets and moons across the Solar System.

Spacecraft orbiting Mars have photographed giant landslides along the cliffs of Valles Marineris, a canyon system much bigger than the Grand Canyon. Radar images from probes circling Venus show rough, fan-shaped zones where rock tumbled down steep volcanic peaks.

Because Mars is very dry and Venus is extremely hot, landslides there happen without rainwater. Geologists study extraterrestrial slides to understand how gravity, rock types, and quakes work together without oceans or plant life.

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  Radar image of a rockslide on Venus

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  An active dust and ice slide on Mars photographed in 2008

Protecting Communities and Slopes