Landslides occur when rock, soil, or debris moves downhill under the force of gravity. They happen when the forces holding a slope together become too weak to resist the pull of gravity, or when conditions increase the force driving material downhill. Heavy rainfall, earthquakes, erosion, volcanic activity, and human construction can all contribute to this loss of stability.
Landslides are a natural part of how Earth’s surface changes over time. They reshape mountains, transport sediment into rivers, and create new landforms. However, when they occur near homes, roads, utilities, or communities, they can cause severe damage, injuries, and loss of life. Understanding why slopes fail, which conditions make failure more likely, and what warning signs to recognize can help people assess and reduce these risks.
How landslides happen
A hillside may appear solid and stable while containing weaknesses that make it vulnerable to movement. Its stability depends on the strength of the soil or rock, the angle of the slope, the amount of water within the ground, and the forces acting on the material.
Two competing forces largely determine whether a slope remains in place. Driving forces pull material downhill, primarily through gravity. Resisting forces, including friction between soil particles, cohesion within the material, and the strength of intact rock, oppose that movement. A landslide occurs when the driving forces exceed the resistance along a potential failure surface.
A failure surface is the boundary along which soil or rock begins to move. It may lie close to the ground surface or extend deep beneath a hillside. Some landslides involve a relatively thin layer of soil, while others displace enormous masses of rock and sediment.
Geologists often describe slope stability in terms of the factor of safety, which compares the forces resisting movement with those driving it. A slope with a factor of safety greater than 1 has more resisting strength than driving force under the conditions being evaluated. When the factor falls below 1, failure becomes mechanically possible. In real landscapes, however, both the material properties and the forces acting on a slope vary, so predicting the precise time of failure can be difficult.
Landslides take several forms. A rockfall occurs when pieces of rock detach from a steep slope or cliff and fall, bounce, or roll downward. A slide occurs when material moves along a relatively distinct surface. In a rotational slide, the moving mass rotates backward along a curved surface; in a translational slide, it moves along a more nearly planar surface. A flow occurs when water-rich soil, sediment, or debris moves more like a fluid.
These categories can overlap. A slide may break apart and become a debris flow as it travels downhill, picking up additional water, rocks, trees, and soil along the way.
Heavy rainfall and groundwater are major triggers
Water is one of the most important factors affecting landslide risk. Rain can weaken a slope even when the underlying rock and soil have remained unchanged for thousands of years.
As water infiltrates the ground, it fills spaces between soil particles and enters cracks in rock. This increases the weight of the material. At the same time, rising water pressure within the soil can reduce the frictional resistance that helps hold the slope together.
The effect of water pressure is particularly important. Soil particles press against one another and resist movement through contact forces and friction. When water pressure rises between those particles, it reduces the portion of the pressure that the particles effectively carry. This can make the soil substantially weaker without any visible change at the surface.
Heavy rainfall can also saturate previously dry soil, raise groundwater levels, and increase pressure along existing slip surfaces. If water accumulates above a relatively impermeable layer, such as dense clay or unfractured bedrock, it may create a weak zone along which the overlying material can slide.
The duration and timing of rainfall matter as much as its intensity. A short, intense storm can trigger rapid runoff and shallow landslides, particularly on steep slopes with loose material. Prolonged rainfall can gradually saturate deeper layers, raising groundwater levels and destabilizing larger masses of earth. A slope may also fail after rain has stopped because water continues moving through the ground and pressure may remain elevated.
Repeated wetting can be especially damaging when a hillside has already been weakened by earlier storms. Once cracks develop or small sections begin moving, additional water may enter the newly opened pathways, further reducing stability.
Earthquakes can destabilize entire slopes
Earthquakes trigger landslides by shaking the ground and changing the forces acting on soil and rock. Even a slope that is stable under normal conditions may fail when subjected to strong or prolonged shaking.
During an earthquake, ground acceleration produces additional forces within a hillside. These forces can exceed the strength of fractured rock, loose soil, or sediment resting on steep slopes. Rock masses may detach from cliffs, and soil layers may slide across weaker underlying material.
Earthquake shaking can also cause liquefaction in susceptible soils. Liquefaction occurs when saturated, loose sediment loses much of its strength because earthquake shaking causes water pressure between particles to rise. The sediment can then behave temporarily more like a liquid than a solid. On a slope, this loss of strength can produce rapid ground movement.
Not every earthquake causes landslides, and the effects are not limited to the area directly above the earthquake’s source. The likelihood of slope failure depends on shaking intensity, slope steepness, rock and soil conditions, groundwater, and the presence of preexisting fractures or unstable deposits.
Earthquakes can also leave slopes weakened long after the shaking ends. Cracked rock, disturbed soil, and loosened debris may remain vulnerable to rainfall-triggered landslides for months or longer, depending on local conditions.
Erosion removes the support that holds slopes together
A slope can become unstable when its base loses support. Rivers, ocean waves, glaciers, and surface runoff all reshape landscapes by removing material from hillsides and their foundations.
A river flowing along the base of a steep bank can erode sediment and rock, leaving the material above unsupported. Coastal cliffs can fail when waves repeatedly undercut their bases. In mountainous regions, streams can erode the toes of slopes, especially during floods.
The lower part of a slope is known as its toe. When erosion removes material from this area, the remaining hillside may become steeper or lose an important source of resistance. Eventually, gravity may overcome the weakened slope.
Erosion does not always cause immediate failure. A hillside may remain standing for years after its base has been undercut, then collapse following a storm, earthquake, or further loss of material.
Water flowing over the surface can also carve gullies, remove protective vegetation, and carry away loose soil. Although these processes may begin gradually, they can increase the likelihood of shallow slides and other forms of slope failure, particularly where the ground is already steep or unstable.
Geological conditions determine which slopes are vulnerable
Not all slopes respond to rainfall, earthquakes, or erosion in the same way. Their underlying geology strongly influences how much stress they can withstand and where a failure is likely to occur.
Rock type is one important factor. Some rocks are strong and relatively resistant to weathering, while others break down easily or contain weak layers. Shale, for example, can split along bedding planes, and some clay-rich rocks weaken when exposed to water. Fractured or weathered bedrock may also provide pathways for groundwater to enter a hillside and create zones of reduced strength.
The orientation of rock layers matters, too. If weak bedding planes or fractures slope in the same direction as the hillside, they may provide a natural surface along which rock can slide. If those planes dip into the hill rather than toward the valley, the same rock structure may pose less of a direct sliding risk.
Soil composition influences stability in similar ways. Loose sand and gravel may drain readily but can fail when steep or poorly supported. Clay-rich soils can become slippery when wet, although their behavior depends on the type of clay, its structure, and its moisture history. Some soils also shrink and crack during dry periods, allowing water to penetrate more deeply when rain returns.
Weathering gradually breaks down rock through physical and chemical processes. Freeze-thaw cycles can widen cracks as water freezes and expands, while chemical weathering alters minerals and weakens rock. Over time, these processes create loose material that may accumulate on steep slopes.
The result is a landscape in which some hillsides remain stable through repeated storms while others fail under relatively modest conditions. Geological weaknesses often determine where a landslide occurs, while rainfall, shaking, erosion, or human activity supplies the immediate trigger.
Volcanic activity and rapid snowmelt can cause landslides
Volcanic landscapes are especially susceptible to several kinds of slope failure. Eruptions can fracture volcanic rock, deposit loose ash and debris, and rapidly alter the shape of a volcano. The steep slopes of volcanic cones may contain thick accumulations of material that are only weakly consolidated.
One particularly dangerous volcanic landslide-related hazard is a lahar, a fast-moving flow of water mixed with volcanic ash, rock fragments, and other debris. Lahars can form when volcanic heat melts snow and ice, when heavy rain mobilizes loose volcanic deposits, or when water mixes with material released during an eruption.
Because lahars can travel down valleys far from a volcano, communities outside the immediate eruption zone may still face serious risks. Their paths are strongly influenced by drainage networks and topography.
Rapid snowmelt can also destabilize nonvolcanic slopes. When large amounts of meltwater enter the ground over a short period, they may saturate soil, raise groundwater levels, and increase pressure within weak layers. Rain falling on existing snow can intensify the process by supplying additional water.
Glaciers contribute to slope instability in other ways. They can erode valley walls, expose steep rock faces, and leave loose sediment behind as they retreat. When ice that once supported or confined a rock mass disappears, the slope may adjust to the changed conditions, sometimes through rockfalls or larger failures.
Human activities can increase landslide risk
Landslides are natural processes, but human activities can make them more likely by changing slope geometry, removing vegetation, redirecting water, or adding weight to unstable ground.
Road construction is a common example. Cutting into a hillside to create a road may remove support from the toe of a slope. If the cut exposes weak rock layers or intersects groundwater pathways, the slope may become more vulnerable to failure. Poorly designed drainage can compound the problem by concentrating water in places where it weakens the ground.
Building on a slope can also change its stability. Structures, fill material, and stored soil add weight, increasing the forces that pull material downhill. Excavation near the base of a hillside can reduce resistance, while poorly placed fill near the top can increase the load on the slope.
Deforestation and some forms of land clearing can contribute to instability. Tree roots help bind soil together, and vegetation can intercept rainfall and influence how water moves through the ground. Removing trees may reduce root reinforcement and expose soil to erosion. However, vegetation is not a universal safeguard: roots cannot stabilize every deep-seated landslide, and some heavily vegetated slopes remain naturally unstable.
Agricultural practices, mining, quarrying, and poorly managed drainage systems can also alter slope conditions. Irrigation may increase soil moisture, while leaking water pipes and septic systems can introduce water into the ground over long periods. Reservoirs may affect nearby slopes by changing water levels and groundwater conditions.
Human activity is rarely the only factor involved. Construction or land clearing may create the conditions for failure, while a storm, earthquake, or gradual increase in groundwater pressure provides the final trigger.
Climate and changing environmental conditions affect landslide hazards
Climate influences landslide risk primarily through its effects on rainfall, snow, ice, vegetation, and erosion. The relationship varies among regions because different landscapes respond to different combinations of weather and geology.
In places where intense rainfall is a major trigger, storms that deliver large amounts of water over short periods can cause rapid slope failures. Prolonged wet periods can also increase risk by allowing groundwater to rise. Changes in the timing and amount of snowmelt can affect slopes in mountainous regions.
Wildfires can increase landslide susceptibility as well. Fire may remove vegetation, alter soil structure, and leave the ground less able to absorb water in some locations. After a fire, intense rainfall can rapidly wash loose sediment downhill and trigger debris flows. The level of risk depends on burn severity, soil properties, slope steepness, and rainfall intensity.
Long-term changes in climate do not affect every region in the same way. Some areas may experience more frequent conditions conducive to rainfall-triggered landslides, while others may see different patterns associated with drought, snowpack changes, or shifting storm behavior. Local geology and land use remain critical in determining whether a particular hillside will fail.
Warning signs that a landslide may be developing
Some landslides occur suddenly, with little obvious warning. Others develop gradually, producing changes in the ground that may signal increasing instability. Recognizing these signs can help people identify a potential hazard, although no single sign proves that a landslide is imminent.
Cracks in the ground are among the most important warning signs. New openings across a hillside, widening cracks in soil, or fractures appearing near the tops of slopes may indicate that the ground is beginning to separate along a developing failure zone. Cracks in roads, driveways, retaining walls, or building foundations can also indicate movement, especially when they appear suddenly or continue to widen.
Changes in the position of objects may provide additional clues. Trees, utility poles, fences, and retaining walls that begin tilting or shifting can reflect ground movement. Doors and windows that suddenly stick, new gaps around foundations, or walls that develop fresh cracks may also warrant attention when they occur alongside other signs of slope instability.
Water-related changes are particularly relevant after heavy rain. New springs, unexpected seepage, unusually wet patches, or sudden changes in the flow of water from a hillside may indicate that groundwater pathways have changed. Streams that become muddy or carry an unusual amount of sediment can sometimes signal erosion or slope movement upstream, although many other processes can produce the same effect.
Small rockfalls, fresh piles of broken rock, bulging ground near the base of a slope, and newly exposed soil on a hillside can also be warning signs. In some cases, people may hear cracking, rumbling, or trees breaking before or during movement. These sounds should be treated seriously when accompanied by visible ground changes or other signs of instability.
Warning signs are not always easy to interpret. Buildings can crack because of settlement unrelated to a landslide, and muddy streams can result from ordinary erosion. Conversely, an apparently unchanged slope can fail rapidly during intense rainfall or an earthquake.
Anyone who notices multiple new signs of movement on or near a slope should move away from the suspected hazard and report the conditions to local emergency management or an appropriate geological or engineering authority. People should not approach a suspected landslide area to investigate cracks, unstable ground, or unusual water flow.
Why landslides can be so dangerous
The danger of a landslide depends on how much material moves, how quickly it travels, the path it follows, and whether people or infrastructure lie in its path. Even a relatively small failure can destroy a building or block a road if it occurs in the wrong location.
Rapid rockfalls and debris flows can be especially hazardous because they may leave little time to react. Debris flows can carry large rocks, trees, and other heavy objects, while their water-rich material can exert substantial force on buildings, bridges, and vehicles. A moving mass may also strike structures directly or bury them beneath sediment.
Slower landslides can cause extensive damage as well. Gradual ground movement can distort foundations, crack roads, damage pipelines, and disrupt water, electricity, and communication systems. A slope may move intermittently over months or years, damaging structures each time movement resumes.
Landslides can also block rivers and streams. The resulting natural dams may create temporary lakes upstream. If a dam breaks, rapidly released water can cause flooding downstream. Landslide debris entering a river can also increase sediment loads, alter channels, and affect aquatic habitats.
The consequences often extend beyond the initial failure. Roads may become impassable, emergency access may be interrupted, and displaced sediment can damage farmland or water supplies. Recovery may require extensive engineering work, slope stabilization, relocation, or long-term monitoring.
The greatest danger often arises where several hazards overlap. A powerful storm can trigger landslides while also causing flooding, and an earthquake can damage buildings and roads while destabilizing surrounding slopes. In mountainous terrain, a single landslide can affect multiple communities along a valley.
Where landslides are most likely in the United States
Landslides can occur in every state, but the types of hazards and their frequency vary with terrain, geology, rainfall, and land use.
The Appalachian Mountains contain steep slopes, deeply weathered rock, and areas with clay-rich soils that can become unstable during prolonged or intense rainfall. Road cuts, development, and drainage changes may further increase susceptibility in some locations.
The Pacific Coast and parts of the western United States have extensive landslide-prone terrain. Coastal cliffs can fail through wave erosion and groundwater seepage, while mountainous areas experience rockfalls, deep-seated slides, and debris flows. Heavy rain can trigger failures in naturally weak or previously disturbed slopes.
In California and other fire-prone parts of the West, intense rain after wildfires can generate debris flows. Steep terrain, loose sediment, and reduced vegetation can combine to produce dangerous runoff and rapid sediment movement.
The Rocky Mountains and other high-elevation regions face hazards associated with steep rock faces, freeze-thaw weathering, snowmelt, and intense storms. Alaska has additional risks associated with glaciers, thawing ground, steep mountain terrain, and changing environmental conditions.
Landslides are not limited to mountainous regions. Riverbanks, bluffs, coastal slopes, road embankments, and hills formed from loose or weak sediment can also fail. Even in areas with relatively gentle terrain, local cuts, fills, drainage problems, or unstable riverbanks may create significant hazards.
A location’s general reputation for landslides is not enough to determine the safety of an individual property. Conditions can change sharply over short distances, depending on the slope’s underlying materials, drainage, and history of movement.
How scientists assess and reduce landslide risk
Geologists and geotechnical engineers investigate landslides by examining the shape of the terrain, the composition of soil and rock, groundwater conditions, and evidence of past movement. They may map cracks, measure slope deformation, analyze soil strength, and determine whether weak layers or fractures could provide a path for failure.
Monitoring instruments can detect changes that are difficult to observe from the surface. Inclinometers measure movement within the ground, while instruments that track groundwater levels help specialists assess how water pressure changes over time. Surveying equipment and satellite-based techniques can measure surface movement across slopes or larger areas.
Rainfall measurements and forecasts can support landslide warnings in susceptible regions. Some warning systems combine rainfall intensity, accumulated precipitation, soil moisture, and local geological conditions to estimate when slopes may become more likely to fail. Such systems cannot predict every landslide, particularly where local conditions are poorly understood or failures occur abruptly.
Reducing risk often begins with avoiding development in hazardous locations. Where construction is necessary, engineers may redesign slopes, install drainage systems, reinforce unstable material, build retaining structures, or remove loose rock. These measures must be tailored to the specific conditions; a solution that works for one slope may be ineffective or even harmful on another.
Good drainage is frequently essential because uncontrolled water can increase pore-water pressure and weaken soil. Properly designed retaining walls, slope reinforcement, and erosion-control measures can improve stability, but they require appropriate engineering and maintenance. Simply covering a slope with vegetation or placing a barrier at its base does not guarantee protection from a major landslide.
Land-use planning also plays a central role. Geological hazard maps can help communities identify areas where landslides have occurred or are more likely to occur. Building restrictions, setbacks from unstable slopes, stormwater management, and emergency planning can reduce exposure even when the underlying hazard cannot be eliminated.
For individuals living near steep slopes or unstable bluffs, practical precautions include keeping drainage systems functional, avoiding unauthorized excavation or dumping of fill, and paying attention to changes in the ground after heavy rain or earthquakes. Property owners who notice persistent movement should seek assessment by a qualified geotechnical professional rather than attempting to stabilize a potentially dangerous slope themselves.
If a landslide appears to be occurring, immediate safety takes priority over property protection. Move away from the moving material and its likely path, follow local emergency instructions, and contact emergency services when people or structures may be in danger. Avoid crossing a landslide or entering an affected area, because additional movement can occur after the initial failure.
Why landslides remain difficult to predict
Scientists understand the physical processes that cause landslides, but predicting exactly when and where an individual slope will fail remains challenging. The difficulty comes from the enormous variation in ground conditions and from changes that may occur deep below the surface.
A slope can contain alternating layers of strong and weak rock, hidden fractures, pockets of groundwater, and old failure surfaces. These features may be difficult to identify without detailed field investigations and subsurface measurements. Rainfall can also infiltrate unevenly, producing localized increases in water pressure that are not apparent from the surface.
Small changes can sometimes trigger a large failure when a slope is already close to instability. In other cases, a hillside may withstand repeated storms before finally moving. The same amount of rainfall can therefore have very different effects in different locations or even on neighboring slopes.
Scientists can often identify areas of elevated susceptibility, recognize conditions that increase the likelihood of failure, and monitor slopes that show signs of movement. They can also estimate risks over particular periods using geological evidence, weather observations, and models. These methods are valuable for planning and warning, but they do not provide perfect forecasts of every event.
The most reliable approach combines an understanding of the landscape’s geology with attention to changing conditions. Rainfall, earthquakes, erosion, volcanic activity, and human disturbance can all supply the trigger, but the outcome depends on the slope’s underlying strength and the forces acting on it. Recognizing that relationship is central to understanding landslides and reducing the harm they cause.
