Landslides are a type of mass movement and are characterized by the sliding of stretches of ground, chunks of rock and other debris along a ridge, hill or mountain slope. They are caused by natural factors, conditioned by the heavy rains and by the characteristics related to the soil and the relief, and also by anthropic factors, among which are the deforestation of slopes and construction in irregular areas. When in urbanized areas, landslides cause great material losses and serious consequences for the population, such as the destruction of their homes, injuries and deaths.
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Summary about landslide
Landslides, a type of mass movement, are caused by both natural and human (or anthropogenic) factors.
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The natural factors that cause landslides are:
heavy rains;
soils that absorb a lot of water and impermeable rocks;
accentuated reliefs.
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The following are anthropic factors that potentiate landslides:
slope deforestation;
constructions on hills and mountains;
Illegal disposal of garbage and other waste.
Landslides are classified, according to the rupture surface, in translational and rotational.
Although they are likely to occur in any area, some measures can be taken to avoid them, such as not make cuts on the slopes, do not remove the vegetation cover, and build paths for the correct drainage of water from the rains.
The consequences of landslides in inhabited areas range from material losses to deaths.
What causes landslides?
Landslides are a type of mass movement characterized by the sliding of solid materials, such as soil, rock fragments and other debris, along a slope. this kind of movement commonly occurs in areas of rugged relief, and is triggered by the presence of cracks in the soil or rock and by the joint action of external factors, both natural and anthropic, that is, of human origin.
→ Natural causes of landslide
The natural causes for the occurrence of landslides are associated with the following elements and phenomena:
Soil type: poorly compact soils, where there is great water infiltration, are susceptible to landslides. The probability of occurrence is even greater in soils that form on impermeable rocks. With that, the water it accumulates in the soil and leaves it with a very high saturation, or soaked, increasing the chances of this substrate breaking down and going down the slope.
Rough relief and slope slopes: the angle of inclination of the slopes of mountains and hills (declivity) is a determinant of mass movements such as landslides. The stability of materials on inclined terrain is lower the greater the angle of inclination. Steep slopes with angles greater than 20º are considered.
Heavy rains: rains very strong and concentrated in a short period make the slopes susceptible to landslides, especially those devoid of vegetation cover. The sudden increase in the volume of water received by the soil causes more intense surface runoff, which generates runoff. This movement can cause the destabilization of the soil, causing it to lose its cohesion and go down to the base of the slopes.
In countries where the occurrence of earthquakes often, earthquakes can also cause landslides.
→ Anthropogenic causes of landslides
As mentioned, natural factors are not the only causes of landslides. Human actions enhance the occurrence of this type of natural disaster, including:
Removal of vegetation cover from slopes: the vegetation helps to reduce the impact of raindrops on the soil, in addition to having a root system that absorbs part of the water that infiltrates the soil, helping it not to become saturated. O logging does away with the protective role that the vegetation cover has.
Slope buildings: the occupation of steep terrain, such as the slopes of hills, common in large urban centers as a result of disorderly urbanization, and, consequently, the works and the construction of houses and other establishments in these areas increases the weight on the ground, which can eventually affect its balance and lead to landslides.
Illegal disposal of garbage and other debris: in addition to the clogging of water drainage routes, the improper disposal of garbage directly on the ground, for example, causes the appearance of gases, such as methane, which cause mass. In rarer cases, explosions associated with the presence of this gas can also cause landslides.
Read too: The dangerous combination of disorderly urban growth and summer rains
What are the types of landslides?
Landslides are classified, according to their rupture surface, into:
Rotational Slips: the rupture surface, the area where the soil breaks down and mass movement begins, appears as a curve towards the interior of the hill.
Translational slips: the failure surface is an inclined plane.
The United States Geological Survey (USGS) explains that, in many cases, the term landslide is used as a synonym for mass movement, and its classification occurs according to the type of material detached. In that sense, it can be divided into several categories. The main ones are:
Fall: breaking of blocks of rock and/or soil that suddenly fall and descend to the lowest areas.
Tipping: a fault causes the detachment of a piece of rock or soil, which is followed by a rotational movement and a tipping of this block forward. A recent example was the tipping of an immense block of rock in the Furnas dam, in the Brazilian city of Capitólio (MG).|1|
Slip: landslide itself.
Debris flow: rock fragments, soil, organic matter and other debris mix with water rushing down a hillside at high speed.
How to avoid landslide?
Landslides are common phenomena in periods of very intense and prolonged rain, as in the summers from tropical countries, such as Brazil. Some measures can be taken in order to prevent its occurrence, such as:
Do not deforest the slopes of hills and mountains, thus maintaining the natural protection of the soil.
Avoid planting large and heavy trees on slopes, such as banana trees, which require a lot of water.
Dispose of garbage correctly, so as not to create an additional weight on the floors.
Create channels for water drainage.
Sewage and water already used must be disposed of in a proper place, not on the ground.
Fix domestic water leaks as soon as possible so that water does not reach the ground.
Do not make cuts on slopes without the prior authorization of public bodies.
If you notice areas of exposed soil without protection on a slope, ask the Civil Defense to cover these places with plastic or another type of barrier to protect the soil in case of rain.
What are the consequences of the landslide?
When in uninhabited areas, landslides are responsible for falling trees; soil scarring and exposure of the underlying rock; silting up of rivers and watercourses, if there are any immediately below the area of occurrence; and death of animals.
In inhabited areas, mainly in cities and urbanized regions, however, landslides are capable of causing great tragedies. They cause:
landslides and the destruction of houses and buildings that were both in the hillside areas and below them, leaving people homeless, injured or dead;
obstruction or destruction of sidewalks, roads and other urban transport routes;
silting up of urban springs;
destruction of electricity, water, gas and sanitation supply networks;
greater susceptibility to the occurrence of new landslides in the affected areas.
Read too: Floods in Brazil — why does this phenomenon occur so often in our country?
Landslide in Brazil
Landslides are recurring in Brazil during the rainy season, which takes place in the summer months, that is, between December and March. According to the Brazilian Institute of Geography and Statistics (IBGE), the regions South It is Southeast concentrate the greatest number of areas where the risk of landslides is high, despite the fact that these events occur in all areas of the country.|2|
The process of urbanization disorganized and often irregular occupation of the slopes of hills and mountains are among the human factors that enhance the landslides in the country, a fact that implies a greater degree of gravity to this type of mass movement when it happens, given the density populational.
A mountain region of Rio de Janeiro is one of the most affected by landslides, which have already resulted in major disasters. The worst of them happened in the cities of Nova Friburgo, Petrópolis and Teresópolis in 2011, when more than 900 people lost their lives and another 35 thousand were left homeless.
Even today, more than a decade later, 99 individuals remain missing. The second worst occurrence in the history of the country happened in the municipality of Caraguatatuba, in Serra do Mar, in São Paulo. In 1967, heavy rains caused a series of landslides throughout the city, leaving more than 400 people dead and 3,000 homeless.3| To learn more about the occurrence of this phenomenon in our country, read: Landslides in Brazil.
Video lesson on landslides in Brazil
Grades
|1| STATE OF MINES. Know what a landslide is and the difference between a fall and a landslide. Estado de Minas, 12 Jan. 2022. Available here.
|2| RODRIGUES, Leo. The Southeast and South regions concentrate areas at risk of landslides. Agência Brasil, 29 Nov. 2019. Available here.
|3| RUN, Daniel. Landslide that devastated Caraguatatuba completes 50 years. g1, 18 Mar. 2017. Available here.
image credits
[1] Thomas Dekiere / Shutterstock
By Paloma Guitarrara
Geography Teacher
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