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What Are Landslides? Understanding the Science, Mechanism, Causes & Types of Landslides

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Introduction


  • On 20 July 2026, a tragic landslide at the "Teesta Hydropower Project Stage-IV canal" near Limachi Village in Sikkim, situated in the lap of the young and geologically active Himalayan mountain ranges, claimed the lives of eight workers, once again highlighting the destructive power of landslides. 
  1. The fragile geology, 
  2. steep slopes, 
  3. intense monsoon rainfall, and 
  4. increasing human interventions frequently trigger these "slope failures", resulting in landslide tragedies, but, unfortunately, this tragedy is part of a much larger global pattern, when every year thousands of people lose their lives to landslides across the world. 

  • According to the Global Fatal Landslide Database, 2024 witnessed 766 fatal landslide events resulting in 4,933 deaths, making it one of the deadliest years in recent records. 
     

  • The trend continued in 2025, when several catastrophic landslides struck Asia and Africa, including a devastating event in Sudan, where a single landslide reportedly claimed around 1,000 lives.
  • When we compile these data over the last 25 years, we find something disturbing about human resources and natural calamities. As shown in the follow infigraphics.

  • The infographics present a grim picture of calamities induced by landslide disasters. These disasters raise an important question: Are landslides merely natural hazards, or are increasing human activities making them more frequent and more destructive?
  • To answer this question, we must understand (a) the science behind landslides, (b) their significance as an exogenic geomorphic process, (c) the mechanism of slope failure, and (d) the natural as well as anthropogenic factors responsible for these disasters. 
  • This article explores these concepts step by step to develop a scientific understanding of one of Earth's most destructive geomorphic hazards.



What is a Landslide? – Understanding the Science Behind It



  • The above infographic explains to us the Earth's gravitational acceleration, which apply on every thing present on the Earth. With this understanding, a landslide is the downslope movement of rock, soil, debris, or a combination of these materials under the influence of Earth's gravitational force. It is one of the most common forms of mass movement (mass wasting) and an important exogenic geomorphic process that continuously reshapes the Earth's surface. 
  • Depending on the type of material involved and the mode of movement, landslides may occur suddenly within seconds or develop gradually over months or even years.
  • The science of landslides begins with one of the most fundamental forces of nature—Earth's gravitational force. Every object on Earth is continuously attracted towards the centre of the Earth due to gravity. 
  • Near the Earth's surface, this gravitational attraction produces an acceleration of approximately 9.81 m/s², giving all objects—including rocks, soil, and debris—their weight. This also explains why the same object has different weights on the Earth and the Moon, even though its mass remains unchanged.

  • As illustrated in the following infographic.


  • This highlights the important distinction between mass and weight. Mass is the amount of matter contained in an object and remains constant regardless of its location in the universe. Weight, on the other hand, is the gravitational force acting on that mass. It depends on the strength of the local gravitational field and can therefore change from one celestial body to another.
  • In simple terms, the greater the gravitational force, the greater the weight of an object; the weaker the gravitational force, the lighter the object becomes. Thus, although a rock has the same mass on the Earth and the Moon, it weighs much less on the Moon because the Moon's gravitational pull is significantly weaker.
  • Understanding this concept is essential because Earth's gravity provides the driving force responsible for the downslope movement of rocks, soil, and debris, making it the fundamental force behind landslides.
  • Therefore, the science behind landslides is fundamentally the science of slope stability. A landslide occurs when the natural balance between Earth's gravitational pull and the forces resisting movement is disturbed, allowing gravity to dominate and initiate the downslope movement of earth materials.

The Mechanism of Landslide?

  • Dive into the following infographics-

  • Before understanding landslides, it is essential to understand the forces that continuously shape the Earth's surface. As illustrated in the above infographic, Earth's geomorphic processes are governed by two broad categories of forces—Endogenic Forces and Exogenic Forces.
  • Endogenic forces originate from within the Earth and are primarily responsible for constructing and uplifting the Earth's crust through processes such as mountain building, volcanism, and tectonic movements. These are therefore regarded as "constructive geomorphic forces".
  • In contrast, Exogenic forces operate at or above the Earth's surface under the influence of the atmosphere, hydrosphere, gravity, and solar energy, and their primary function is to wear down, transport, and redistribute Earth's materials, gradually -
  1. reducing relief and
  2. lowering elevated landforms. Collectively, these processes are known as denudation, and the forces responsible are called denudational (exogenic) forces.
  • Among these denudational processes, landslides represent one of the most important forms of mass movement (mass wasting). Their geomorphic role is to transfer weathered rock, soil, and debris from higher elevations to lower elevations, thereby contributing to the long-term reduction and reshaping of the Earth's relief.

  • The science behind landslides begins with one of the most fundamental forces of nature—Earth's gravitational force. We have discussed that every object on Earth is continuously attracted towards the centre of the Earth due to gravity, and near the Earth's surface, this attraction produces a gravitational acceleration of approximately 9.81 m/s², giving rocks, soil, and other earth materials their weight.

  • This continuous gravitational attraction exerts a "downslope pull", known as the "driving force", on mountain slopes and hillsides. Opposing this movement are the "resisting forces", including friction between particles, cohesion, the shear strength of rocks and soils, vegetation roots, and favourable geological structures. Now, the point of order is that as long as these opposing forces remain in balance, the slope exists in a state of "dynamic equilibrium" and remains stable.

  • However, this equilibrium is not permanent. The following infographic illustrates the factors resposible to disturbe this equilibrium.
        
  • Here, the "natural processes" such as-
  1. intense rainfall, 
  2. earthquakes, 
  3. weathering, 
  4. river erosion, and 
  5. volcanic activity, together with "anthropogenic activities" including 
  6. deforestation, 
  7. hill cutting, 
  8. mining, 
  9. road construction, and 
  10. unplanned urbanisation, gradually weaken the slope. 
  • These factors either increase the downslope driving force or reduce the resisting forces, making the slope increasingly unstable.
  • The mechanism of a landslide begins when this weakening continues over time. Progressive deformation leads to the development of a "failure plane" (slip surface) beneath the ground, separating the unstable mass from the stable bedrock. Once the resisting forces become insufficient to counter the downslope pull of gravity, the slope fails. We can better understand this through the following infographics, which describe sequential processes.

  • The detached rock, soil, or debris then begins to move downslope under the continuous influence of Earth's gravity. Depending on the nature of the material, the amount of water present, and the rate of movement, this process may occur as a rock fall, rotational slide, translational slide, debris flow, mudflow, earthflow, or debris avalanche, as shown in the following infographics.
       
Types of Landslides


  • Therefore, landslides are not random natural disasters but the outcome of a "progressive loss of slope stability". They occur when the natural equilibrium between the driving force generated by Earth's gravity and the forces resisting movement is disturbed, allowing gravity to dominate and initiate the downslope movement of earth materials.
Note- 
  • With a clear understanding of the science and mechanism of landslides, the next blog in this series will shift from theory to geography. We will examine-
  1. Exploring the natural and anthropogenic factors responsible for intensive landslide.
  2. The global spatial distribution of landslides and identify the regions most vulnerable to slope failures.
  3. Covering India with 
  • landslide-prone areas, 
  • their causes, 
  • associated threats, and 
  • the strategies for mitigation, risk reduction, and 
  • sustainable mountain development.
 

We may conclude

  • Just as an effective treatment begins with an accurate diagnosis, effective landslide mitigation begins with a scientific understanding of the phenomenon. Unless we first identify the fundamental characteristics of landslides—their nature, the science behind them, and the mechanism through which they occur—we cannot develop effective strategies to reduce their impacts.
  • This article has laid that scientific foundation. While landslides are an inevitable part of Earth's natural geomorphic processes, their consequences become far more devastating when natural systems are disturbed through unsustainable human activities.
  • With this understanding, we are now better prepared to move from understanding to mitigation. In the next blog, we will focus on the prevention, mitigation, and management of landslides.


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