What is a Mass Movement?
Mass Movement, also known as mass wasting, represents the downhill movement of cliff material under the persuasion of gravity. It depicts the bulk movements occurring in soil and rock residue down slopes in response to rapid or steady sinking of the Earth’s surface in a vertical direction or the pull of gravity. Previously, the word mass wasting was used to reflect different processes by which huge masses of crustal substances are moved by gravity from one place to another.
Terms Used for Mass Movement Geography
More recently, the mass movement meaning or the term mass movement has been used as a replacement to include mass wasting processes and the sinking of constrained areas of the Earth’s ground surface. Mass movements on slopes and sinking mass movements are frequently fed by water and the importance of both types is the part each plays in the remodeling of landforms.
What is Mass Wasting?
Mass wasting, which is identical with slope failure” is the non-fulfillment and downslope movement of rock or unconsolidated substances in response to gravity. If you have heard of the term landslide, it is almost similar to mass wasting, but not totally since some people conserve “landslide” for relatively rapid slope failures, while others do not.
Classification of Mass movements
Mass Movements Can Be Divided into Two Major Classes:
Slow movements
Rapid movements
Factors Triggering Mass Wasting
Previously, the word mass wasting indicated a variety of processes by which large masses of crustal substances are moved by gravity from one place to another. Latterly, the word mass movement has been substituted to involve mass wasting processes and the sinking of restrained areas of the Earth’s ground surface. That said, Mass movements on slopes and sinking are often sustained by water and the importance of both types is the part each plays in the alteration of landforms.
Features of Mass Movements
Mass movement is also known as Mass Wasting, bulk movements of rock and soil
Debris down slopes in response to the pull of gravity, or the rapid or steady sinking of the Earth’s surface in a principally vertical direction is influenced with many factors.
It is the movement of masses of bodies of mud, soil, rock, bedrock residue, which commonly occurs along steep-sided hills and mountains due to the gravitational pull.
Gravity imposes its force on all matter, both bedrock and the elements of weathering. Therefore, weathering is not crucial for mass movement though it aids mass movements.
Mass movements that are sliding of massive amounts of rock and soil are observed in landslides, mudslides, and avalanches.
These are very active over weathered slopes rather than over unweathered materials.
The ice, water or air does not carry debris with them from place to place but conversely, the residue may transport with it ice, water, or air.
Mass movements do not appear under erosion though there is a shift of substances from one place to another.
Heave, slide and flow are the three forms of movements.
Causes Preceding Mass Movements
There are many activating factors preceding mass movements which are as follows:
Elimination of aid from below to substances above through natural or artificial means.
Removal of substances or load from over the original slope surfaces.
Haphazard elimination of natural vegetation.
An upheaval in height of slopes and gradient.
Overfilling through the addition of substances naturally or by artificial filling
Overburdening because of heavy rainfall, lubrication, saturation of slope materials.
Epodes of earthquakes and explosions etc.
Tremendous natural seepage.
Extreme drawdown of water from reservoirs, rivers, streams, lakes leading to a gradual outflow of water from beneath the slopes or river banks.
FAQs on Mass Movement
1. What is mass movement in geography?
In geography, mass movement, also known as mass wasting, refers to the downslope movement of rock, soil, and regolith (weathered material) under the direct influence of gravity. Unlike other erosion processes, it does not require a transporting medium like water, wind, or ice to move the material, although water can play a significant role in triggering it.
2. On what basis is mass movement classified?
Mass movement is primarily classified based on three main criteria which help in understanding its nature and impact:
Type of Material: Whether the material involved is unconsolidated debris (like earth or mud) or consolidated bedrock (rock).
Type of Motion: The way the material moves downslope, which can be a fall, slide, or flow.
Rate of Movement: The speed at which the movement occurs, ranging from extremely slow (e.g., creep) to very rapid (e.g., rockfall or avalanche).
3. What are the main types of mass movement?
Mass movements are broadly categorised by their motion. The main types include:
Slides: This is when a block of material moves as a coherent unit along a distinct surface or plane. A common example is a landslide.
Flows: In a flow, the material moves like a viscous fluid. Examples include mudflows (fine material), debris flows (coarse material), and creep (very slow soil flow).
Falls: This involves the free-fall of detached rock or debris from a steep cliff or slope. A rockfall is a classic example.
Slumps: A type of slide where the material moves along a curved, rotational surface, often creating a crescent-shaped scarp at the top.
4. What are the primary causes that trigger mass movement?
Mass movements are triggered when the force of gravity overcomes the slope's internal resistance. Key causes include:
Water Saturation: Heavy rainfall or snowmelt adds weight and reduces friction between particles, making slopes unstable.
Steep Slopes: Over-steepened slopes, whether natural or due to human activity like road cuts, are inherently less stable.
Seismic Activity: Earthquakes and volcanic eruptions can shake the ground, dislodging large amounts of material.
Removal of Vegetation: Deforestation or wildfires remove plant roots that bind soil together, increasing erosion and slope failure risk.
Human Activities: Construction, mining, and improper land use can alter slopes and drainage patterns, leading to instability.
5. How does water play a crucial role in causing mass movement?
Water is one of the most important factors in triggering mass movement. It destabilises slopes in several ways. Firstly, it adds significant weight (load) to the soil or rock mass. Secondly, it fills the pore spaces between particles, reducing the internal friction that holds the material together. This lubrication effect allows particles to move past each other more easily. In some cases, excessive water can create high pore water pressure, which pushes particles apart and effectively neutralises the slope's shear strength, leading to sudden failure like a mudflow or slump.
6. What is the key difference between a landslide and a mudflow?
The key difference lies in the material's consistency and type of motion. A landslide typically involves the movement of a relatively coherent block of rock, debris, or earth along a well-defined slip surface. The material moves more or less as a single unit. In contrast, a mudflow behaves like a liquid, consisting of a high concentration of fine-grained particles like silt and clay mixed with water. It flows rapidly down channels or valleys, behaving as a viscous fluid rather than a solid block.
7. Can human activities increase the risk of mass movement? Provide examples.
Yes, human activities can significantly increase the frequency and magnitude of mass movements. Key examples include:
Deforestation and Land Clearing: Removing trees and vegetation for agriculture or urbanisation destroys the root systems that bind soil, making slopes vulnerable.
Construction: Cutting into the base of a slope for roads or buildings (undercutting) removes its support, while piling material at the top adds weight, both promoting instability.
Mining and Quarrying: These activities can fracture rock and create artificially steep and unstable slopes.
Improper Water Management: Leaking water pipes, artificial irrigation, or diverting drainage can saturate slopes that were previously stable.
8. Why is soil creep considered a significant geomorphic process despite being extremely slow?
Although soil creep is the slowest form of mass movement and often imperceptible on a day-to-day basis, it is a significant geomorphic process because of its widespread and continuous nature. Over many years, its cumulative effect can transport vast amounts of surface material downslope. Evidence of creep is visible in tilted trees, fences, and utility poles, and the buckling of retaining walls. This slow, steady movement is a major factor in shaping hillslopes and is a fundamental process in landscape evolution.
9. What is the difference between slow and rapid mass movements? Give an example for each.
The primary difference is the rate of motion. Slow mass movements occur at very low velocities, often millimetres or centimetres per year, making them difficult to observe directly. Their effects are only visible over long periods. A prime example is soil creep, the gradual downslope movement of soil and regolith. Rapid mass movements are sudden, fast, and often catastrophic, occurring at speeds of metres per second. They pose an immediate threat to life and property. A clear example is a rockfall, where rocks detach from a cliff and fall rapidly to the base.
