5.8. Mass Movements

These movements transfer the mass of rock debris down the slopes under the direct influence of gravity. That means, air, water or ice do not carry debris with them from place to place but on the other hand the debris may carry with it air, water or ice.

Mass movements refer to the downward transfer of rock debris under the force of gravity. Unlike other processes where agents like water or wind move materials, in mass movements, gravity is the primary driving force. The materials only move downwards, and they may carry air, water, or ice as they slide down, not the other way around.

Imagine a pile of sand on a sloped surface. When you push the sand, it does not roll uphill. Instead, it slides down, illustrating how gravity acts on the material directly. Similarly, rocks and soil can move down a slope when gravity overcomes their resistance.

The movements of mass may range from slow to rapid, affecting shallow to deep columns of materials and include creep, flow, slide, and fall.

Mass movements can vary significantly in their speed and the amount of material they affect. Creep is a very slow movement that may go unnoticed, while flow and slide refer to more rapid varieties of mass movement. For example, a 'flow' might involve mud that moves quickly down a slope, while a 'slide' occurs when a block of earth suddenly drops down a hill. Meanwhile, a 'fall' involves rocks or debris that fall freely from a height.

Consider a wet sponge on a slope. If you slowly squeeze it, the water creeps slowly down the slope (like creep). If you tilt it suddenly, water rushes down fast (like flow), while a sudden drop of the sponge off the edge of a table (like fall) leads to a quick fall to the ground.

Gravity exerts its force on all matter, both bedrock and the products of weathering. Mass movements are aided by gravity and no geomorphic agent like running water, glaciers, wind, waves and currents participate in the process of mass movements.

The fundamental factor that triggers mass movements is gravity, which pulls materials downwards. Unlike erosion, where agents like water and wind are involved, mass movements are primarily driven by the force of gravity acting on the material itself. For mass movements to occur, the weight of the material must exceed its resistance to sliding down slope.

Think of standing a stack of books vertically on a shelf. If you add another book on top, at some point, the bottom books will no longer support the weight, and the stack will collapse (just like materials collapsing down a slope). Gravity pulls everything down!

Weak unconsolidated materials, thinly bedded rocks, faults, steeply dipping beds, vertical cliffs or steep slopes, abundant precipitation and torrential rains and scarcity of vegetation etc., favour mass movements.

Certain conditions make mass movements more likely to occur. For instance, steep slopes have less resistance to sliding. Unconsolidated or loosely packed materials (like sand or loose soil) are more susceptible as they don't hold together well. Heavy rainfall can saturate the ground, weighing down the materials and reducing their cohesion, increasing the risk of landslides.

Picture a pile of gravel on a steep hill after a heavy rainstorm. The water seeps into the gravel, making it heavier and causing it to slide down. Similarly, gardens with loose soil on a slope may experience soil slips during heavy downpours because the roots that normally keep the soil in place are washed away.

Several activating causes precede mass movements. They include (i) removal of support from below to materials above; (ii) increase in gradient and height of slopes; (iii) overloading through addition of materials; (iv) heavy rainfall, saturation and lubrication of slope materials; (v) occurrence of earthquakes.

Various factors can trigger mass movements; for example, if the soil at the base of a slope is eroded or removed, the top material may lose its support and fall. Furthermore, increasing the slope's steepness can also lead to instability. Overloading the slope with additional weight or intense rainfall can saturate the soil and make it slippery, leading to a higher chance of collapse.

Think of a teeter-totter designed for balanced play. If one child moves too far to one side or if too many children sit on one side, the other side can tip over. Similarly, when a slope is overloaded or becomes too steep, it can tip into a mass movement.

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