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Context: The death count due to a massive landslide at a Territorial Army Camp in Manipur's Noney district has increased to 20, officials said.

About: A landslide generally refers to the downhill movement of rock, soil, or debris. The term landslide can also refer to the deposit that is created by a landslide event.

The Role Of Gravity: Landslides nearly always move down a slope. This is because the force of gravity—which acts to move material downhill—is usually counteracted by two things: (1) the internal strength of the material, and (2) the friction of the material on the slope. A landslide occurs because the force of gravity becomes greater than either friction or the internal strength of the rock, soil, or sediment.

The Role Of Water: The addition of water to the material on a slope can make landslides more common. This is because water adds significant weight to the slope as it seeps into the ground, becoming groundwater, and adding to the gravitational force. Water also lowers the strength of the material which can make it less able to withstand the force of gravity. Water also reduces friction, making it easier to move material downhill. These processes help to explain why landslides are much more common during the rainy season, and especially common during or right after large storms.

The Role Of friction: The amount of friction between a deposit of rock or soil and the slope that it rests on plays a large role in when landslides happen. Imagine trying to slide a large rock along a flat surface—it's very difficult because of the friction between the rock and the surface. Pushing the rock is easier if the surface slopes downhill or is slippery. The same is true for landslides—steeper slopes have less friction, making landslides more common. Any change to the Earth's surface that increases the slope (for example, river incision or the removal of material at the base of a slope by humans) or that reduces the friction of a slope (such as the addition of water) can increase the likelihood of a landslide.

Types:

1. Shallow landslides are rooted in the soil layer and often form slumps along roadways or fast-moving debris flows down valleys. These types of landslides are often called 'mudslides' by the news me- dia. Shallow landslides also occur as flows, slides, or rockfalls and topples.
2. Deep-seated landslides are rooted in bedrock, are often slow moving, and can cover large areas and devastate infrastructure and housing developments. Deep-seated landslides usually occur as translational slides, rotational slides, or large block slides. Deep- seated landslides are typically much larger than shallow landslides.

Background:

• The entire Himalayan tract, hills/mountains in sub-Himalayan terrains of North-east India, Western Ghats, the Nilgiris in Tamil Nadu Konkan areas are landslide-prone.
• The Geological Survey of India (GSI) has done a national landslide susceptibility mapping for 85% of the entire 4,20,000 square km landslide-prone area in the country. The areas have been divided into different zones according to the propensity of the disaster.

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