The correct option is 1 and 3 only.
Explanation
Debris avalanches are rapid mass movements involving the sliding of soil, rock, and organic material down a slope under the influence of gravity. These events are classified as exogenic geomorphic processes and are influenced by lithology, tectonic stability, and climatic factors.
Statement-wise Analysis:
- Statement 1 is Correct: The Himalayas are tectonically active young fold mountains composed largely of sedimentary rocks. These rocks are often unconsolidated, fractured, and structurally unstable. Combined with steep slopes, these factors make the region highly prone to frequent debris avalanches and landslides.
- Statement 2 is Incorrect: Debris avalanches are not absent in the Nilgiris. Although the Nilgiris are composed of harder rocks (such as granite and gneiss) and are tectonically more stable than the Himalayas, they possess a thick mantle of weathered material due to the tropical climate. Heavy rainfall and steep slopes frequently trigger debris avalanches in this weathered layer.
- Statement 3 is Correct: Heavy rainfall is a critical trigger for mass movements. Water infiltration increases the pore water pressure within the slope material, reducing the friction (shear strength) between particles. Additionally, the weight of the saturated soil increases, promoting slope failure.
Key Takeaway:
Landslides in the Himalayas are primarily driven by tectonic instability and sedimentary lithology, whereas in the Western Ghats and Nilgiris, they are driven by heavy rainfall acting on steep slopes with thick weathered soil covers.