Why the Wayanad landslide was so destructive: a new study points below the surface

The devastating 2024 Wayanad landslide cannot be blamed on extreme rainfall alone. A new study published in the peer-reviewed journal Landslides finds that geological, geomorphological and structural factors also shaped the disaster. Intense monsoon rain triggered the collapse, but the underlying rock determined where the landslide began, how far it travelled, and why it became one of the most destructive events ever recorded in the Western Ghats.

The disaster struck on the night of July 30, 2024. Nearly 573 millimetres of rain fell within 48 hours, causing a massive slope failure in the upper catchment of the Punnapuzha river. What began as a slope collapse turned into a high-speed debris flow that ran nearly eight kilometres and descended about 768 metres, sweeping through Punchirimattam, Mundakkai and Chooralmala while carrying enormous quantities of rock, soil and vegetation.

The study was carried out by researchers from the University of Kerala, the Indian Institute of Science Education and Research Mohali, and Savitribai Phule Pune University. They conducted field investigations across the affected valley in April 2025, mapping the geology from Chooralmala to Punchirimattam and analysing rock samples in the laboratory. Because the landslide crown was still unsafe to reach on foot, they used drone-mounted LiDAR and high-resolution aerial imagery to study the upper failure zone.

What they found beneath the surface explains a great deal. The hills are underlain by ancient crystalline rocks that have been deformed repeatedly over hundreds of millions of years. That history left the rock threaded with natural planes of weakness in the form of shear zones, fractures and foliations. Rainwater seeping into those fractures over long periods caused extensive chemical weathering, turning large volumes of hard rock into soft, deeply weathered material hidden below the surface.

The landslide, the study suggests, began inside one such highly weathered shear zone near the crown, at a point where a first-order stream crossed the weakened rock. During the extreme rainfall, water rapidly infiltrated the interconnected fractures and raised the water pressure inside the slope. Once that pressure exceeded the strength of the rock mass, a large block detached and the landslide was under way.

The geology also explains the scale of the destruction downstream. As the debris rushed downhill, narrow sections of the valley underlain by stronger rocks such as metagabbro and granite acted as natural barriers. Debris piled up behind these constrictions and formed short-lived natural dams. When those dams gave way, they released powerful surges of water, mud and boulders, intensifying the damage further down the valley.

The researchers who led the work, Y. Anilkumar and K.S. Sajinkumar of the Department of Geology at the University of Kerala, described the finding as the interplay of climate and geology. Extreme rainfall was the immediate trigger, but the pre-existing geological framework determined where the slope failed, how the debris moved, and why the event was so exceptionally destructive.