EXPLAINER: How glaciers can burst and send floods downstream

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This frame grab from video provided by KK Productions shows a massive flood of water, mud and debris flowing at Chamoli District after a portion of Nanda Devi glacier broke off in Tapovan area of the northern state of Uttarakhand, India, Sunday, Feb.7, 2021. (KK Productions via AP)

The floods that slammed into two hydroelectric plants and damaged villages in northern India were set off by a break on a Himalayan glacier upstream. Here's a look at how glaciers and glacial lakes form and why they may sometimes break:

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HOW GLACIERS AND GLACIAL LAKES FORM

Glaciers are found on every continent except Australia and some are hundreds of thousands of years old. A large cluster of glaciers are in the Himalayas, which are part of India’s long northern border. Sunday’s disaster occurred in the western part of the Himalayas.

Glaciers are made of layers of compressed snow that move or “flow” due to gravity and the softness of ice relative to rock. A glacier's “tongue” can extend hundreds of kilometers (miles) from its high-altitude origins, and the end, or “snout," can advance or retreat based on snow accumulating or melting.

“Ice may flow down mountain valleys, fan out across plains, or in some locations, spread out onto the sea,” according to the National Snow and Ice Data Center.

Proglacial lakes, formed after glaciers retreat, are often bound by sediment and boulder formations. Additional water or pressure, or structural weakness, can cause both natural and manmade dams to burst, sending a mass of floodwater surging down the rivers and streams fed by the glacier.

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