EXPLAINER: How glaciers can burst and send floods downstream

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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WHY DID THIS GLACIER BURST?

It's not yet known what caused part of the Nanda Devi glacier to snap off Sunday morning, sending floodwater surging downstream toward power plants and villages in India’s northern state of Uttarakhand.

Seismic activity and a buildup of water pressure can cause glaciers to burst, but one particular concern is climate change. High temperatures coupled with less snowfall can accelerate melting, which causes water to rise to potentially...