What actually caused the disastrous avalanche in India

The video footage is violent and unsettling.

On Sunday, a torrent of ice, water, mud, and debris surged through a steep river valley in the northern Indian state of Uttarakhand. At least 32 people are dead, with nearly 200 missing.

The tragic event happened in a warming Himalayan region where glaciers are rapidly melting and retreating. Yet glacier experts say the flood likely wasn't triggered by the collapse of or "burst" from (not a thing) a glacier itself, though the heating climate may have played a role in the greater episode. Earth scientists will learn more in the days and weeks ahead, but satellite evidence points to a high-elevation landslide causing an avalanche of rock, glacier ice, and snow.

"The disaster was triggered by a rock-ice-snow avalanche. This is what we know with quite good confidence," explained Mohd Farooq Azam, an assistant professor in the Discipline of Civil Engineering at the Indian Institute of Technology Indore. Azam researches glaciers in the Indian Himalayas.

This avalanche was an extreme event. On a steep ridge at some 5,600 meters (18,370 feet) in elevation, a collapsed mountain slope pulled down a glacier that was perched on the mountain, and the resulting rock-ice-snow mass hit the valley floor at some 3,500 meters. Then the mass all flowed downwards through the valley, said Azam.

In such a rapidly moving avalanche, great friction instantly melts ice and snow, which is where some of the surging water seen in the videos came from, explained Joseph Shea, an assistant professor of environmental geomatics at the University of Northern British Columbia who has spent years visiting and researching Himalayan glaciers.

The full source of the huge amount of water seen in the videos, however, is an ongoing investigation, said Azam, noting Indian scientists are at ground zero assessing the event. The avalanche could have picked up ample amounts of ice from the valley floor.

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In a region with rapidly melting ice, the natural question is how the relentlessly warming globe might have contributed to this chaotic event.

Major avalanches happen naturally, especially, on steep, glaciated slopes that are prone to rock falls. "It is very difficult to ascertain that this particular disaster happens because of climate change only," explained Azam. Mountain landslides that result in big debris flows are normal and well documented. A somewhat similar landslide happened in Nepal in 2012.

But, crucially, climate change has enhanced glacier and snow melting in the Himalayan region, Azam said, adding that "the frequency of mountain hazards has also increased due to climate change."

See Also: Where to see Earth's dying glaciers

This could make a major rockfall or landslide around a glacier more likely. As shrinking glaciers retreat, they can leave unstable slopes open and vulnerable to rockfalls, explained Shea. What's more, a warming climate creates more opportunities for high elevation places to freeze and thaw, freeze and thaw, and so on. This allows water to seep into cracks, freeze, and expand, which then opens up ever-larger cracks. "It eventually releases these big blocks," said Shea. At some point, gravity wins.

Both of these processes could have potentially contributed to the cascading rock-ice-snow avalanche, said Shea.

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The ensuing surge of rock, ice, and water eventually flowed through a hydroelectric river dam, trapping many workers in a tunnel. Rescues are underway, though lives have already been lost.

"It's a tragic event," said Shea.

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