The Khumbu Icefall is a chaotic jumble of house-sized ice boulders set with a hair trigger.
The world's 14 "eight-thousanders" -- mountains taller than 8,000 meters (26,247 feet) -- are all located in Asia. On one hand, they really beautify a horizon, but on the other they present a fierce, at times fatal, challenge to mountain climbers. Beauty can, indeed, be deadly. Here's the "baby" of the bunch, Shishapangma in Tibet, peaking at 8,027 meters (26,335 feet).
This is Gasherbrum II (we'll have another Gasherbrum coming up shortly), on the border of Pakistan and China. It's 8,035 meters up in the sky (26,361 feet) and is sometimes known as K4. These mountaineers are near the summit.
Ed Darack/Science Faction/Corbis
On the left side of this picture we see the sheer mass of Broad Peak, the 12th highest mountain on the planet at 8,047 meters or 26,394 feet above sea level.
As promised, here's another Gasherbrum: Gasherbrum I. (Gasherbrum, translated from the Tibetan language Balti, means "beautiful mountain.") It also goes by the name of K5, lives along the China-Pakistan border and is 8,080 meters (26,444 ft) high.
The Himalayan mountain range Annapurna, in Nepal, is seen here from Pkhara, about 124 miles (200 kilometers) west of Kathmandu. Annapurna is considered one of the most dangerous for climbers; first crested in 1950, it has since been climbed by more than 100 people but taken 53 lives along the way.
This somewhat unsettling photo was taken in 1931 by mountaineers at a base camp on Pakistan's Nanga Parbat, the ninth-tallest eight-thousander at 8,126 meters (26,660 feet). The area captured in the picture is known as the Nanga Wall.
Association Chantal Mauduit Namaste/Corbis/Sygma
Eighth-tallest of the eight-thousanders is Nepal's Manaslu, at 8,163 meters (26,781 feet).
The Dhaulagiri mountain range in the Himalayas sports a rather volcanic look in this picture, with the sun brushing its top. But Nepal's 8,167-meter (26,795-foot) monster is of course quite chilly on top. Dhaulagiri's south face is considered by mountaineers to be a next-to-impossible climb, and no one has ever topped the mountain from that side.
Clouds hover over snow-covered Cho Oyu mountain in Tibet. The sixth-tallest mountain stands 8,201 meters tall (26,906 feet), and the "Mountain Goddess" (in Tibetan translation) is considered one of less-challenging climbs among the eight-thousanders (if you don't consider climbing ANY mountain a challenge, that is!).
Next in the eight-thousander club is Makalu, the fifth-highest mountain in the world, looming on the border between Nepal and China. It's 8,463 meters (27,766 feet) tall and is another tough climb, with 22 deaths tallied against its 206 successful climbs.
Pal Teravagimov Photography
Standing 8,516 meters tall (27,940 feet), Lhotse, fourth-tallest, rests on the border of Nepal and Tibet. It was first climbed in 1956.
Kangchenjunga, in Nepal, is the world's third-tallest mountain, edging Lhotse by just 71 meters, standing 8,587 meters tall (28,169 feet). It's a prominent mark on the horizon in Darjeeling, the tea-growing region.
K2, the second-tallest mountain on the planet, is 8,611 meters up in the clouds (28,251 feet) along the China-Pakistan border. Climbers know it for its incredibly difficult ascent routes; in 2008, an ice fall on the treacherous slopes took the lives of 11 climbers.
And now we reach the Big Daddy in the worldwide mountains club. That, of course, would be Mount Everest. Its name alone is synonymous with challenging feats, as climbing it continues to this day to be a dicey endeavor, though it draws people year after year to attempt the ascent. And what a climb: Mount Everest stands 8,848 meters tall (29,029 feet). It was famously crested for the first time in 1953 by New Zealand climber Edmund Hillary and Nepalese Sherpa mountaineer Tenzing Norgay.
The Himalayan ice avalanche that killed at least 12 Nepalese guides earlier today happened in a place where both the sun and the Earth conspire to increase risk and where many other lives have been lost in past years.
The Khumbu Icefall, where the avalanche reportedly occurred, is basically a glacier that's in a constant state of flux as it grinds down one of Mount Everest's flanks. On the surface, it consists of giant blocks of ice that are very unstable and shift regularly -- sometimes catastrophically. When that happens, the river of ice becomes more like a stampede of ice boulders.
The difference is that ice has the ability to self-lubricate when it falls or flows, Turnbull said. This drops the friction inside the flow and makes it move more easily. The self-lubrication comes from the collision of ice particles, which causes small amounts of short-lived melt water on their surfaces -- the same way ice skate blades create a tiny area of pressurized melt water that makes skating possible.
“When an icefall falls to pieces, it's with big chunks of ice,” said the University of Nottingham's Barbara Turnbull, a fluid dymanicist with a special research interest in ice and snow. “So it's more like a rockfall.”
But even without that lubrication, the house-sized blocks of ice in the Khumbu Icefall are primed to fall because the glacier itself is buckling and turning over steep, uneven ground. This creates constant stress in the ice, which means avalanches have a hair trigger.
Primed as it is to fail, there are at least a couple of things that set an avalanche in motion in an icefall, Turnbull explained. One trigger, also seen in snow avalanches, is people. As small a thing as a person tramping on the surface of such an unstable area can be enough to compress and shift some layers of snow or ice, causing a small amount of ice to melt, and the avalanche begins, Turnbull said.
Another very well known trigger is the sun. When it rises in the morning and shines on the glacier, the sunlight starts melting some of the ice. That meltwater percolates down and weakens the glacier, making it even more unstable. This is also a common trigger for snow avalanches, Turnbull said.
This danger of the warm sun is why Everest climbers always try to get through the Khumbu Icefall before sunrise.
The Khumbu Icefall is a chaotic jumble of house-sized ice boulders set with a hair trigger.Corbis
Other common triggers for snow and ice avalanches are rainfall, earthquakes and even animals. These are unlikely to have been triggers this morning at Khumbu Icefall.
A factor that may be playing a role is climate, which as changed a lot since the first Everest expeditions almost a century ago. Warmer temperatures and changes in snowfall just add another layer to the already complex picture of the Khumbu Icefall.
"Global warming isn't helping," Turnbull said.
Worldwide, most mountain glaciers are receding due to human-caused climate change, according to climate scientists and glaciologists. Those in the Himalayas are no exception, although there are regional variations there, according to Ann Rowan of the British Geological Survey.
"We see acceleration of mass loss since the 1990s in the eastern Himalaya glaciers," said Rowan.
That's the part of the Himalaya that includes Everest. It's also the part that's most affected by the Indian monsoons, she said.
So with all of this instability on so many levels, why do people still traverse the deadly Khumbu Icefall? It's simply matter of it being the least dangerous of some very dangerous options, Turnbull said. It's the lesser evil, but evil nonetheless.