To backcountry skiers and snowmobilers, slab avalanches are known as the ones that kill.

They account for nearly all the avalanche deaths in North America.

“The reason slab avalanches are so deadly — and the reason we’re so worried about them — is they can break over a large area instantaneously,” said Mark Staples, an avalanche specialist with the Gallatin National Forest Avalanche Center in Bozeman.

“They break instantly, like a pane of glass, and often there’s no way to escape. People get caught in the middle, and it’s breaking up all around them.”

Slab avalanches can accelerate faster than anyone can ski or snowmobile away from them.

“It can be going 60 mph in just a couple of seconds and can reach speeds of over 100 mph by the time it reaches the bottom of its path,” he said.

In the Rockies, the recipe for a slab avalanche starts with a little bit of snow in late fall or early winter and then really cold weather.

This winter, the cold weather kicked in on Dec. 5, after early season storms in October and November.

“That cold weather turns the snow into a weak, sugary snow, we call faceted snow. That’s the foundation on which we’re building our snowpack,” Staples said.

The poor foundation forms a weak layer. As more snow accumulates in December, and the skiing or snowmobiling gets better, a cohesive slab forms on top of the weak layer.

One avalanche expert describes the weak layer, the early season snow, as being like a pile of potato chips. The slab layer is like a brick sitting on top of those potato chips.

Others describe seeing a slab avalanche as like watching a magazine slide off a tipped coffee table.

The third “recipe” ingredient is a steep slope of 30 to 45 degrees. The final ingredient is the trigger.

“Usually that’s us,” Staples said. “That weak layer is stressed, and we hop on that weak snow and we tip the balance.”

More than 90 percent of the time, the victim, or someone in the victim’s group, triggers that avalanche, Staples said.

Often the trigger point occurs below the fracture line, making escape difficult. Slabs can teeter on the verge of catastrophe for days or even months before stress causes a fracture.

Southwestern Montana typically has a few avalanche deaths every year, Staples said.

This winter, the first death came on Dec. 10, 2009, when a world-class ice climber, Guy Lacelle, died in an avalanche in Hyalite Canyon. Billings snowmobiler Scott Hall Herren became the second fatality, when an avalanche struck near Cooke City.

During the winter of 2008-2009, all three avalanche fatalities in Montana’s backcountry occurred on one deadly day, Jan. 17. 2009.

On that day, an avalanche at Crown Butte, near Cooke City, broke off a slab 15 feet deep that ran more than 1,000 feet downhill and killed one snowmobiler.

Another snowmobiler, highmarking south of Mount Jefferson, about 30 miles west of West Yellowstone, was killed by an avalanche estimated at 50 feet wide. That avalanche also traveled about 1,000 feet.

The third snowmobiler, who died at Black Butte, in the Gravelly Range, was killed by an avalanche that slid about 75 feet downhill.

It’s not just big avalanches that kill, Staples said. Most fatal slides are small to medium size, with more than half of fatalities resulting from slides less than 20 feet wide and 1,000 vertical feet.

Staples advised anyone who recreates in the backcountry in winter to take special avalanche courses. The basic rules include:

• Go one at a time, so only one person is exposed, while others watch from a safe location.

• Carry and know how to use rescue gear.

• Look for recent avalanche activity. It’s the No. 1 sign that snow is unstable.

Last winter 26 avalanche fatalities occurred nationwide, including the three in Montana and five in Wyoming, which had highest death toll among the 50 states.

During some winters, the national avalanche death toll is pretty evenly split between snowmobilers and nonmotorized backcountry users.

But, when the Colorado Avalanche Information Center compiled statistics solely on snowmobile avalanche fatalities between 1997 and 2007, an interesting statistic showed up. During that decade, there were more snowmobile fatalities from avalanches in the state of Montana than any other state, including Alaska.

 

Doug Chabot, director of the Gallatin National Forest Avalanche Center, describes what happened when a world-class ice climber died in an avalanche on Dec. 10, 2009 in Hyalite Canyon, outside Bozeman.  The climber, Guy Lacelle, died in an avalanche triggered by other climbers during the first day of the Bozeman Ice Festival at Hyalite, which promotes ice climbing and safe climbing techniques.  

 

Doug Chabot testing the crown of an avalanche at Crown Butte, near Cooke City.

Doug Chabot tours an avalanche site, triggered just half an hour before, where a snowmobiler got caught up in it, but made it out alive.