A massive landslide, one of the biggest ever seen in North America and which registered on seismographs in Canada and the United States, has covered a more than 5-mile run of the Johns Hopkins Glacier in Glacier Bay National Park and Preserve like chocolate frosting on cake.

Coursing down an unnamed valley that descends beneath 11,750-foot Lituya Mountain in Alaska's Fairweather Mountains, the muddy flow sloshed back and forth through the twists and turns of the valley, rattling and shaking the surrounding ridges and mountain flanks, which contributed their own snowy avalanches along the route.

While the exact volume of the slide has not been calculated, when it broke off the upper reaches of Lituya Mountain it registered as a relatively small, but noticeable, earthquake. But it wasn't the result of an earthquake.

“This thing is huge. It’s 9 kilometers long, so 5.5 miles long," Dr. Marten Geertsema, a research geomorphologist for the provincial Forest Service in British Columbia, said Tuesday from his Prince George office. "On the Canadian side it triggered a 3.7-magnitude earthquake. The (U.S. Geological Survey) recorded it at 3.4. That’s quite large for a seismic signal (from a landslide).

"... If someone was trekking up this glacier when it happened, they would have been toast."

The landslide occurred shortly before 2:30 p.m. local time on June 11. Its immensity wasn't fully comprehended until Drake Olson, an air taxi pilot, few over it July 2 and returned home with some incredible photographs.

"It spilled on top of the glacier and then it just rolled along the glacier," Lewis Sharman, a Park Service ecologist at Glacier Bay, said Tuesday. "The travel angle was calculated at about 13.7 degrees, which is not very steep, yet this stuff rolled down the glacier more than five miles.”

The slide is not readily visible from the ground or from boats on Glacier Bay, as it's estimated to be 12-15 miles "up glacier" from the bay and deep within the Fairweather Range. Since the glacier moves at a pace of 10-15 feet per day, park officials add, it could be years before this debris reaches the snout of the glacier.

So far a cause for the slide hasn't been determined. Whatever the cause, a massive slice of rock and ice peeled off Lituya Mountain.

“I haven’t had a good look at the mountain, but it’s a combination of ice and rock," said Dr. Geertsema. "And a big piece of that mountain broke off. But there was a large chunk of glacier sitting up there as well. That’s fairly common in that type of area."

The geomorphologist said the slide might have been caused by a breakdown in the permafrost on the mountain. "With permafrost degradation there’s a whole complex suite of fractures that develop. Some of them dialate, so they open and close, but some of them are permanent," he said. "And over time it progressively weakens, the rock mass. It could have been just ready to go. It’s hard to know what the factors are.”

Dr. Geertsema said he's aware of a similar, but much larger, slide that occurred in Russia and which ran about 20 miles.

"But this is the biggest one I’ve seen in North America," he added. "There might have been ones that are larger, I just don’t know about them.”

Is the landslide evidence of global warming? Is it a rare, but naturally occurring phenomena?

“I think it’s unusual in terms of its size. But they certainly happen," said Dr. Geertsema. "Right now I’m plotting all of these things. I’ve been doing some work in Switzerland and also here, looking at mountain permafrost degradation. That may or may not play a role in these things."

Another fascinating aspect of the slide is that it pushed a powerful "air blast" down the valley in front of it. In places the blast is thought to have tossed rocks and other debris as much as 1,600 feet above the slide's surface. Though the speed of the air blast hasn't been calculated, the Canadian geomorphologist said some have been clocked as fast as 200 mph.

“There was this big dust cloud, this air blast that traveled above the flowing rock mass. According to the pilot who sent me the picures, Drake Olson, there’s a lot of fist-sized stones that were ejected above the dust cloud as well," said Dr. Geertsema.

Back at Glacier Bay, ecologist Sharman said geomorphologists anticipate such landslides "becoming a little more frequent as we have warmer than usual temperatures, we have more melting events, and glaciers and surface sediments become better lubricated on surface terrain at elevation."

“Geomorphologists are not surprised that they’re starting to see more of these events. In any case, they’re rare of this size,” he said.

Traveler footnote: This landslide shares a name, but nothing else, with a 1958 event on the park's western coast. "On July 9, 1958, a magnitude M 7.9 earthquake on the Fairweather Fault triggered a rock avalanche at the head of Lituya Bay," according to USGS reports. "The landslide generated a wave that ran up 524 m on the opposite shore and sent a 30-m high wave through Lituya Bay sinking two of three fishing boats and killing two persons."