A massive landslide has covered about 5 1/2 miles of Johns Hopkins Glacier in Glacier Bay National Park in Alaska, park officials say.

The National Parks Traveler Web site said a huge chunk of the slope of 11,750-foot high Lituya Mountain sloughed off June 11 and cascaded into the valley below. It slid down with such force it registered as a small earthquake.

"This thing is huge. It's 9 kilometers long, so 5.5 miles long," Marten Geertsema, a research geomorphologist for the provincial Forest Service in British Columbia, told the Traveler Tuesday. "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 cause of the slide, which was first photographed July 2 by an air taxi pilot, Drake Olson, hasn't been determined.

Geertsema said a breakdown in the permafrost on the mountain could be to blame.

"With permafrost degradation there's a whole complex suite of fractures that develop," he said. "Some of them dilate, so they open and close, but some of them are permanent. 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."

He called it biggest slide he's seen in North America, though he said one in Russia covered 20 miles.

Geertsema said it's not known whether global warming played a role in the landslide.