One “glacial outburst” of churning meltwater and boulders damaged the Westside Road on Thursday and terrifying two hikers, who at first mistook the sound for that of a train.

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This year’s dry, hot conditions are likely to blame for a series of “glacial outbursts” on Mount Rainier that sent blasts of muddy water, rock and boulders barreling down the volcano’s western flank beginning Thursday morning, says the geologist for the national park.

The powerful surges gouged out channels, swept away trees and shook the ground hard enough to register on seismometers, said Scott Beason.

“The sound of boulders rolling and smashing — it just got louder and louder and you could hear trees snapping,” said Beason, who was on the park’s Westside Road when one of the flows raced through the Tahoma Creek drainage.

No one was hurt. Park staff surveyed the area by helicopter and were able to account for all hikers in the area.

But water and debris flooded over portions of the Westside Road, which will be closed at least through the weekend.

Eighteen-year-old Zachary Jones, of Snoqualmie, was hiking up the road about noon Thursday when he and a companion heard a noise they at first mistook for a train. When the road emerged from the woods and the creek came into view, they were stunned to see a muddy flow churning in their direction.

“The rumble was getting louder and trees were falling down and it looked like a big pile of rubble was raging down the dry creek bed,” said Jones, who captured the event on video. “We saw huge boulders, half the size of a Volkswagen bug, just raging down and falling over each other.”

The pair had no idea what was happening.

“I thought it might be an earthquake,” Jones said. “It was pretty terrifying.”

Then a park employee came running down the road, yelling for them to evacuate.

The glacial outbursts and debris flows originated at an elevation of 6,800 feet on the South Tahoma Glacier. They are the first such outbursts in the park in nearly a decade, Beason said. But he wasn’t at all surprised.

“Based on the summer we’ve had, and based on the recession of the glaciers, I was telling (colleagues) that this is the year when we’re going to have a debris flow.”

Like all of Rainier’s glaciers, South Tahoma has been melting back rapidly.

Sections of the glacier’s “toe” are now stagnant — cut off from the upper reaches of the ice. Apparently one of those sections, measuring about a half-acre, broke away at about 9:40 a.m. Thursday, unleashing trapped meltwater.

A series of more than 30 glacial outbursts from the South Tahoma Glacier between the 1980s and early 1990s carved out the deep channel that hikers cross via a suspension bridge over Tahoma Creek that’s part of the Wonderland Trail.

It’s possible another cycle has started, Beason said. “This might be a kind of recurring thing now,” he said.