Fury Lurks Beneath Mt. Shasta’s Calm Exterior

Take it from the U.S. Geological Survey, stately Mt. Shasta someday will turn mean.

Folks living near the 14,152-foot peak beside Interstate 5 in Northern California may feel the ground start to shake. They may see a menacing glow at the summit of Shasta or atop one of the satellite volcanoes near it.

Hot lava and volcanic ash will begin melting the snow and the ice locked in Shasta’s five glaciers. Flash floods and mud slides will pour down the slopes. People in towns like Weed, Mt. Shasta and McCloud will scurry for safety.

The catch is that nobody knows when.

“The past record suggests that Mt. Shasta will erupt again, conceivably in the near future. There is no known way to predict when the next eruption will occur,” said USGS researcher C. Dan Miller in a report written in 1980.

“And that’s the way it still is,” says James C. Blodgett, a hydrologist in the USGS’ Sacramento office.

Since the disastrous eruption of Mt. St. Helens in Washington in May, 1980, USGS scientists have been taking a close look at Shasta and four other dormant volcanoes in the Cascade Range of the Pacific Northwest.

The Mt. St. Helens disaster killed 57 people and flattened 200 square miles of timberland. The eruption blew off the upper 1,300 feet of the mountain. Economic damage was estimated at $3 billion.

Besides Shasta, the mountains in the USGS study project are Mt. Rainier and Mt. Baker in Washington, and Three Sisters and Mt. Hood in Oregon.

Though short-term eruption forecasts remain beyond the reach of scientists, they believe Shasta has erupted about once every 600 years during the last 4,500 years. The last one could have occurred about 200 years ago, though nobody is sure.

But in the course of the recent studies, geologists have learned things about Shasta that weren’t known before.

The five peaks were chosen partly because of the large quantities of frozen water stored on their slopes, which pose the dangers of floods and mud flows in eruptions.

In that respect, Mt. Rainier dwarfs all the others. In winter, Rainier’s flanks are covered by an estimated 156 billion cubic feet of snow and ice. Reduced to water, it would equal the average flow of the Columbia River for 24 days.

Shasta’s snow and ice pack is only 4.7 billion cubic feet, but it still stores around 100,000 acre-feet of water, more than the amount consumed yearly by the city of San Francisco.

Shasta’s neighbor, 10,457-foot Mt. Lassen, 70 miles to the southeast, gets less attention despite its eruption in 1915. There was no loss of human life, though the Lassen outburst destroyed some farms and killed livestock. The smaller amount of water in Lassen’s winter snowpack makes it less threatening.

To most people Shasta looks stable, but to the trained eyes of Blodgett and his USGS co-workers, it’s a mountain on the move.

Under the snow and ice, Shasta’s sides are covered with the loose debris from past eruptions. Glaciers and melting snow shift the rocky debris around. At intervals, the debris and the water merge to trigger massive “debris flows” down the creeks that drain Shasta’s glaciers.

“Debris flows look like concrete that hasn’t set,” Blodgett said. Boulders and trees are snapped off by the river of mud and carried along.

These debris flows are called “cold flows,” to distinguish them from the “hot flows” in eruptions that would bring down hot mud. The timing of cold flows is impossible to predict. The last one occurred in 1985. Cold flows are not considered dangerous by USGS researchers because they don’t escape the neighborhood of the mountain. Hot flows in an eruption could move “tens of miles” outside the Shasta vicinity, geologists say.

As a result of the recent studies, Blodgett and two other researchers, W. R. Osterkamp and C. R. Hupp of the USGS Washington office, estimate that large cold flows have occurred on Shasta about 70 times in the last 500 years.

Trees are one of the keys to tracing the history of Shasta’s cold flows, Blodgett says. A few of the fir and pine trees around the mountain survive for up to 500 years. Researchers study the rings in trunks of old trees close to Shasta’s creeks for evidence of past distortions and damage.

Nowadays, core samples of a living tree’s trunk can be taken without hurting the tree. Scientists also study fallen trees around the creeks.