Could Mount Shasta be next Mount St. Helens?

When a series of small earthquakes over a two-week period rattled the southwestern flank of Mount Shasta last September it caught the attention of geologists. Such events can be a precursor to a volcano rumbling to life.

Although the earthquakes near McCloud ended almost as soon as they started, the Cascade Range sits on an active fault zone. The last major eruption was Mount St. Helens in 1980. Mount Shasta is the second-most southern peak in the range and is considered dormant but not extinct.

For a long time, 1786 was assumed to be last time Mount Shasta erupted. But in a new report, geologists are retracting that event from the record. The source always had been a nautical record by a French exploration team at sea that noted a plume of smoke and ash over the mountain. Turns out it likely was a large wildfire, according to the United States Geological Survey.

In an updated list by the USGS, Mount Shasta is ranked fifth out of a list of 18 volcanoes in the country that pose a “very high threat.” Kilauea on the island of Hawaii is ranked first.

“The youngest eruption that we can confirm happened was about 3,000 years ago,” said Dr. Jessica Ball with the California Volcano Observatory based in Menlo Park. Although on average Mount Shasta erupts every 600 to 800 years, Ball said “volcanoes don’t care about averages. You have periods where it is more active and periods where it is less active.”

Still, Mount Shasta is considered a relatively young volcano in geologic terms. It has “molten magma in its plumbing system,” Ball said. “It’s literally hot inside.”

Those climbing the mountain often will encounter a strong sulfur smell near the summit of the 14,179-foot peak. In his famous essay “Snow-storm on Mt. Shasta” first printed in Harper’s Magazine, John Muir described surviving a wicked storm by hunkering down for the night in a volcanic vent. He was badly scalded and spent several days recuperating at the old Sisson Inn at the foot of the mountain.

“Mount Shasta tends to have explosive eruptions,” Ball said. “In an explosive event, if you are in the danger zone, you might not be able to escape, as people found out with Mount St. Helens.”

For those who live next to the volcano — especially on the north side in the Lake Shastina area — remnants from the last eruption are easily visible, ranging from lava tubes to mounds left by pyroclastic flows, or lava. More common today are sudden mud events.

There is nothing currently about Mount Shasta that suggests any sort of unusual behavior, according to Ed Venske with the Smithsonian Institution’s Global Volcanism Program in Washington, D.C. Venske said the fact that Mount Shasta is listed as the fifth-most dangerous volcano by the USGS is a “hazard assessment tool” that takes into account 24 different factors, such as proximity to population centers or aircraft flight patterns.

“It is not an eruption forecast tool,” he said.

Venske said that in addition to earthquakes and increased emissions of gases, volcanologists study something called deformation, or changes to the shape of the mountain.

“This is the kind of stuff detected with sensitive instruments or a satellite pass over a long period of time,” he said. “These are very, very tiny changes. They are not going to be visible to the naked eye.”

On a larger, planetary scale are events that continue to shape and change the Cascade Range, which is part of the Pacific Ocean’s Ring of Fire. The driving force is a collision between two massive parts of the Earth’s crust: The Juan de Fuca ocean plate, which meets and then plunges below the North American Plate. The results are both earthquakes and volcanoes.

Since plate tectonics are a continual force in constant motion, so is the need to plan for the inevitable outcome. In the initial environmental impact report for the Mount Shasta Ski Park in 1985, for example, the resort was designated as “Blast Zone 3.” An earthquake swarm three years later in the nearby Medicine Lake area was considered a significant geologic event.