Alaska Science Forum

This article is provided as a public service by the Geophysical Institute, University of Alaska Fairbanks, in cooperation with the UAF research community. Ned Rozell is a science writer at the institute.

Like a just-awakened giant with a rumbling stomach, Mount Iliamna is demanding attention.

Frequent earthquakes have rattled within the snowy dome of the 10,000-foot volcano in the past few weeks, making scientists at the Alaska Volcano Observatory take a closer look at the mountain, which sits about 75 miles across Cook Inlet from the town of Kenai.

Researchers, such as Geophysical Institute Volcano Seismologist Steve McNutt, are keeping both eyes on seismometers attached to Mount Iliamna to see if its behavior evokes a feeling of deja vu. McNutt and Geophysical Institute graduate student John Benoit recently detailed the habits of volcanoes that erupted from 1979 to 1989. They found the sleeping giants often go through the same rituals before waking with a bang to spew ash, hot gases, and molten rock.

After sifting through a decade of information, McNutt and Benoit noticed a pattern. Volcanoes often go through the following steps before erupting:

• Step 1: High-frequency earthquakes (during which the ground shakes very fast) increase within a volcano during the earliest stages of activity. This swarm of earthquakes is caused as molten rock, also known as magma, forces itself upward.

   

• Step 2: As the volcano progresses toward eruption, different kinds of earthquakes shake the volcano. These low-frequency earthquakes, which vibrate slower than the first type, happen when molten rock invades spaces between rocks and form magma-filled cavities. These chambers slowly resonate in response to underground impulses, much like a bell rings when struck by a clapper.

   

• Step 3: The number of earthquakes often decreases after seismometers scratch out the slew of low-frequency earthquakes, but seismologists don't relax. Volcanoes typically quiet down right before they erupt.

   

• Step 4: After a volcano seems to snooze, the buildup of gases and the reaction of hot magma with cold ground water often cause volcanic tremor, slight earthquakes that signal an eruption can happen at any time.

   

• Step 5: The pattern completes itself when the magma spews forth from a volcano (often quite explosively), and gases and ash are belched from the vent.

As this column was written, Mount Iliamna was at the first step of McNutt and Benoit's model. In late August, an average of 10 high-frequency earthquakes each day were shaking the mountain, with a high of 28 earthquakes on August 13.

This first step to eruption--a swarm of high-frequency earthquakes--can last an undefined amount of time, McNutt said. When Alaska's Mount Spurr erupted in 1992, high-frequency earthquakes shook the mountain for 10 months before it blew. Other volcanoes have completed the entire sequence--from high-frequency swarm to eruption--in less than a day.

Scientists don't expect Mount Iliamna to run though it's pre-eruption rituals in a day. Despite Mount Iliamna's grumblings, volcanologists Guy Tytgat and Kay Lawson felt safe enough to recently install two more seismometers (instruments that measure ground movement) on the volcano and tune up the four seismometers that were already cemented into the mountain. The instruments send radio signals of the earth's movement back to the Alaska Volcano Observatory, made up of a team of scientists in Anchorage and Fairbanks who get funding from the U.S. Geological Survey. AVO scientists work at the Geophysical Institute, the U.S. Geological Survey, and the Alaska Division of Geological and Geophysical Surveys.

Although Mount Iliamna has taken the first step to eruption, it may calm down and remain quiet for another 100 years. It's just as likely to explode in clouds of ash and spit up molten rock in a few weeks or months. Such is the mystery that makes the science of predicting volcanic eruptions a constant challenge.