# Volcano Watch — Mount St. Helens: One year and counting

Release Date:

Almost exactly one year ago today, the first signs of reawakening were detected at Mount St. Helens in Washington. On September 23, 2004, a small earthquake swarm occurred beneath the volcano. This was not unexpected. Earthquake swarms have been common at Mount St. Helens since the end of the 1980-1986 eruptive activity.

High angle view of Mount St. Helen's crater and new dome. North is to the bottom. (Credit: John Ewert and Jim Vallance, USGS. Public domain.)

And, in fact, scientists initially thought that the quakes were caused by rainwater, dropped by a major storm the week before, mixing with hot rock beneath the volcano.

Sure enough, the earthquakes tapered off within 24 hours. But three days later, on Sunday September 26, 2004, the staff of the Cascades Volcano Observatory (CVO) was called to the office. The earthquakes had resumed, and they were bigger than before.

Over the next several days, CVO launched a full response to the quakes, which steadily increased in magnitude and frequency, placing additional seismometers on the volcano, establishing new GPS sites, and measuring gases and temperatures in the crater. On September 29, scientists conducting an observation flight in a helicopter noticed a large patch of cracked and uplifted glacial ice in the back of the volcano's crater, confirming that the earthquakes were caused by magma pushing up from below.

CVO immediately declared that minor explosions were likely to occur within the next few days as the rising magma encountered groundwater. Sure enough, a small explosion occurred from the growing bulge in the crater on Friday, October 1, 2004, exactly one week after the first earthquakes were detected.

HVO geologist Mike Poland, who was working at CVO at that time, remembers the day vividly. "Several other scientists and I were at a local pizza parlor, trying to have lunch and discussing the activity away from the huge media presence back at the office. While we were eating, I noticed several people standing around the television, and I heard the words 'steam and ash' come from the TV. Sure enough, the small explosion that we had been predicting was underway. After a few minutes of staring at the TV screen, somebody pointed out that we could actually just walk outside and see the ash plume with our own eyes. We had to shake off our excitement and get back to the office pretty quick. I never did finish that slice of pizza."

Additional minor explosions occurred throughout the following week, and by October 11, 2004, lava had reached the surface.

Since that time, lava has continuously extruded into the crater, forming a new dome that is already 2/3 the size of the dome that formed in the 6 years after the catastrophic 1980 eruption. Amazingly, only a few small explosions have occurred over the course of the activity, none of which have posed a significant hazard. And despite the steady decrease in seismicity and deformation rates over the past year, perhaps signaling the waning of the eruption, the dome continues to grow, slowly filling the crater of the volcano.

Of course, long-lived eruptions are nothing new to those of use who live in Hawaii, where Kīlauea has been erupting for over 22 years. But it is important not to let these continuous extrusions of lava, be they on the mainland or right here at home, lull us into a false sense of security. The activity of a volcano can change at any time, and major explosions at Mount St. Helens are still a possibility, just as changes in the style of the eruption may occur at Kīlauea.

However, thanks to the comprehensive set of monitoring instruments that are deployed at both volcanoes, including GPS sensors, seismometers, tiltmeters, gas sensors, remote cameras, and other devices, volcanologists at CVO and HVO are well prepared to detect any signs of impending changes in eruptive activity, be the eruption in its 22nd year, or in the first. And counting.

### Volcano Activity Update

Eruptive activity at Puu Oo continues. On clear nights, glow is visible from several vents within the crater and on the southwest side of the cone. Lava continues to flow through the PKK lava tube from its source near Puu Oo to the ocean, with few surface flows breaking out of the tube. Flows are visible intermittently on the steep slope of Pulama pali and on the coastal plain. As of September 22, lava is entering the ocean at East Laeapuki, in Hawaii Volcanoes National Park.

The embayment left by the 11-acre bench collapse at East Laeapuki on August 27 was mostly refilled by September 9, although small bench collapses continue to occur. Large cracks cross both the old and new parts of the bench.

Access to the ocean entry and the surrounding area remains closed, due to significant hazards. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.

One minor earthquake was felt on Hawaii Island during the week ending September 21. At 2:29 p.m. on September 18, a magnitude-3.0 earthquake located 9 km (5 miles) west-southwest of Pahala at a depth of 6 km (4 mi) was felt in Naalehu, Ocean View, and Pahala.

Mauna Loa is not erupting. During the week ending September 21, seven earthquakes were recorded beneath the summit area. All were short-period, shallow (less than 6 km) events. Inflation of Mauna Loa continues.