Early Sunday morning, January 16, 2005, an explosive event punctuated the ongoing eruption at Mount St. Helens. The effects were only local in their extent. We were "in the dark," so to speak, because the event occurred at night and during poor weather. Our only insight at the time came from the complete loss of radio transmissions from three of our instruments. Some other within-crater stations posted intermittent transmissions in the seconds following the event. Visual confirmation of the explosion was obtained during an overflight on Wednesday, January 19.

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Mount St. Helens, January 19, 2005. Oblique aerial view southward to Mount St. Helens, January 19, 2005. Ash from the January 16, 2005 explosion can be seen darkening the snow in the crater and on the volcano's east flank. Image taken by John Pallister.

The explosion began at 03:18 PST January 16, 2005, judging from seismic records. It produced a shower of ballistic blocks directed to the east and a layer of ash that blanketed the crater floor east, south, and west of the new dome. The explosion appears to have originated from the vent area at the north end of the growing lava dome, on the basis of the ash and block deposits. No conspicuous crater is visible, although some areas were masked by steam during the overflight.

Helicorder traces from NED and YEL seismic stations, showing the lengthy seismic record and associated data dropouts from the explosion on January 16, 2005.

Most blocks ejected by the explosion fell just east of the vent, pockmarking the snowy surface with craters as large as 1 m in diameter. Some of these blocks damaged a new instrument package, including a camera and volcanic gas sensor that had been flown into the crater on the previous Friday (January 14). Another GPS receiver, MID-9, was nearby and damaged, too, interrupting its 81-day string of data gathering.

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Remnants of gas-sensor and video station damaged by ballistic fallout on January 16, 2005. Box is 1.5 m wide. The station has been separated from its floor and tilted 10-20 degrees east. Its camera is missing, fiberglass housing is broken, gas intake tube is disconnected, and three holes penetrate the roof. Impact craters, visible throughout the field of view, are most notable at lower left and far right. Image taken January 19, 2005, by Dan Dzurisin.

A third instrument lost to the event was a GPS receiver that had been installed January 14. This unit, CDAN, had been lowered onto the active dome, in order to track the growth of the dome. The loss of CDAN, although a bummer, has a humorous side. It was named for an internationally known volcanologist, C. Dan Miller, who retired recently from the Cascades Volcano Observatory. Miller had a lengthy distinguished service; CDAN also had distinguished service but retired early. Nothing was seen of young CDAN during the overflight, so we suspect that the explosion toppled it into a pile of rubble near the presumed vent.

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New GPS receiver, with code name CDAN, on January 14, 2005, two days preceding the explosion. This instrument, lowered into place by helicopter, was lost during the explosion. Presumably it toppled downslope and ultimately into rubble. Image by John Pallister.

Seismic records from within-crater seismometers provided us with several clues for interpreting the event even before the weather improved sufficiently for an overflight. Tremor-like seismicity began with slow onset at 3:18 a.m. at the nearest station, increased in amplitude, and then died slowly, with the entire sequence lasting approximately 20 minutes. The seismic traces resembled rockfalls, but whereas rockfalls commonly go quiet in less than one minute, these signals lasted nearly 20 minutes. Thus an explosion was suspected early in the investigation.

Another line of evidence was the intermittent transmission, or dropout, of some seismic data from stations that normally are reliable. The dropouts could have several explanations, but their restricted occurrence to the 20 minutes of anomalous seismicity suggested a volcanic cause--that abundant airborne ash had interfered with the radio transmissions. Sunday came and went with no visual proof, but a reasonable interpretation was that an explosion had left its mark in the crater, on the seismic records, and on the damaged instrument cases. The January 19 overflight verified the sequence of events.

Post-mortem for CDAN. The newest GPS spider on the new dome retired January 16 between its last call home at 3:08 a.m. and its missed call-in at 3:38, local time, after less than 36 hours of service. In the preceding 24 hours it had moved south-southeast 8.3 m as it rode the back of the growing dome away from the vent. Its vertical change was upward 4.7 meters. If the explosion had occurred a week later, CDAN's rate of displacement might have carried the instrument safely beyond the damaging ballistics. Instead it tumbled to oblivion and is now remembered only in fable and song.

Left: Incandescence viewed by U.S. Forest Service webcam on Thursday evening, January 13, 2005. Area of incandescent rock corresponds to gash indicated on oblique view B, taken the next day during overflight (see enlarged image below). A thermal imaging camera captured a similar perspective, C. Below: Mount St. Helens' dome as seen from the west, January 14, 2005. Gash is visible on the new dome. Image taken by Matt Logan and Jim Vallance.

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Helicorder traces, January 13, 2005. Helicorder traces from seismometers BLIS and NED, showing rockfall that began at about 8:40 p.m. on January 13, 2005. Signals labeled "Earthquakes" provide a useful contrast between the vibration created by the rockfall and the shaking from earthquakes.