Volcano Watch: Remembering Mount St. Helens Eruption in AprilApril 17, 2020, 8:00 AM HST (Updated April 17, 2020, 7:05 AM)
Volcano Watch is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates.
Mount St. Helens was exploding! The first eruption in the Cascades since 1914–1917 (Lassen Peak) started on March 27, 1980. April became a frenzied, exciting, challenging, sometimes frustrating, once-in-a-lifetime experience for several scientists with experience at the US Geological Survey (USGS) Hawaiian Volcano Observatory (HVO), called on to measure the deforming volcano.
Their work augmented seismic monitoring by University of Washington geophysicists and was coordinated with other USGS scientists having previous geologic knowledge of the volcano. Everyone tried to make sense of what was happening.
Forty years ago, no scientists in the USGS and academia were adept at monitoring all types of active volcanoes. Their expertise was gained from working on Kīlauea and Mauna Loa, not from steep composite volcanoes that dominate the Cascades. Equipment was remedial by today’s standards, computers were not in general use, and satellite observations were limited. There was no cache of equipment or experience to lean on. We were on our own.
By early April, a growing bulge high on the north flank of the volcano was readily apparent, cracking glaciers and leaving a crater behind as it moved northward. This itself was alarming, but was it a shallow feature or only the tip of deeper, larger deformation that might reach beyond the volcano?
To answer this question, we used ice-covered Spirit Lake north of the volcano as a large liquid tiltmeter. We nailed wooden yardsticks to tree stumps or dock piers around the lakeshore where open water was present. Using helicopter hops, we read water levels at six sites in about 20 minutes and calculated their differences. The ice cover damped wave oscillations, and yardsticks could be read to 1/16 inch (about 2 mm), enabling detection of tilt across the lake to about 2 microradians (roughly one ten thousandth of a degree) To our relief, repeat measurements until the ice melted in mid-month showed no change.
We could thus focus deformation measurements on the bulge itself. The flat parking lot at Timberline campground just northeast of the bulge was perfect for measuring tilt, using a method developed at HVO. We drove nails into the pavement at the tips of a triangle about 10 m (33 feet) on a side and leveling determined their relative elevations.
Repeated leveling, often during snowstorms, found changes in elevation caused by tilting ground. Seven relevelings (March 30–April 30) showed an overall tilt away from the bulge at about 2 microradians per day. This small tilt was further evidence that deformation was concentrated in the bulge itself.
Huge tilts of tens of microradians lasting only a few minutes were superimposed on the overall tilt. The parking lot was swaying back and forth, probably because of jerky movement of the bulge itself. Such tilts amazed us, but tests confirmed they were real.
To provide continuous tilt data, electronic platform tiltmeters were installed in nearby areas in late April. Instrument problems and sites made unstable by thawing ground limited their use.
Meanwhile a search was underway for an electronic distance meter (EDM) to make measurements of the bulge itself—our Holy Grail. Powerful EDMs were expensive and not readily available. An instrument was located at the Smithsonian Institution and a loan arranged. Measurements began on April 20.
But measurements were not straightforward. An EDM requires a target that reflects a laser back to the instrument. Normally, costly glass prisms were used, but anything on the bulge had to be expendable, that is, cheap. HVO had experimented with plastic highway reflectors and found them suitable for short distances. Could 5–8 such plastic reflectors clustered together work at distances of 2–4 km (1–2.5 miles)?
Yes, they could. Several reflectors were screwed to a board, which was bolted onto a steel signpost driven into the ground at helicopter-accessible sites on and near the bulge. These makeshift targets, the loaned EDM, and an old-fashioned optical theodolite allowed us to measure bulge movement of up to 1.5 m (almost 5 feet) per day, define the limits of the bulge, and otherwise provide scientists with reliable data as Mount St. Helens tore apart over the next month.
At the time, we measured our progress in blood, sweat, and—after the deadly eruption on May 18—tears. Forty years later, we can add a measure of pride for what was accomplished under extraordinary circumstances.