Mount St. Helens Precursory Activity

Scientist in Cougar on the morning of April 7. Officials moved the roadblock on State Route 503 several miles west two days earlier, cutting off tourist access to the small town. USGS photo courtesy of Pete Lipman.

A Mountain in Motion - Bad weather obscured the volcano part of the week making visual observations difficult. Explosions continued at irregular intervals. Overall earthquake activity began to settle into a pattern that would continue throughout April. Episodes of harmonic tremor continued intermittently as USGS scientists determined that parts of the mountain had moved up or out as much as 300 feet. The rapid change in shape, or deformation, of the north flank would continue up to May 18. National Guard troops stepped in to help local authorities staff roadblocks, several of which were moved further away from the volcano. Local agencies responded to the possibility of a major eruption and the USFS declared Mount St. Helens a "geologic area."

Earthquakes and explosions were outward signs of processes taking place inside the volcano. USGS scientists had watched the crater grow and the north flank crack and swell, but they could not be sure of just how much change had taken place. On April 7 aerial photographs were taken and used to make a topographic map of the volcano. Similar photos taken on August 15, 1979, were used to make a second topographic map. When the new (1980) map was compared to the old map, it showed that parts of the north flank had moved outwards by over 300 feet! Although the aerial photos were taken on April 7, the new data on the bulge was not available to scientists until April 23.

Map view of elevation changes (in feet) on the north flank of Mount St. Helens between August 1979 and April 7, 1980. Dashed line outlines the May 1980 crater. Red contours are negative values, blue contours are positive values (figure modified from USGS Professional Paper 1250, p.126).

Daily Log

April 5 - At least three explosions sent ash to about 15,000 feet. Low concentrations of sulfur dioxide were detected in erupted plumes. Four earthquakes of magnitude 4.0 or larger were recorded. At 6:00 a.m. sixty Washington National Guard troops took up positions at roadblocks. Closures on State Routes 503 and 504 were moved further away from the mountain to provide better control and place the most congested viewing areas off limits. The roadblock on 503 east of Cougar was moved to the junction of the old Lewis River Road (near Jack's Sporting Goods), effectively cutting off tourist traffic to the town.

National Guard troops staff roadblock on State Route 504. USGS photo courtesy of Don Swanson.

April 6 - A few insignificant explosions occurred under clouds and rain. Earthquake activity remained about the same as the past few days. There were 54 earthquakes with magnitudes greater than 3.0 recorded, including 6 with magnitudes greater than 4.0. No harmonic tremor was recorded.

April 7 - The crater's dimensions measured approximately 1700 feet long by 1200 feet wide and 500 feet deep. Scientists obtained their first clear view of the vent at the bottom of the deepest part of the crater. A single circular "throat" about 20 feet in diameter was visible. They also reported the first observations of substantial ponding of water. Two pools of muddy water large enough to float chunks of ice disappeared with each explosion then returned as more water accumulated after the explosion subsided. The overall seismicity remained about the same and harmonic tremor was recorded for the first time in two days. Earthquakes were centered beneath the north slope at depths ranging from one half to three miles. USGS scientists installed a new tiltmeter at the Timberline Viewpoint.

Eruption at dawn on April 7 viewed from Coldwater I. USGS Photo courtesy of Rick Hoblitt.

Aerial view of crater. By April 7 the crater measured 1700 feet long, 1200 feet across and 500 feet deep. USGS photo courtesy of Richard Waitt.

April 8 - At 8:22 a.m. a series of explosions were sighted which lasted at least four hours. It was the longest uninterrupted sequence of explosions to date. During the explosions part of the upper Wishbone Glacier caved into the crater, widening the rim by more than 300 feet. There were 38 earthquakes with magnitudes greater than 3.0, including 4 with magnitudes larger than 4.0. Scientists were uncertain about what the volcano would do next. Most preferred to withold judgement until more data were available. The National Oceanagraphic and Atmospheric Association (NOAA) began a week long study to compare the emissions from St. Helens to those of industrial sources. Officials moved the roadblock on State Route 503 from Jack's Store back to the Swift Canal east of Cougar after local merchants threatened to sue them over loss of business. The superintendent of the Toutle Lake School District announced evacuation plans for schools in the event of a large eruption of Mount St. Helens.

April 9 - Clouds obscured the mountain, but light ashfalls accompanied by the odor of sulfur were reported near Timberline and Spirit Lake from at least three separate explosions. Seismometers recorded 37 earthquakes larger than magnitude 3.0, including 6 larger than magnitude 4.0. A new tiltmeter at Timberline began sending continuous data to Vancouver. The USFS oficially designated Mount St. Helens a "geologic area," which protected the forest around the mountain from future land use changes. Geologists reported melting at the base of the Wishbone Glacer. They expressed concerns that continued melting could cause the glacier to slip down into the North Fork Toutle River. Business and traffic returned to Cougar following the relocation of the 503 roadblock. Fire district officials renewed concerns that tourist traffic may create problems should an evacuation be needed.

On calm days the white steam portion of erupted plumes rose as high as 10,000 feet above the crater. USGS photo courtesy of Richard Waitt.

On windy days the plume drifted downwind. At times it would hug the ground on the lee side of the cone and deposit ash as it moved downslope. USGS photo courtesy of Richard Waitt.

April 10 - Weather cleared to permit aerial observations of continued explosions from a circular vent about 30 feet across in the western part of the crater. Harmonic tremor was detected for the first time in three days. Steve Malone of the University of Washington reported that there had been 101 earthquakes with magnitudes larger than 4.0 since March 27.

The Cost of an Eruption - Explosions and earthquakes had settled into a monotonous pattern with no end in site. Mounting expenses began to affect the level of monitoring and maintenance of roadblocks. Since March 20 the USFS estimated expenditures at $157,000. Skamania and Cowlitz Counties had spent $17,000 and $10,000, respectively, for their part in maintaining the roadblocks leading to the mountain. The National Guard estimated costs of $3,800 per day since April 5 for their part in staffing roadblocks. Washington State Patrol spent over $5,000 for their part of this effort. In order to reduce costs, the USFS grounded both observer planes, but kept one on standby in the event of an eruption. They also closed the press center at the Shilo Inn, ending twice daily briefings. Days before the USGS had reduced the number of scientists monitoring the volcano from 30 to between 5 and 10.

The USFS provided information on the explosions of Mount St. Helens to visitors at the Yale boat launch, one of several safe viewpoints. Photo courtesy of USFS.

USFS interpreter gives a talk about the continued eruptions of Mount St. Helens at the Yale boat launch. Photo courtesy of USFS.

April 11 - A moderate sized explosion was observed in the morning hours. Two new steam vents were noted on the northwest flank just below the crater rim. Ponded water was also visible on the crater floor. A magnitude 4.9 earthquake was recorded in the afternoon, and along with the one on March 28 is the strongest to date. Tiltmeters on the north and east flanks show swelling and subsidence over minutes to hours, but the net changes are inconclusive. Washington State Department of Transportation posted signs for four safe viewpoints along Interstate 5. They were located near exit 14 (Ridgefield Junction), exit 63 south of Toledo, exit 76 south of Chehalis, and the Yale boat launch.

Phreatic explosion. USGS photo courtesy of Rick Hoblitt.