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Report on St. Helens (United States) — April 1985

St. Helens

Scientific Event Alert Network Bulletin, vol. 10, no. 4 (April 1985)
Managing Editor: Lindsay McClelland.

St. Helens (United States) Activity at background levels; magnetic data

Please cite this report as:

Global Volcanism Program, 1985. Report on St. Helens (United States) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 10:4. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198504-321050

St. Helens

United States

46.2°N, 122.18°W; summit elev. 2549 m

All times are local (unless otherwise noted)

No gas-and-ash emissions from the dome were observed, and none was detected by seismographs through early May. Maximum displacement rates were 2-3 mm/day. SO2 emission averaged 30 ± 5 t/d in April, lower than the average February and March rates of 50 ± 10 t/d.

The following is a report from Daniel Dzurisin and Roger Denlinger.

"Measurements of total magnetic field intensity on and near the lava dome are providing unique insights into the dome's internal structure and cooling history. Permanent changes in magnetic intensity occur on and near the dome during episodes of rapid growth, and secular increases occur on the dome as its exterior cools and becomes permanently magnetized."

"Magnetic intensity data with a precision of 0.25 gamma are collected simultaneously at two base stations on the volcano's flanks, and at fixed stations on the dome and surrounding crater floor. Identical proton precession total field magnetometers are used at the base stations and in the crater. The base stations are automated and transmit data to CVO once each minute; crater stations are measured sequentially by a field crew using portable instruments. The base station record is subtracted from the crater data to remove diurnal variations to an empirical accuracy of about 2 gammas."

"Two types of changes in magnetic field intensity have been detected since measurements began in March 1984. The first type occurs at stations near the dome during rapid endogenous or exogenous growth of the dome. Permanent decreases in magnetic intensity of a few to a few tens of gammas accompanied dome extrusions in March and September 1984. No comparable changes occurred during a relatively passive extrusion in June 1984 [but see SEAN 10:10], or during a period of rapid endogenous growth that followed in early July. Parts of the dome were displaced by several tens of meters during the March and September extrusions, but displacements during the June and July events were considerably smaller. We tentatively attribute this first type of magnetic change to large displacements of the cooled magnetic exterior of the dome, relative to nearby magnetic monitoring stations."

"A second type of change occurs only on the dome, where the magnetic field intensity at most stations has increased steadily since measurements began there in December 1984. Rates of increase vary from 0.1 to 2.2 gammas per day on different parts of the dome, but do not change significantly with time at any one station. We tentatively attribute these secular increases in field strength to cooling and magnetization of the outer parts of the dome."

"To better understand this second type of magnetic intensity change, we have made a preliminary magnetic intensity map of the dome, and have repeatedly measured several short magnetic profiles. The magnetic intensity map shows a broad positive anomaly of about 1,000 gammas amplitude associated with the dome. A magnetic profile with more closely spaced stations across the September 1984 lobe reveals local anomalies with wavelengths of a few to a few hundred meters, with a maximum amplitude of about 700 gammas. We tentatively attribute the long wavelength anomaly to a cooled magnetic exterior enclosing the lobe's hotter, non-magnetic interior. The strength of the long wavelength anomaly increased by as much as 100 gammas from February to April 1985, presumably owing to continued cooling. More detailed magnetic profiles centered at each magnetic monitoring station on the dome tell a similar story. Measurements are made at 1-m intervals along N-S and E-W profiles about 20 m long, at sensor heights of 2.5, 3.7, and 5.0 m above the ground. Typically, short-wavelength (1-10-m) anomalies decay rapidly with increasing height, but they do not change significantly with time. Instead, the magnetic intensity along the entire profile increases uniformly with time, implying the growth of broader anomalies at depths of a few tens of meters."

"During the next year, we plan to improve our data base on the dome, and to begin quantitative modelling and interpretation of our results. The existing magnetic intensity map is not sufficiently detailed to distinguish the magnetic signatures of various lobes comprising the dome, so we will make a more detailed map this summer. We will also numerically model the development of magnetic anomalies associated with the September 1984 lobe and the dome as a whole, to estimate the downward migration rate of permanent magnetization, and the implied volumetric cooling rate of the dome. Combined with results from other concurrent geophysical studies, the goal of this research is to characterize the thermal structure of the lava dome and its temporal evolution. Our results may eventually bear on such diverse topics as the rheology of the dome and the volcanic hazards implications of its continued growth."

Geological Summary. Prior to 1980, Mount St. Helens was a conical volcano sometimes known as the Fujisan of America. During the 1980 eruption the upper 400 m of the summit was removed by slope failure, leaving a 2 x 3.5 km breached crater now partially filled by a lava dome. There have been nine major eruptive periods beginning about 40-50,000 years ago, and it has been the most active volcano in the Cascade Range during the Holocene. Prior to 2,200 years ago, tephra, lava domes, and pyroclastic flows were erupted, forming the older edifice, but few lava flows extended beyond the base of the volcano. The modern edifice consists of basaltic as well as andesitic and dacitic products from summit and flank vents. Eruptions in the 19th century originated from the Goat Rocks area on the N flank, and were witnessed by early settlers.

Information Contacts: D. Dzurisin, R. Denlinger, D. Swanson, J. Sutton, USGS CVO, Vancouver, WA; C. Jonientz-Trisler, University of Washington.