DESCRIPTION:
Lava Dome Measurements using EDM's and Theodolites

MSH84_USGS_scientists_survey_the_dome_05-26-84.jpg
U.S. Geological Survey geologists use a theodolite and EDM (Electronic Distance Meter) to measure angles and slope-distances to the lava dome. Changes in these angles and distances are used to calculate "deformation rates". An increase in deformation rates is an indication that magma is slowly entering the dome. In the early 1980s deformation rates often reached 30 feet per hour (10 meters/hour) as magma rose and the dome expanded before extrusion started. During the winter months, the instrument stations often had to be dug out of the snow before measurements could be made.
USGS Photograph taken on May 26, 1984, by Lyn Topinka.
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... Slope distance and vertical angle measurements to targets and prisms on the dome ... to determine whether any part of the dome is swelling. Before the August 1982 lobe appeared on the surface, measurements to the dome showed that the west and southwest sides of the dome were growing upward and outward at rates of as much as 22 meters per day. These measurements are made from numerous sites circling the dome. Although many instrument stations are not habitable during the winter months, generally one or two stations can be dug out and occupied.

Measurements show that the dome expands as magma moves up into it before eruptions. Repeated surveys using an electronic distance meter (EDM) and theodolite between points on the crater floor and targets placed on the dome reveal movements that speed up as the eruption nears. A target on the west side of the dome was moving roughly 2 centimeters per day 2 weeks before the May 14, 1982, eruption; these movements increased to about 200 centimeters per day by May 13. Such accelerations were frequently used to predict eruptions in 1982.

Generally, all sides of the dome were monitored in 1981 and 1982 from four to five stations on the crater floor. Experience has shown that the dome deforms differently at each of the stations, thus making it necessary to monitor more than one side for reliable predictions. During the winter months, one and sometimes two sides of the dome can be monitored; it is no longer possible to monitor the east sector of the dome because of hazardous rockfalls from the dome and the east crater wall. This technique has become essential fro predicting the most recent eruptions because most of the ground cracks and thrust faults have been buried by the dome and associated rockfall debris. This type of measurement also documents movements of the dome during prolonged eruptions, such as that of 1983, and is used to foretell significant changes in such continuous activity.

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