Using an EDM to measure deformation of the lava dome at Mount St. Helens, Wasington
Surveys Reveal Spreading of the Lava dome Before Eruptions of Mount St. Helens
Hot new lava glows atop the lava dome in the crater of Mount St. Helens during an eruption that began on 21 October 1986. With a temperature of about 930° C, the glowing lava is about 200 m in diameter! Eruptions of viscous lava are difficult to photograph, but this spectacular time-lapse image captures the moment scientists had predicted the week before with help from measurements made using an electronic distance meter (EDM).
Magma First Rises into Dome
Before magma can erupt from the dome, it has to rise into the dome's core. As magma moves into the dome, one or more sides of the dome spreads sideways away from its center. An electronic distance meter (EDM) was used to measure the "spread" of the lava dome, which often began 3 to 4 weeks before an eruption. To measure the dome's spreading, we installed several benchmarks on the crater floor over which we would set the EDM (see below). When the dome was active between 1981 and 1986, we worked in the crater several times a week in order to survey the dome regularly.
EDM Measures Dome Spreading
Waiting for a new benchmark to be installed high on the side of the lava dome in the crater of Mount St. Helens, a scientist on the crater floor follows the progress of his colleague climbing up the steep west side of the dome. Small short-range EDM's like this one atop the tripod can be used for measuring distances to benchmarks less than about 2 km away.
Benchmarks Placed on Dome
As viewed from the EDM site on the crater floor shown above, bright orange-colored targets dot the side of the dome. This image was taken with a 135 mm telephoto lens. The cliff near the top of the dome is about 50 m tall, which is the edge of a lava flow erupted on 20 March 1982.
Can you find the scientist near the top of the dome? Look at the base of the lava flow left of the large crack (or see large-sized image). The scientist and orange-colored targets are about 200-250 m from the EDM station on the crater floor.
Eruption Prediction Issued
This graph shows the closing distance between two benchmarks located on the dome and at the EDM station on the crater floor before the May 14, 1982, eruption (vertical red line). As magma pushed the side of the dome outward, the distance to the EDM station became smaller. The steep curves indicate that the dome moved outward at increasing rates right up to the eruption. For example, the dome was moving about 2 cm/day on May 2; the movement increased to about 200 cm/day by May 13!
On the basis of these measurements and increasing earthquake activity, scientists issued a prediction on May 11 that an eruption would occur "within the next week, possibly in the next few days." The eruption occurred 3 days later.