DESCRIPTION:
Mount St. Helens North Flank Bulge

[Image,53K,JPG]
Bulge on Mount St. Helens North Flank.
-- USGS Photo by P.W. Lipman, April 27, 1980

A magnitude 4.2 (Richter Scale) earthquake on March 20, 1980 ... preceded by several much smaller earthquakes beginning as early as March 16, was the first substantial indication of Mount St. Helens' awakening ... on March 27, Mount St. Helens began to spew ash and steam ... Through April 21, Mount St. Helens intermittently ejected ash and steam in bursts ... Visible eruptive activity ceased temporarily in late April and early May. Small steam-blast eruptions resumed on May 7, continued intermittently for the next several days, and ceased again by May 16. During this interval, the forceful intrusion of magma into the volcano continued with no respite, as was shown by intense seismic activity and visible swelling and cracking of the volcano. The swelling was easily measurable and affected a large area on the north face of Mount St. Helens; this area became known as the "bulge," the initial growth of which probably began during the first eruption (March 27) or perhaps even a few days before. Through mid-May about 10,000 earthquakes were recorded. The earthquake activity was concentrated in a small zone less than 1.6 miles directly beneath the bulge on the north flank of Mount St. Helens.

A comparison of aerial photographs taken in the summer of 1979 with those taken during and after April 1980 showed that by May 12 certain parts of the bulge near the summit were more than 450 feet higher than before the magma intrusion began. Repeated measurements begun in late April with precise electronic instruments that shoot a laser beam to reflector targets placed on and around the bulge showed that it was growing northward at an astonishing rate of about 5 feet per day. The movement was predominantly horizontal-clear evidence that the bulge was not simply slipping down the volcano's steep slope. As the bulge moved northward, the summit area behind it progressively sank, forming a complex down-dropped block called a graben. These changes in the volcano's shape were related to the overall deformation that increased the volume of the mountain by 0.03 cubic mile by mid-May. This volume increase presumably corresponded to the volume of magma that pushed into the volcano and deformed its surface. Because the intruded magma remained below ground and was not directly visible, it was called a cryptodome, in contrast to a true volcanic dome exposed at the surface.

In mid-April, 10-12 days after monitoring of the lake began, several different geologists viewing the volcano noticed that its upper north flank had apparently deformed relative to its pre-1980 shape. Earthquakes recorded on portable seismographs were located beneath the north flank, so it seemed reasonable that the deformation was related to shallow intrusion of magma into the edifice. But the observed deformation was obvious on glaciers, which apparently were buckling and bulging upward. Was this deformation confined to the glaciers and the result of rapid advance caused by subglacial melting, or was the underlying edifice also taking part in the deformation.

The obvious way to test for deformation of the edifice was to make repeated measurements to targets on snow-free parts of the volcano. Quickly a standard theodolite used by most surveyors was obtained and angles measured to prominent natural features on the volcano. The data suggested outward movement of the north flank, but aiming on natural features was difficult and subject to considerable error. Within a day or two an electronic distance meter (EDM) arrived, and reflectors were placed on the mountain. The EDM is a sophisticated instrument, but the targets were nothing more than clear plastic highway reflectors screwed to boards that were lashed to steel fence posts hammered into the ground. A combination of the theodolite and EDM measurements to these targets soon indicated that the north flank was moving northward at a steady rate, so that clearly the volcanic edifice was deforming in response to intrusion of magma (Lipman and others, 1981). The theodolite measurements to the wooden targets provided sufficient data by themselves to define the rapid displacements of as much as 2.5 meters/day. The EDM data were useful adjuncts to the theodolite measurements but were not necessary to trace the movements at all but the least sensitive targets.

Magma rising beneath a volcano forcefully displaces the surrounding rock, and the resulting deformation can be measured at the ground surface. ... In late April 1980, distance and angle measurements were initiated from instrument sites on the apron of the volcano to targets on its flanks. This monitoring documented a remarkable bulging of the north flank as magma intruded the volcano before the large landslide and explosion of May 18, 1980. The need to monitor all sides of a volcano was indicated by the localized development of the bulge, which covered 3-4 square kilometers, extended nearly 2 kilometers downslope from the summit area, and was mostly confined to a 60 degree radial sector of the cone. Points on the bulge moved tens of meters northward, whereas points just off the bulge and elsewhere on the cone were nearly immobile. After the May 18 events, geodetic monitoring of the volcano's flanks suggested slight horizontal expansion before other explosions in 1980 and slight contraction afterward. ...

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