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Geologists first observed cracks on the crater floor in mid-September 1980.
Would they continue to widen, or had they formed quickly in response to some
short-lived but unknown event? Simple measurements provided the answer. Short
stakes of steel reinforcement rod (rebar) were pounded into the ground on either
side of several cracks, and the distances between the stakes were measured with
a carpenter's steel tape. Repeated measurements showed that some of the cracks
widened with time, and that the rate of widening accelerated before the
explosive event of October 16-18 and the dome-growth event of
December 27, 1980-January 3, 1981 (Swanson and others, 1981).
During the December-January event, geologists were surprised to find two thrust faults on the crater floor north and northwest of the dome. The thrust faults faced outward, away from the dome. The same questions arose with these faults as with the cracks: were they still moving and, if so, might they provide an indication of future eruptive activity? Again rebar stakes were driven into the upper and lower lates of each thrust, and the distance between them was measured with the steel tape. The distance between each pair of stakes shortened with time and indicated that the upper plate was moving across the lower plate. Repeated measurements of the thrusts and associated radial cracks soon showed a distinctive pattern. Rates of displacement were slow after a dome-building episode but increased nearly exponentially as the onset of the next dome-growth event neared (Chadwick and others, 1983). This pattern and various other details of the entire deformation process of the crater floor were nicely traced with the simple rebar-and-tape method and formed the basis for a series of predictions of dome growth in 1981-1982 (Swanson and others, 1983). Moreover, the simple measurements with a steel tape provided the primary data for two interpretative papers about how and why the crater floor deformed (Chadwick and others, 1988; Chadwick and Swanson, 1989). |
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