Measurements of change in the width of cracks on the crater floor of Mount St. Helens, Washington, were a major tool used in predicting dome-building episodes during 1980-1982. These measurements were made by field crews using a steel tape measure to determine the distance between lengths of reinforcing rod driven into the ground on opposite sides of cracks. In 1981, a telemetered, electronic tape measure was installed on the crater floor to monitor crack movement continuously. However, electronic and physical problems kept this and a second displacement meter from being viable predictive tools. In 1985, using new techniques and instrumentation, a third displacement meter was installed on the dome, and it successfully monitored the widening of a crack days before a dome-building episode. By 1989, there were two displacement meters of this type monitoring cracks on the dome.
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