Three USGS geologists measure the distance across a crack on the cr...

The U.S. Geological Survey established both periodic and continuous 24-hour monitoring programs at Mount St. Helens to study and predict eruptions. In this slide, geologists used a steel tape to measure the distance across a crack on the crater floor. Widening of cracks was an indication that magma was rising and deforming the area, leading to an eruption. These cracks were generally radial to the dome, like spokes of a wheel.