David A. Johnston

179

David A. Johnston : biography

18 December 1949 – 18 May 1980

On April 17, a bulge was discovered on the mountain’s north flank, suggesting that Mount St. Helens could produce a lateral blast.Fisher, p. 91. Johnston was one of few people who believed this, along with a professor of geology at a Tacoma community college, Jack Hyde. Observing that Mount St. Helens did not possess visible vents, Hyde suggested that pressure would increase until the mountain exploded. Because Hyde was not a part of the USGS or in a position of responsibility, his opinion was generally dismissed.Bryson, p. 221. However, both were to be proven correct. Rising magma under Mount St. Helens had veered off to the north flank, creating a growing bulge on the surface.

Final signs and primary blast

Given the increasing seismic and volcanic activity, Johnston and the other volcanologists working for the USGS in its Vancouver branch prepared to observe any impending eruption. Geologist Don Swanson and others placed reflectors on and around the growing domes,Parchman, pp. 108–109. and established the Coldwater I and II observation posts to use laser ranging to measure how the distances to these reflectors changed over time as the domes deformed. Coldwater II, where Johnston died, was located just north of the mountain. To the astonishment of the USGS geologists, the bulge was growing at a rate of 5 to 8 feet (1.5 to 2.4 meters) per day.

Tiltmeters installed on the volcano’s north side displayed a northwest trending tilt for that side of the mountain, and a southwest trending tilt was observed on the south side. Worried that the amount of pressure on the magma underground was increasing, scientists analyzed gases by the crater, and found high traces of sulfur dioxide. After this discovery, they began to regularly check the fumarolic activity and monitor the volcano for dramatic changes, but none were observed. Disheartened, they instead opted to study the growing bulge and the threat an avalanche could have for humans relatively near the volcano. An evaluation of the threat was carried out, concluding that a landslide or avalanche in the Toutle River could spawn lahars, or mudflows, downstream.

At that point, the previously consistent phreatic activity had become intermittent. Between May 10 and May 17, the only change occurred on the volcano’s north flank, as the bulge increased in size. On May 16 and 17, the mountain stopped its phreatic eruptions completely.

The active Mount St. Helens was radically different from its dormant form, now featuring an enormous bulge and several craters. In the week preceding the eruption, cracks formed in the north sector of the volcano’s summit, indicating a movement of magma from the bulge and towards the caldera.

At 8:32 a.m. local time the next day (18 May), an earthquake measuring magnitude 5.1 rocked the area, triggering the landslide that started the main eruption. In a matter of seconds, vibrations from the earthquake loosened of rock on the mountain’s north face and summit, creating a massive landslide. With the loss of the confining pressure of the overlying rock, the caldera of Mount St. Helens began to rapidly emit steam and other volcanic gases. A few seconds later, it erupted laterally, sending swift pyroclastic flows down its flanks at near supersonic speeds. These flows were later joined by lahars. Before being struck by a series of flows that, at their fastest, would have taken less than a minute to reach his position, Johnston managed to radio "Vancouver! Vancouver! This is it!" to his USGS co-workers. Seconds later, the signal from the radio went silent. Initially, there was some debate as to whether Johnston had survived; records soon showed a radio message from fellow eruption victim and amateur radio operator Gerry Martin, located near the Coldwater peak and further north of Johnston’s position, reporting his sighting of the eruption enveloping the Coldwater II observation post. As the blast overwhelmed Johnston’s post, Martin declared solemnly, "Gentlemen, the uh, camper and the car sitting over to the south of me is covered. It’s gonna get me, too. I can’t get out of here …" before his radio went silent.