Mount St. Helens—the most active Cascade volcano
The 1980 eruption of Mt St Helens was the most deadly and costly volcanic event in U.S. history, which is not to say that it will be the most dangerous volcano in the future—that honor may go to Mt Rainier. In contrast to Hawaiian shield-type volcanoes, whose eruptions can damage property but are easy to get away from, Cascade composite-type volcanoes are explosive with pyroclastic flows that move very quickly and can be difficult to escape. Fortunately, Mt St Helens erupted toward the north, where the population density was low.
Thirty eight years later, the effects of the eruption are still evident. One of the aspects that made this eruption so dangerous was the lateral blast that removed the volcano’s northern flank and shot out pyroclastic material horizontally rather than vertically. This view toward the south, from the Johnston Observatory that is named for a geologist who perished while monitoring the volcano, shows where the top and northern flank of the volcano were removed. Plants have resumed growing, but it will be a long time before the thick forest of the past returns. Within the volcano’s crater, a lava dome has built up and small eruptions occurred there during the period of 2004-2008. Today the volcano sleeps tranquilly, but future eruptions are certain.
The first part of the 1980 eruption was a gigantic landslide that released built-up pressure (like popping the soda-can top) and caused huge chunks of the volcano to be carried downslope. Some of these chunks were buried by the subsequent flow of hot clasts and ash but the hummocky (i.e., lumpy) topography is still visible in some parts of the landscape. Some of landslide pieces blocked drainages and created lakes. Coldwater Lake is one example of a lake born in 1980—today it is a beautiful site that provides habitat for many organisms.
The force of the lateral blast blew down most trees on the north side—snapping them at the bottom like they were toothpicks. The remnants of these downed trees are still visible near the Johnston Observatory.
The heat from erupting hot lava clasts melted ice and snow on the volcano’s summit, and the landslide and pyroclastic flows were next followed by devastating muddy debris flows that sped rapidly down the Toutle River valley, overflowing the river’s bank and, in the process, taking out bridges and carrying away people’s homes. The river remains choked with sediments today, as flowing water erodes into the pumice plain below the volcano and carries the material downstream. This view of the Toutle River is looking southeast, toward the north-side blast zone. Note the old road in the valley that was partially wiped out by the eruption and debris flows. The new road is higher up and was carefully engineered to withstand future eruptions—it even won an award from the Association of Engineering Geologists!
We camped (Cougar Park) on the south side of Mt St Helens, where there is the best spring-time access to the mountain’s summit. A warm-up hike along Lewis River took us through lush fern-bottomed forests that were not impacted by the 1980 eruption, which blew toward the north.
We climbed to the top of Mt St Helens via the Worm Flows trail. Happily, the clouds cleared during our 1800-meter (6000-foot) ascent, and both Mt Adams (in photo below) and Mt Hood displayed their gorgeous volcanic shapes. We were not able to look down into the crater at the summit of Mt St Helens because the snow cornice that lined the crater’s edge was too dangerous to walk on.
Here is our climbing group on the way down from the summit, with Mt Hood in the distance. The view is to the south; Portland is a large city not far west of this volcano.
The trek down was faster because we could glissade in channels on the snowy upper part of the slope. Great fun! Mt Adams is also visible in this photo, which is courtesy of Jay Ach.
On the south slope there is evidence of past eruptions and water-caused erosion into the volcanic lava and deposits. This view is to the north, back up toward the summit. Photo courtesy of Jay Ach.
As catastrophic as the 1980 eruption was, the amount of material emitted was relatively small compared to other prehistoric eruptions. For example, the eruption that removed most of the volcanic edifice of Mt Mazama and created Crater Lake 7000 years ago emitted 150 times as much material as the 1980 eruption of Mt St Helens, and the eruption of the Yellowstone “super volcano” about 640,000 years ago erupted about 2,500 times as much material.
Nature can certainly be hazardous, but in the process, stunning landscapes are created that we can enjoy safely most of the time.