IAVCEI 2017 pre-conference field trip – pyroclastic density current deposits from the May 18, 1980 Mount St. Helens eruption

Posted by lcb14 at Feb 06, 2018 12:46 PM |
MPhil Researcher Gregor Hahn reports how the Whittaker Award in Geology last year helped fund a place on the 2017 pre-conference IAVCEI field trip to examine pyroclastic density current deposits of the famous 18th of May 1980 eruption of Mount St Helens.
IAVCEI 2017 pre-conference field trip – pyroclastic density current deposits from the May 18, 1980 Mount St. Helens eruption

Gregor Hahn

Beside attending and presenting my research at the International Association of Volcanology and Chemistry of the Earth’s Interior 2017 Scientific Assembly held in Portland, Oregon, USA in Mid-August, I had the amazing opportunity to attend a four-day pre-conference field trip to examine pyroclastic density current deposits of the famous 18th of May 1980 eruption of Mount St Helens, Washington, USA. Luckily the eruption is very well documented and the pyroclastic density current deposits are well exposed. This makes the May 1980 eruption a key event for our modern understanding of how a volcano explosively erupts and how transport and deposition mechanisms work within density currents. These mechanisms, along with the current mobility and runout distances, were the main subjects of the field trip. The group consisted of about 30 volcanologists from around the world; all of whom are researching large scale pyroclastic processes. Established field geologists, experimental and numeric modelling geologists as well as several early career scientists participated, bringing a variety of knowledge and viewpoints together into the field, to see and discuss processes and features that are highly complex and sometimes still poorly understood.

Figure 1. Aerial image of Mount St Helens showing the crater and the large pyroclastic density current deposits to the left of the image.
Figure 1. Aerial image of Mount St Helens showing the crater and the large pyroclastic density current deposits to the left of the image.

The field trip set of early on the 10th of August in Portland. Several stops were made along the way to Mount St Helens, to firstly discuss and familiarise yourself with the different eruptive and evolutionary stages of the volcano, which has been active since at least 240,000 years. This included stops at well exposed earlier eruption deposits, as well as at several lavas and at Ape Cave, a several kilometre long and about 2,000 years old lava tube. In the evening the field group arrived at the Mount St Helens Institute field camp at Cascade Peaks, where tents and dinner were provided.

The main pyroclastic deposits of the May 1980 eruption were visited on day hikes on the 11th to 13th of August. Each day was subdivided into several stops along the pyroclastic deposits both close to and further away from source (proximal to distal) within the main blast zone north of Mount St Helens. Several units of pyroclastic density current deposits of the highly explosive eruption overlay debris avalanche deposits, of one of the largest ever recorded landslides. The deposits are very well exposed, revealing a large variety of transport and deposition regimes during the current movement. The group had several long and informative discussions during each stop, with people arguing from different perspectives and coming up with different ideas and solutions. Each outcrop also varies and changes every year due to snow melt and its accompanying floods. For this reason erosion constantly removes interesting features, but also reveals new structures along the route. Also the exposure of unit boundaries changes constantly requiring an expert eye in some localities.

Beside the amazing exposure of the pyroclastic deposits, a walk up into the crater of Mount St Helens was one of the highlights of the field trip. Until 2008 a new lava dome was growing within the crater, and my continue as well to grow in future. This lava dome is surronded by a new glacier that started to form in the crater since the eruption of 1980.

Figure 2. Exposure of the pyroclastic deposits in the blast zone of the May 1980 eruption with the field group in front, discussing the structures and mechanisms.
Figure 2. Exposure of the pyroclastic deposits in the blast zone of the May 1980 eruption with the field group in front, discussing the structures and mechanisms.

Furthermore, the group was joined by a highly professional media team filming and photographing the field group. Therefore, the media team was well equipped with video and photo cameras to interview the participants as well as with a drone for bird’s eye shots. A short film about the pre-conference field trip will be arranged soon.

The field trip allowed me to train my understanding of transport and deposition related processes. In context to my own research the deposits of Mount St Helens are very young and show many similar but also several different features, helping me to improve my knowledge about field identification criteria for the huge variety of pyroclastic density current deposits. Furthermore, I had the amazing opportunity to compare my ancient deposits to modern examples as well as to meet and discuss my research with many other volcanologists working on pyroclastic density current deposits.

Figure 3. Group photo in front of Mount St Helens (picture by M.Wordell).
Figure 3. Group photo in front of Mount St Helens (picture by M.Wordell).

Many thanks to Geology for the support through the Whittaker Award enabling me to partake in this fabulous field trip!

Gregor is currently studying for his PhD in the School of Geography, Geology and the Environment - "High-temperature deformation of tuffs at explosive volcanoes: a field and structural investigation."

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