Volcanic Eruptions and What Triggers Them

DOI: 10.2113/gselements.13.1.3

The north slope of Mount St. Helens erupted catastrophically at 8:32 a.m. on 18 May 1980 in southern Washington state, about 50 miles northeast of Portland (Oregon, USA). This eruption was preceded by a magnitude 5.1 earthquake and a subsequent landslide that are thought to have triggered the main eruption. Although relatively “minor” compared to other US eruptions (e.g. Yellowstone Supervolcano in Wyoming, USA), Mount St. Helens was the deadliest and most economically destructive eruption in United States’ history (Tilling et al. 1990): it killed 57 people, including U.S. Geological Survey (USGS) volcanologist Dr. David A. Johnston who was monitoring the volcano 6 miles north of Mount St. Helens. It’s somewhat ironic that Dave Johnston was killed “by an unusual eruptive event that was largely unanticipated, in magnitude or style, except perhaps by Dave himself” (Hildreth 1980).

Several months before the May eruption, there were at least four warning signs that major changes were occurring beneath Mount St. Helens (Tilling et al. 1990). These changes included (1) increased seismic activity; (2) increased volcanic activity, including the formation of a bulge on the north flank of Mount St. Helens; (3) phreatic and other minor eruptions; and (4) changes in gas composition. As a result of these warning signals, Johnston and USGS coworkers were able to convince civil authorities to close Mount St. Helens to the public prior to the May eruption, which undoubtedly saved many lives. According to USGS volcanologist Wes Hildreth, Dave Johnston hoped that “systematic monitoring of fumarolic emissions might permit detection of changes characteristically precursory to eruptions” (Hildreth 1980).

I remember the Mount St. Helens eruption vividly for several reasons. One was that I met Dave Johnston several years before the eruption, when he interviewed for a faculty position in the Department of Geology at Stanford University (California, USA). I remember his interview talk, his stage fright, and how enthusiastic he was about his PhD work on the Cimarron Volcano in southwest Colorado (USA). The second reason was that I was teaching an introductory geology class to about 175 Stanford undergraduates during spring 1980. I had given a lecture on volcanoes the Friday before the Mount St. Helens eruption, including the Cascade Range volcanoes—talk about almost perfect timing! Most mornings, about 100 students would be in their seats awaiting the start of the 8 a.m. lecture, and another 60 or so would stumble in over the next 15-20 minutes, many still half asleep. However, at 8:00 a.m. on Monday, 19 May 1980, all 250 seats in the lecture hall in “Geology Corner” were occupied. I started that class by telling the students that what little I knew about the Mount St. Helens eruption largely came from talking with ­several USGS friends at the Menlo Park (California) office, and reiterating that this volcano, which had been inactive for the past 123 years, was part of the Cascade Range of mostly andesitic stratovolcanoes that have been active over the last 37 My because of subduction processes. These tend to erupt violently because of the higher viscosity and water content of their magmas, in contrast to basaltic shield volcanoes, such as in Hawaii.

The third reason the eruption of Mount St. Helens continues to occupy a prominent location in my memory is that my son, Michael, and his wife and two of my grandkids live in Bend (Oregon, USA) near Mount Bachelor and the Three Sisters Volcanoes (North, Middle, South), which are also part of the Cascade Range. South Sister has recently shown signs of tectonic uplift and, thus, may still be active (Dzurisin et al. 1997).

It’s been 36 years since Mount St. Helens erupted catastrophically, yet volcanologists still have a way to go in understanding how to predict volcanic eruptions and what triggers them. After reading the six articles in this issue of Elements, however, I see that significant progress has been made over the past 36 years in understanding the mechanisms of volcanic eruptions relative to the earlier views that I had (and taught) of magma accumulation, storage, and transport in Earth’s crust back in 1980. Bruce Marsh’s magma “mush column” hypothesis (Marsh 1981) and its application to large silicic systems (Bachmann and Bergantz 2004; Hildreth and Wilson 2007) represent major advances in our understanding of magma accumulation and rheology, as does the new understanding of the roles that magma storage and volatile phases play in triggering eruptions of different types. Moreover, the articles in this issue review ways of testing hypotheses about various triggering mechanisms based on integrated geological, geochemical, mineralogical, petrological, and geophysical studies both of ancient eruptions and of active volcanoes. In addition, the use of modern satellites to monitor deformation of volcanoes worldwide at millimeter to meter spatial resolution represents a major technological advance. There appears to be significant hope for developing new quantitative, and predictive, approaches to volcanic eruptions because we are understanding better the processes that trigger them. However, as pointed out by one of the articles in this issue, and the Perspective column, point to the “unknown unknowns” about how volcanoes work, and these will test our current understanding with regard to predicting the timing and style of future volcanic eruptions.

Prospective volcanologists reading this issue have much exciting work to look forward to.


Bachmann O, Bergantz GW (2004) On the origin of crystal-poor rhyolites: extracted from batholithic crystal mushes. Journal of Petrology 45: 1565-1582

Dzurisin D, Stauffer PH, Hendly JW III (1997) Living with Volcanic Risks in the Cascades. U.S. Geological Survey Fact Sheet 165-97, http://pubs.usgs.gov/fs/1997/fs165-97/

Hildreth W (1980) David Alexander Johnston, 1949-1980. In: Guides to Some Volcanic Terranes in Washington, Idaho, Oregon, and Northern California. US Geological Survey Circular 838, United States Geological Survey http://npshistory.com/publications/geology/circ/838/memoriam.htm

Hildreth W (2007) Quaternary Magmatism in the Cascades—Geologic Perspectives. U.S. Geological Survey Professional Paper 1744, http://pubs.usgs.gov/pp/pp1744/

Hildreth W, Wilson CJN (2007) Compositional zoning of the Bishop Tuff. Journal of Petrology 48: 951‐999

Marsh B (1981) On the crystallinity, probability of occurrence, and rheology of lava and magma. Contributions to Mineralogy and Petrology 78: 85-98

Newhall C, Dzurisin D (1988) Historical Unrest at Large Calderas of the World. U.S. Geological Survey Bulletin 1855, https://pubs.usgs.gov/bul/1855/report.pdf

Tilling RI, Topinka LJ, Swanson DA (1990) Eruptions of Mount St. Helens: Past, Present, and Future. U.S. Geological Survey, unnumbered series, https://pubs.usgs.gov/gip/msh/contents.html