Slab windows through time

by Arianna Soldati, GSA Science Communication Intern – Ph.D. Candidate in Volcanology University of Missouri

Plate tectonics – the notion that the outermost shell of our planet is broken into a dozen major mobile plates – is a foundational idea in geology. It is a very elegant theory, and – at the grand scale – a simple one that most people have at least heard of. However, the detailed concepts necessary to fully explain geologic occurrences are complex and largely unfamiliar to the general public.

Derek Thorkelson, a geology professor at Simon Fraser University in Burnaby, British Columbia, focuses on one of those concepts: slab windows. In plate tectonics, when two plates collide the denser plate dives, or subducts, underneath the other. A slab window occurs where a subducting plate tears or breaks-off, or where two plates subduct along a spreading ridge. This process alters mantle circulation and melting processes in the region, changing the chemistry of magmatism observed at the surface. From British Columbia to California to the Basin and Range, many volcanoes are slab window-related. Hence, understanding slab windows is key to assessing volcanic hazards in our continent. “The Cenozoic development of the Americas is largely due to slab windows,” points out Thorkelson.

Figure: An overview of ridge-trench interactions in modern and ancient settings. Credit: Sisson, V.B., Pavlis, T.L., Roeske, S.M. and Thorkelson, D.J., 2003. In: V.B. Sisson, S. Roeske, T.L. Pavlis (Editors), Geology of a transpressional orogen developed during ridge-trench interaction along the North Pacific margin. Geological Society of America Special Paper 371, pp. 1-18.

Yet most people have never even heard of slab windows, and struggle to grasp this concept. Lay or non-scientific audiences are typically satisfied with a simplified understanding of plate tectonics and thus not concerned about integrating it with the additional complexity of slab windows. Geologists also did not take on to slab windows well initially.

Plate tectonics was widely accepted by the scientific community in the 1970s, but when Bill Dickinson, geology professor at the University of Arizona, introduced the concept of slab windows in 1979, his idea was met with suspicion. Geologists did not appreciate having their fundamental understanding of plate tectonics challenged.

Thorkelson got involved in 1989. As a master’s student working on Eocene rocks in southern British Columbia, he was faced with anomalous rock compositions that the tectonic understanding of the time could not explain. Slab windows could.

But working on slab windows entailed fighting an uphill battle. There was still a real pushback to the idea from the scientific community, and it would continue for almost 15 years. Thorkelson was harshly criticized when giving talks on the topic. “Ideas are dangerous,” sighed Thorkelson, recalling those times early in his career.

“Throughout it all Bill, who was so successful in so many ways, probably giggled to himself, knowing his colleagues would have to come around,” reflected Thorkelson. In 1992 Dickinson sent Thorkelson – a Ph.D. student at the time – a letter. “It was a short, handwritten letter, in which he thanked me for my contributions to the field.” It was a defining moment, the one that drove Thorkelson to continue pursuing this line of research.

Finally, in the 2000s there was mounting geochemical and seismic evidence for the existence of slab windows, and the geologic community did come around. Like many brilliant ideas, slab windows stood the test of time and became a successful paradigm. Thorkelson finally no longer has to talk people into it – “it’s very nice.”

At the Geological Society of America 2017 Annual Meeting in Seattle, slab windows were under the spotlight, and Thorkelson was an invited speaker, introduced as a pioneer. In his talk, titled “Slab windows: from understanding the Cenozoic to solving the Precambrian,” the collaborative nature of his science shone through. “You have to be willing to let other people modify your research to move forward,” said Thorkelson, indicating the integration of different observational and modeling techniques as the next step in slab windows research. “Never doubt your ability to make an impact in a field,” he added. “I wasn’t trained in slab windows – nobody was! – and I was still able to make meaningful contributions to it.”

The GSA Science Communication Internship was a program offered at the GSA Annual Meeting in Seattle, WA, designed for student attendees interested in science communication as a possible alternative career path.  Interns were paired with GSA’s Science Communication Fellow in order to gain experience in making science clear and exciting, under the tutelage of a professional writer.  Students were assigned to conduct interviews with presenters at the meeting and to compile summaries capturing the significance of the presenters’ work for a non-technical audience.  Media assignments and mentoring were useful learning experiences and exposure opportunities for students seeking to expand their knowledge into geoscientific reporting.