A Papua New Guinea rock that sank more than 90 kilometers into Earth challenges theories of its interior

A small rock from a bay in Papua New Guinea has given geologists an unusual record of a trip more than 90 kilometers into Earth, and its temperature history does not match what current models predict. The University of Göttingen reports that the rock sank into a subduction zone about four million years ago, reached roughly 800 degrees Celsius, or 1,472 degrees Fahrenheit, at depth, and has since come back to the surface. Lead author Jan Schönig called the pattern of heating unexpected.

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The rock is an eclogite, a dense metamorphic rock formed at high pressure, and it came from an outcrop at Taleba Bay on Goodenough Island in Milne Bay Province. The study appeared in the journal Nature Geoscience, and ScienceDaily carried the university’s account on Oct. 21, about two weeks after the university issued its own press release.

How the rock records its depth

Göttingen’s press release says the evidence for the depth is a set of tiny coesite crystals, each smaller than one micrometer, trapped inside garnet. Coesite is a form of silica that forms only at extreme pressure, so finding it locked inside another mineral indicates the rock once sat at ultrahigh pressure. From those clues the team reconstructed the temperature history of the rock on the way down. The researchers conclude that the rock was subducted to at least 90 kilometers, a depth of about 56 miles. A photo caption in the release goes further and says the coesite proves a depth above 90 kilometers, while the main text uses the more careful language of a conclusion drawn from the evidence, and this article follows the main text.

The ScienceDaily account says the sample was examined with several advanced analytical techniques and does not name them, so the specific instruments and the exact uncertainties on the temperature estimates are not given here. Those details sit in the paper, which Schönig and seven co-authors wrote, and the figures quoted in the press materials are rounded.

The hot-to-cold temperature shift

The surprise is in the temperatures. At about 45 kilometers, half of the final depth, the rock had already reached about 700 degrees Celsius, or 1,292 degrees Fahrenheit, only about 100 degrees cooler than its peak near 800 degrees at 90 kilometers or more, with another 45 kilometers still to go. Temperature normally climbs much faster with depth in thermal models of subduction zones, and here it rose far more slowly through the second half of the descent.

Schönig, of the Geoscience Center at the University of Göttingen, described the finding in the ScienceDaily account as a change from hot to cold subduction and said it was unexpected, because it contradicts current thermal models of subduction zones. In the paper’s framing, the rock appears to have experienced hot conditions at shallow depths and then cooler ones as it went deeper, which is why the university titled its release “Two kinds of subduction recorded in one rock.”

Two explanations the authors offer, and what is still open

The study does not settle why the rock’s journey looks the way it does. According to the university’s account of the research, there are several possible explanations, and the team offers two as hypotheses. The first is that plates may generate more heat at shallow depths than previously thought, possibly through shearing stress along the contact between them.

The second is that the subduction zone had not existed long enough to cool down to the temperatures conventional models predict at that stage. The authors say both processes may also have contributed together, and the press materials do not rank the three possibilities. The two ideas point in different directions: one concerns how much heat the contact between plates generates, and the other concerns the age of the subduction zone itself, so they would carry different implications for other subduction zones if either held up. Testing them would take comparison with rocks from other places, which this single sample cannot supply.

That caution matters, because one sample from one island cannot show how common this behavior is. The release does not claim the pattern applies to subduction zones in general, and it subtitles its account as an unusual journey for the most recently formed ultrahigh-pressure eclogite, which makes the sample a comparatively young record of the process and one that formed only about four million years ago. The authors say the work supports a better understanding of subduction and its links to earthquakes, volcanism and the long-term carbon cycle, and the research was funded by the German Research Foundation.

The paper, titled “Geothermal gradient change during subduction recorded by ultrahigh-pressure eclogite,” lists Schönig, Silvio Ferrero, Kerstin Gresky, Dominik Sorger, Axel Schmitt, Bernd Wunder, Suzanne Baldwin and Adam Holt as authors, and its DOI record is the place to check the full data. Schönig works in the sedimentology group of the Geoscience Center in Göttingen.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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