A rock from northern Papua New Guinea records a surprising thermal reversal during subduction: it experienced about 800°C at roughly 45 kilometres underground, then about 100°C colder conditions after descending beyond 90 kilometres. The result challenges simple expectations about how temperature changes with depth, but the cause remains uncertain.
What did the rock record?
The rock’s reconstructed history shows it was hotter at a shallower point in its descent and cooler after reaching greater depth. Curtin University reports approximately 800°C at about 45 kilometres below the surface, followed by conditions roughly 100°C colder at depths greater than 90 kilometres. The release does not provide uncertainty ranges, so these figures should be read as approximate.
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The finding comes from a rock carried downward in a subduction zone in northern Papua New Guinea, where the Australian and Pacific plates move toward one another. The result is a record from one rock, not evidence that every subduction zone follows the same temperature pattern.
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How did researchers infer the rock’s conditions?
Researchers found tiny inclusions of coesite and zircon trapped inside garnet. According to Curtin University, coesite indicates pressures consistent with depths of at least 90 kilometres, while zircon provides age information. The university release does not describe the full analytical workflow or report the age result.
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The study was led by the University of Göttingen and included researchers from Curtin University’s School of Earth and Planetary Sciences. Its paper, “Geothermal gradient change during subduction recorded by ultrahigh-pressure eclogite,” was published in Nature Geoscience (DOI: 10.1038/s41561-026-02110-1). Curtin University’s report quotes co-author Dr Axel Schmitt: “Finding this change from relatively hot conditions at shallower depth to colder conditions deeper down was unexpected.”
Why could a deeper rock be cooler?
The release offers two possible explanations, not a confirmed cause. One is that intense shearing where the plates meet generated extra heat at shallower depth. Another is that the subduction zone had not yet cooled to the lower temperatures expected. Neither possibility is established as the mechanism behind this rock’s thermal history.
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The observation challenges simple subduction-zone thermal models that would not anticipate this reversal. It does not show that temperature always falls with depth or that a single explanation applies across different subduction zones. The detailed model comparisons and uncertainty estimates are not given in the university release.
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Subduction zones are associated with earthquakes and volcanic activity, and they carry rocks and elements deep into Earth as part of the long-term carbon cycle. The study may help researchers understand how subduction operates over long timescales, but this one rock does not itself change earthquake or eruption forecasts, nor does it establish a new carbon-cycle estimate. Curtin University describes those broader roles as context for why subduction matters.
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