A rock from northern Papua New Guinea records an unusual change during subduction: it was about 800°C at roughly 45 kilometres below the surface, then about 100°C colder after it descended beyond 90 kilometres. The finding challenges the expectation of a steadily increasing temperature with depth, but researchers have not established why the reversal occurred.
What the rock recorded
The rock formed part of a subduction zone, where the Australian and Pacific plates move toward one another and one plate carries rock downward. Curtin University’s 2026 account describes two approximate points in this rock’s thermal history:
| Approximate depth | Recorded temperature | What the source reports |
|---|---|---|
| About 45 km | About 800°C | Curtin University reports this as a temperature recorded at shallower depth; it provides no uncertainty range. |
| More than 90 km | About 100°C colder than the shallower condition | Curtin University reports the deeper condition comparatively, without a formal uncertainty range. |
These are approximate values, not exact readings with published error bars in the university release. The important result is the direction of the change: the rock was hotter at the shallower point and cooler after descending farther.
How researchers inferred the depth and thermal history
The evidence comes from tiny mineral inclusions trapped inside garnet in the rock. Curtin University says coesite indicates pressures consistent with depths of at least 90 kilometres, while zircon provides age information. Together, these inclusions helped researchers reconstruct the rock’s journey through different conditions underground.
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The university release does not detail the full analytical workflow or report the age result. The study is titled “Geothermal gradient change during subduction recorded by ultrahigh-pressure eclogite” and was published in Nature Geoscience. It was led by the University of Göttingen and involved researchers including Curtin University’s School of Earth and Planetary Sciences. Curtin University’s announcement identifies the paper’s DOI as 10.1038/s41561-026-02110-1.
Why might a deeper rock be cooler?
The reported pattern is unexpected, but the cause is unresolved. The researchers suggest two possible explanations:
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- Extra heat from intense shearing: friction or deformation where the plates meet could produce additional heat at a shallower level.
- A subduction zone that has not fully cooled: the zone may not yet have reached the lower temperatures expected at greater depth.
These are possibilities, not confirmed explanations. The release says the result challenges simple expectations about temperature with depth; it does not establish that either mechanism caused this rock’s thermal history or that the same pattern occurs in other subduction zones.
What the finding does—and does not—tell us
Subduction zones are associated with earthquakes and volcanic activity, and they transport rocks and elements deep into Earth as part of the long-term carbon cycle. Understanding how conditions change as material descends can help researchers study these processes over long timescales.
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This single rock does not, by itself, change earthquake or eruption forecasts, nor does the report quantify an effect on carbon-cycle estimates. The result is evidence about one rock’s recorded history and a challenge to current models of subduction-zone temperatures—not a prediction of a particular hazard.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What is not established in the published announcement
Curtin University’s release does not give the exact field locality, sample age, detailed mineral chemistry, thermobarometric method, formal uncertainty estimates, or comparisons among models. Those details should not be inferred from the approximate temperatures and depths reported in the announcement.
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