Rock from Papua New Guinea cooled as it moved deeper underground, study finds

An international research team studied a rock from northern Papua New Guinea, where the Australian and Pacific tectonic plates move toward each other, causing rocks to be carried deep underground through a process known as subduction, according to Phys.org. The researchers found the rock experienced temperatures of about 800°C (1,472°F) at a depth of 45 kilometers (28 miles) below the surface. It became about 100°C colder after reaching depths of more than 90 kilometers (56 miles).

Study co-author Dr. Axel Schmitt of Curtin University said the finding was unexpected because temperatures would normally be expected to gradually increase with depth. "At about 45 kilometers (28 miles) deep, the rock was surprisingly hot, but as it traveled deeper, it entered an environment that was cooler than we would normally expect at that depth," Schmitt said.

The researchers reconstructed the rock's history by analyzing tiny inclusions of the minerals coesite and zircon trapped inside the mineral garnet, according to Phys.org. Coesite forms under extremely high pressures, providing evidence the rock reached depths of at least 90 kilometers (56 miles). Zircon provided age information on when this happened.

Schmitt said the findings challenged current models of the thermal conditions within subduction zones. He said one possibility is that intense shearing where the tectonic plates meet generates additional heat at relatively shallow depths. Another possibility, he said, is that the subduction zone had not yet cooled to the lower temperatures expected.

Schmitt said subduction zones are sites of earthquakes and volcanic activity and play an important role in Earth's long-term carbon cycle, carrying rocks and elements deep into the planet. He said the findings suggest the thermal structure of these zones may be more complex than previously thought. "Understanding these temperature changes helps us build a clearer picture of the extreme conditions deep inside Earth, where tectonic plates collide and drive processes that shape the planet's surface," he said.

The study, by Jan Schönig and colleagues, was published in Nature Geoscience in 2026, with DOI 10.1038/s41561-026-02110-1. Phys.org reported the discovery of the unexpectedly hot rock challenges scientists' understanding of how heat is distributed deep inside the planet, with potential implications for how they understand earthquakes, volcanic activity and the movement of carbon through Earth.