An international research team led by the University of Göttingen found that the rock traveled more than 90 kilometers underground approximately four million years ago. During its descent, it experienced unusually high temperatures at relatively shallow depths before entering conditions associated with a comparatively cold type of subduction.

The discovery raises new questions about how heat is distributed inside Earth and how temperatures change as tectonic plates descend.

Rock preserves clues from deep inside Earth

Earth’s outer shell is divided into giant tectonic plates that move slowly across the planet. When two plates converge, one can be forced beneath the other and descend into Earth’s interior. This process is known as subduction.

Subduction zones play a major role in shaping the planet. They are associated with powerful earthquakes and volcanic activity, while also helping regulate Earth’s climate over millions of years by recycling materials such as carbon dioxide within the planet’s interior.

The immense pressure in subduction zones can transform rocks into a distinctive red and green material known as eclogite. Researchers used several advanced analytical techniques to study one such rock from Papua New Guinea.

Inside the rock, they discovered tiny crystals of coesite, a mineral that forms only under extremely high pressure. Because these crystals become trapped inside other minerals as rocks form, they preserve valuable evidence of the conditions the rock experienced deep underground.

By examining these microscopic clues, scientists determined that the rock descended at least 90 kilometers below Earth’s surface, where temperatures reached approximately 800 degrees Celsius (1,472 degrees Fahrenheit).

Rock records an unexpected shift from hot to cold subduction

Researchers reconstructed the rock’s temperature history and found that it had already reached about 700 degrees Celsius (1,292 degrees Fahrenheit) at a depth of approximately 45 kilometers (28 miles).

In other words, the rock was only about 100 degrees cooler at half its maximum depth, even though it still had another 45 kilometers to descend.

This result was unexpected because existing models predicted a different distribution of heat within the subduction zone. The rock appears to have experienced unusually high temperatures at shallow depths before entering an environment with a relatively cold thermal pattern farther below. Its temperature did increase with depth, but the rate of warming was much lower than scientists typically expect.

“The discovery of this change from hot to cold subduction was unexpected, as it contradicts current thermal models of subduction zones,” explains Dr. Jan Schönig from the Geoscience Center of the University of Göttingen.

Scientists have previously identified rocks containing evidence of unexpectedly high temperatures. However, this is the first time researchers have been able to reproduce such a clear transition between different thermal conditions as a rock descended from shallow to deeper parts of a subduction zone.

What could explain Earth’s hidden heat?

Researchers have identified several possible explanations for the unusual temperature patterns.

One possibility is that tectonic plates generate substantially more heat at relatively shallow depths than scientists previously believed. As plates move against each other, intense shear forces can produce additional heating along their boundaries.

Another explanation is that the subduction zone was not yet mature enough to reach the lower temperatures predicted by conventional models.

Both processes may have contributed to the rock’s unusual thermal history.

Why subduction temperatures matter

Understanding temperature differences within subduction zones is important because these regions influence some of Earth’s most powerful geological processes. More accurate thermal models could help scientists better assess conditions linked to earthquakes and volcanic activity.

The discovery may also improve understanding of major geochemical cycles, particularly the long-term carbon cycle. This cycle helps regulate Earth’s climate by moving carbon between the surface and the planet’s interior.

By revealing how conditions change as tectonic plates descend, the study gives scientists a new way to investigate how subduction has operated over millions to billions of years and how these processes continue to shape Earth today.

This study was funded by the German Research Foundation (DFG).