A rock plunged 90 kilometers into Earth and revealed something unexpected

An international research team led by Göttingen University found that the rock traveled more than 90 kilometers (56 miles) underground about four million years ago. Along the way, it experienced unusually high temperatures at shallower depths before entering conditions associated with a relatively cooler type of subduction.

The discovery raises new questions about how heat is distributed inside Earth and how tectonic plates behave as they descend. The findings were published in Nature Geoscience.

A Strange Discovery Deep Beneath Earth's Surface

Earth's outer shell is divided into enormous tectonic plates that slowly move across the planet. When two plates converge, one can be forced beneath the other and sink into Earth's interior. This process is called "subduction."

These regions play a major role in shaping our planet. They are associated with powerful earthquakes and volcanic activity, but they also help regulate Earth's climate over millions of years by recycling materials such as carbon dioxide through the planet's interior.

The immense pressure found in subduction zones can transform rocks into a distinctive red and green material known as "eclogite." Researchers examined one of these rocks from Papua New Guinea using several advanced analytical techniques.

Inside the rock, they discovered tiny crystals of coesite, a mineral that can form only under extremely high pressure. These crystals had become trapped within another mineral as the rock formed, preserving valuable evidence of the conditions it experienced deep underground.

By studying these microscopic clues, the scientists determined that the rock had descended at least 90 kilometers beneath Earth's surface, where temperatures reached approximately 800°C (1,472°F).

Even more surprising was what they uncovered about its earlier journey.

An Unexpected Shift From Hot to Cold Subduction

The researchers reconstructed the rock's temperature history and found that at a depth of approximately 45 kilometers (28 miles), it had already reached temperatures of around 700°C (1,292°F).

That meant the rock was only about 100°C cooler at half its maximum depth, even though it still had another 45 kilometers to descend.

This was unexpected because existing models predict a different distribution of heat within subduction zones. The rock appears to have experienced unusually hot conditions at shallower depths before entering an environment with a relatively cooler temperature pattern farther down. Although its actual temperature increased with depth, the rate of warming was much smaller than scientists would normally expect.

Dr. Jan Schönig at Göttingen University's Geoscience Center explains: "Finding this change from hot to cold subduction was unexpected as this contradicts our current thermal models of subduction zones."

Scientists have previously discovered rocks containing evidence of unexpectedly high temperatures. However, this is the first time researchers have been able to reconstruct such a clear transition between different thermal conditions as a rock descended from shallower to much greater depths.

What Could Explain Earth's Hidden Heat Mystery?

The researchers have identified several possible explanations for the unusual temperature pattern.

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

Another explanation is that the subduction zone had not yet existed long enough to develop the cooler temperatures predicted by conventional models.

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

Understanding these temperature differences is important because subduction zones influence some of Earth's most powerful geological processes. More accurate models could help scientists better evaluate the conditions associated with earthquakes and volcanic activity.

The findings could also improve our understanding of major geochemical cycles, particularly the long-term carbon cycle, which 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 discovery offers scientists a new way to investigate how subduction has operated over millions to billions of years and how these processes continue shaping Earth today.

The research received funding from the German Research Foundation (DFG).