Deep within a small piece of eclogite from Papua New Guinea, sealed crystals of coesite have kept a record of a journey no human could witness: a slab of Earth's crust that plunged more than 90 kilometers into the planet about four million years ago. Researchers led by Göttingen University have read that record and found a rhythm of heat that does not match what the models predicted, with the rock arriving hot at shallow depth and then warming only slowly as it sank further. In a planet whose surface quakes, erupts, and slowly breathes carbon through its interior, such an unexpected pattern is
A Rock Sunk 90 Kilometers Into Earth Revealed Hotter-Than-Expected Subduction Conditions
Only about 100°C cooler at half its maximum depth
Mark
Why does one rock from Papua New Guinea matter this much?
Mimi
Because it recorded its own journey. Tiny coesite crystals, trapped inside another mineral, show it went at least 90 kilometers down at about 800°C.
Mark
And the odd part is the temperature along the way?
Mimi
Yes. At 45 kilometers it was already near 700°C, only about 100°C cooler than at its deepest, with another 45 kilometers still to descend.
Luke
Is that a measurement or a reconstruction? Those are different things.
Mimi
A reconstruction, built from the mineral evidence using several analytical techniques. It's the team's inference of the thermal history, not a thermometer reading.
Mark
So what explains it?
Mimi
Two ideas. Plates may produce more shear heating at shallow depth than thought, or the subduction zone was too young to have cooled down. Possibly both.
Luke
So the cause is still open. The reporting doesn't pick one.
Mimi
Correct. What's reported is that the pattern contradicts current thermal models.
Luke
And it's one rock. Does it say anything about subduction zones generally?
Mimi
The source doesn't claim that. It calls this the first clear hot-to-cold transition reconstructed, which is why more samples will matter.
Mark
And the link to earthquakes and climate?
Mimi
Better models could help assess earthquake and volcano conditions and the carbon cycle, but that's a potential benefit, not a result yet.
Il Polso
- A rock that sank 90 kilometers into Earth has defied the temperature models scientists rely on to explain how tectonic plates behave underground.
- Within the rock, microscopic coesite crystals act as sealed time capsules, revealing it reached about 800°C at its deepest point.
- By the halfway mark near 45 kilometers, the rock was already around 700°C, leaving almost no room for the gradual heating that models expect.
- Researchers are weighing two explanations, extra shallow heat from grinding plate boundaries or a subduction zone too young to have cooled, and will test both against other rocks.
- The outcome could sharpen forecasts of earthquakes and volcanoes and deepen understanding of how carbon is recycled through Earth over geological time.
Deep within a small piece of eclogite from Papua New Guinea, sealed crystals of coesite have kept a record of a journey no human could witness: a slab of Earth's crust that plunged more than 90 kilometers into the planet about four million years ago. Researchers led by Göttingen University have read that record and found a rhythm of heat that does not match what the models predicted, with the rock arriving hot at shallow depth and then warming only slowly as it sank further. In a planet whose surface quakes, erupts, and slowly breathes carbon through its interior, such an unexpected pattern is a reminder that the machinery beneath our feet still holds surprises worth patiently learning.
Inside a red and green rock from Papua New Guinea, researchers found crystals too small to see without instruments, and those crystals have begun to rewrite part of the story of how heat moves through the planet. The rock is eclogite, and it once descended more than 90 kilometers beneath the surface before being carried back up.
The study was led by Göttingen University with an international team and published in Nature Geoscience, supported by the German Research Foundation. The rock's journey took place roughly four million years ago, during subduction, when one tectonic plate slides beneath another and sinks into the interior.
Using several advanced analytical methods, the team found tiny coesite crystals trapped within another mineral. Coesite forms only under extreme pressure, so these crystals preserved proof of the conditions the rock endured. They pointed to depths of at least 90 kilometers, where temperatures reached about 800°C.
The surprise lay in the earlier stretch of the journey. At about 45 kilometers, the rock was already near 700°C, only about 100°C cooler than at its deepest point, yet it still had nearly as far to travel. Its temperature rose with depth, but far more gently than scientists would normally expect.
Dr. Jan Schönig of Göttingen University's Geoscience Center described the shift from hot to cold subduction as unexpected, since it runs against current thermal models. The team proposes two possibilities: plates may generate far more heat near the surface through intense shearing, or the subduction zone may not have been active long enough to cool as conventional models assume. Both could be at work.
The implications extend well beyond this single rock. Subduction zones drive powerful earthquakes and volcanoes and help regulate climate by recycling carbon dioxide through the interior. Sharper temperature models could improve how scientists assess these hazards and the long-term carbon cycle. The next step is testing whether these explanations hold across other rocks and other subduction zones.
Inside a red and green rock from Papua New Guinea, researchers found crystals too small to see without instruments, and those crystals have rewritten part of the story of how heat moves through the planet. The rock is a piece of eclogite, and it once sank more than 90 kilometers (56 miles) beneath the surface before being brought back up.
The work was led by Göttingen University and involved an international team. It was published in Nature Geoscience, with funding from the German Research Foundation (DFG). The rock's descent happened roughly four million years ago, during subduction, the process in which one tectonic plate is forced under another and sinks into Earth's interior.
The researchers studied the sample with several advanced analytical techniques. Inside it they found tiny crystals of coesite, a mineral that forms only under extremely high pressure. The crystals had been trapped within another mineral as the rock formed, which preserved a record of the conditions it met underground. From that evidence, the team concluded the rock had reached at least 90 kilometers down, where temperatures hit about 800°C (1,472°F).
The surprise came when they reconstructed the earlier part of the trip. At about 45 kilometers (28 miles), half of its maximum depth, the rock had already reached around 700°C (1,292°F). It was only about 100°C cooler there than at its deepest point, even though it still had another 45 kilometers to go. Its temperature did keep rising with depth, but far more slowly than scientists would normally expect.
Existing models predict a different distribution of heat inside subduction zones. The rock appears to have passed through unusually hot conditions at shallow depth, then entered an environment with the cooler temperature pattern of a relatively cold subduction zone. Dr. Jan Schönig of Göttingen University's Geoscience Center said that finding a change from hot to cold subduction was unexpected, because it contradicts current thermal models. Other rocks have previously shown evidence of unexpectedly high temperatures. This is the first time researchers have reconstructed so clear a transition between thermal conditions as a rock moved from shallow to much greater depth.
The team offers two possible explanations. Tectonic plates may generate considerably more heat at shallow depths than assumed, with intense shearing along their boundaries as they grind against each other. Or the subduction zone may simply not have existed long enough to develop the cooler temperatures that conventional models predict. Both processes could also have played a part.
The stakes reach beyond one rock. Subduction zones are linked to powerful earthquakes and volcanic activity, and they help regulate climate over millions of years by recycling materials such as carbon dioxide through Earth's interior. More accurate temperature models could help scientists better evaluate the conditions behind earthquakes and eruptions, and refine their picture of the long-term carbon cycle.
The researchers say the finding gives them a new way to investigate how subduction has operated over millions to billions of years. The next step is testing whether the two explanations hold up against other rocks and in other subduction zones.
Citazioni salienti
Finding this change from hot to cold subduction was unexpected as this contradicts our current thermal models of subduction zones.— Dr. Jan Schönig, Göttingen University Geoscience Center
Vuoi la storia completa?
Leggi l'originale su Science Daily ↗
Science & Space