Beneath the restless surface of the modern world lies a slow, long memory written in ancient stone. A team studying more than a thousand rocks from Australia, Africa, North America and Europe has found that a chemical signature carried by the earliest continental crust fades away around two and a half billion years ago, as though the planet's habit of building land had changed its nature. Read this way, the finding suggests that the great slow dance of tectonic plates, which raises mountains and shapes the conditions for life, may have taken up its rhythm at that moment, even though what first
A Chemical Fingerprint in Ancient Rocks Points to Plate Tectonics Starting 2.5 Billion Years Ago
Around 2.5 billion years ago, that fingerprint vanishes.
Mark
Why is it so hard to say when plate tectonics started?
Mimi
Because tectonics erases its own history. Colliding plates drag old crust into the mantle, and the mountains they raise erode away. What's left is mostly chemistry inside surviving rocks.
Mark
And what chemistry did they use?
Mimi
A trace left when new crust forms by melting older crust. They compare it to inherited DNA, and it survives in feldspar and some sulfide minerals.
Mark
What did they find?
Mimi
Rocks older than about 2.5 billion years carry the signature of crust formed around four billion years ago. Then it vanishes, in more than a thousand rocks across four continents.
Luke
Careful with the wording, though. The disappearance is the observation. Calling it the start of plate tectonics is the authors' interpretation, and the source itself says tectonics "may have" begun then.
Mimi
Fair. They do cite other changes at the same time: crust thickness and composition, rock chemistry, and the first supercontinent.
Luke
Still, those are listed rather than detailed here, so I can't judge how tightly they tie to this one fingerprint.
Mark
Does the study say what caused the shift?
Mimi
No. The trigger is unknown, and the team says they're still working on it.
Luke
And the dataset is a patchwork of newly analysed and previously published rocks. Whether the sampling covers enough of the planet isn't something this piece lets us check.
Der Puls
- A chemical fingerprint that has survived billions of years suddenly disappears from the rock record about 2.5 billion years ago.
- Researchers sifting through more than 1,200 rocks across four continents found no trace of the ancient crust signature in younger samples.
- The authors read the vanishing as a shift from recycling one ancient reservoir of crust to blending old and new material, a hallmark of modern plate edges.
- The question of what ignited plate tectonics is still unresolved, and the team is pressing on to find the trigger.
Beneath the restless surface of the modern world lies a slow, long memory written in ancient stone. A team studying more than a thousand rocks from Australia, Africa, North America and Europe has found that a chemical signature carried by the earliest continental crust fades away around two and a half billion years ago, as though the planet's habit of building land had changed its nature. Read this way, the finding suggests that the great slow dance of tectonic plates, which raises mountains and shapes the conditions for life, may have taken up its rhythm at that moment, even though what first set it moving remains an open question.
Finding the moment when Earth's surface first began to move is hard, because the same forces that build continents also grind their history away. Plate collisions drag old crust back into the mantle, and mountains wear down into the sea. What sometimes survives is a chemical inheritance, locked into minerals such as feldspar and certain sulfides, that carries the memory of older rock into newer rock much as DNA passes from parent to child.
A research team set out to read that inheritance across more than 1,200 rocks drawn from ancient landmasses in Australia, Africa, North America and Europe, spanning nearly four billion years. Rocks older than about 2.5 billion years carry the imprint of very ancient crust, possibly formed not long after Earth itself, which was melted and remelted for well over a billion years to build the early granite-rich continents. Then the signature stops. Hundreds of younger rocks never show it again.
The authors interpret that silence as a turning point. Earlier, the same old material kept cycling with little fresh input. Afterward, crust began forming from a mixture of old and new sources, the pattern seen today where volcanoes draw up new magma while older crust is recycled. Shifts in rock chemistry, crustal thickness and the formation of Earth's first supercontinent point the same way.
What set plate tectonics in motion remains unknown, and the researchers are still searching for that cause. For now, their work offers a clear marker in time, one that may mark when the planet's living surface began to take shape.
The evidence for when Earth's surface began to move is hard to find, because the process that built the modern planet is also very good at destroying its own record. A team of researchers has now tried to get around that problem by reading a chemical trace that survives in some of the oldest rocks on Earth, and they report that it disappears abruptly about 2.5 billion years ago.
The stakes are considerable. Plate tectonics, the movement of enormous slabs of crust, builds mountain ranges, triggers earthquakes, and shapes oceans and continents. Over deep time it also helps regulate Earth's climate and magnetic field. Many scientists think it supplies the long-term stability that keeps the planet life-friendly, in contrast to tectonically "dead" worlds such as Venus and Mars. Mountains erode, and that erosion supplies nutrients that feed marine life. For much of its early history, the authors note, Earth may have been mostly ocean, without colliding plates to raise tall mountains.
Whether tectonics has run through all of Earth's 4.5-billion-year history or switched on at some point has been argued for a long time. The difficulty is that where plates collide, old crust is dragged back down into the mantle, and crust pushed up into mountains wears away. Often what survives is a chemical fingerprint locked inside rocks that have endured for billions of years.
The fingerprint the team studied comes from how new continental crust forms. It does not always crystallise fresh from mantle magma. Often it is made by melting older rock already sitting in existing crust, and that leaves a lasting chemical trace in the new rock, a bit like DNA inherited from parents. In some minerals, specifically feldspar and certain sulfide minerals, the trace can survive largely undisturbed for billions of years.
The researchers analysed more than 1,200 rocks. Some were newly analysed by the team, and others were compiled from earlier studies. They came from ancient landmasses across Australia, Africa, North America and Europe, and together they span nearly four billion years of Earth's history.
The pattern was striking. Rocks older than about 2.5 billion years carried the signature of very ancient crust that appears to have formed around four billion years ago, not long after Earth itself formed. That material was melted and remelted repeatedly for well over a billion years, producing the granite-like rocks that made up much of the earliest continents. Then, around 2.5 billion years ago, the fingerprint vanishes. The team searched hundreds of younger rocks and never found the signature of those ancient crustal reservoirs again.
The authors interpret the disappearance as a fundamental reorganisation in how Earth builds continents. Before 2.5 billion years ago, their results suggest, the same ancient material was melted over and over with little input from other sources. Afterward, the planet began making crust from a mix of old and new sources, which is what is seen today at plate edges, where volcanoes bring fresh molten rock up from the mantle while older crust is recycled. On that reading, plate tectonics may have begun at that point.
They point to other changes around the same time: shifts in rock chemistry, alterations in the thickness and composition of the crust, and the formation of Earth's first supercontinent. Taken together, they argue, these mark a turning point in how the planet works, one that helped create the surface environment life needed.
What set tectonics in motion is still unknown. The authors say they are continuing to chase that answer, and for now present their result as one more piece of the puzzle.
Bemerkenswerte Zitate
The authors believe the shift 2.5 billion years ago is when plate tectonics may have begun, though the trigger remains unknown.— Study authors, The Conversation
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