Mysterious bubbles inside the Earth have triggered plate tectonics | Geology

About 4,500 million years ago, a Mars-sized object slammed into the newly formed Earth, creating our own Moon. Now, a team of scientists proposes that this giant impact did even more: the collision left behind mysterious bubbles in the Earth’s interior that may have helped usher in plate tectonics – the geological process that feeds earthquakes, volcanoes and, in general, allows the existence of life on our planet.

The idea, developed using computer models in a new study published in the journal Geophysical Research Lettersis an attempt to answer one of the most fundamental questions about our planetary home.

We live on the only planet known to have a surface made of rocky plates that slide and collide against each other in the boundary zones as the superheated interior moves. This subterranean drama normally goes unnoticed on the scale of human time, except when an earthquake occurs or a volcano erupts. But most experts agree that this process is absolutely essential for life as we know it, because it helps the planet cycle carbon, which is important for maintaining a habitable climate.

What experts do not agree on is the origin of plate tectonics.

The new scientific article builds on an earlier idea that attempts to explain a geological enigma. For decades, geologists have speculated about mysterious bubbles deep within the Earth’s mantle detected through seismic images. These huge, dense bubbles appear to be made of different material from the surrounding mantle, raising questions about what they are and how they formed.

A hypothesis – published in the magazine Nature last year – offers an explanation, but also raises some questions. It suggests that, after the object that formed the Moon collided with our planet, pieces of that object ended up intact inside the Earth.

The new paper takes this idea a step further: About 200 million years after the impact, these submerged bubbles may have helped create hot plumes deep inside Earth that disrupted the newly formed surface, rupturing the crust and allowing plates to form. rings to sink – a process called subduction.

This process, say the study authors, may explain the fact that the oldest minerals on Earth are zircon crystals that appear to have been subducted more than 4 billion years ago. Furthermore, they suggest that it may have contributed to the emergence of modern plate tectonics.

“The giant impact is not only the reason for our Moon’s existence, if that is the case, but it also established the initial conditions for our Earth”said Qian Yuan, a geoscientist at the California Institute of Technology (USA) and one of the authors of the paper.

A time almost impossible to know?

Geologists who were not involved in the research say this model is intriguing, but it raises a number of questions.

Taras Gerya, geoscientist at the Federal Institute of Technology in Zurich (Switzerland), said, for email, that the idea of ​​an initial subduction phenomenon triggered by vigorous mantle plumes shortly after the impact that formed the Moon is credible and is supported by models and some geochemical data. But he added that he is not sure whether this would have led to modern plate tectonics or whether it would have resulted in rapid global recycling of the entire crust, similar to what may have happened on the inhospitable planet Venus.

Michael Brown, a geoscientist at the University of Maryland, said it is not clear how a circular subduction zone would lead to global plate boundaries and the mosaic of rocky plates that exist in modern plate tectonics.

“We have to take into account that there is not enough evidence to really know what the tectonic mode was in the Archaic”an eon that lasted between 4000 and 2500 million years ago, he also said Michael Brown. “Therefore, from a philosophical point of view, it is almost certainly unknown and unknowable. I think that sometimes this point is lost.”

T. Mark Harrison, a research professor at the University of California, Los Angeles, recently wrote an article titled “We don’t know when plate tectonics began”. He questioned the model’s assumptions, pointing out geochemical inconsistencies that cast doubt on the giant impact theory itself.

“If we didn’t have plate tectonics, we wouldn’t be having this conversation, because our species wouldn’t have emerged.”he said Mark Harrison. “The best thing I can tell people is that my generation (…) failed to solve the most interesting question in science, which is knowing how and under what conditions life arose.”

For future scientists, Mark Harrison has a message: “We leave you a small wrapped gift.”