Minerals trapped within tiny crystals that have survived the grinding of continents for billions of years can help reveal the origins of plate tectonics, and perhaps even provide clues to how complex life arose on Earth. .
The theory of plate tectonics, which describes how the earth’s crust separates into plates that float and slide over a layer of malleable rock below, was widely accepted by science about 50 years ago. The process is believed to have shaped the world around us by allowing continents to form, huge mountain ranges to rise when they collide, volcanic islands to be created, and catastrophic earthquakes to be triggered.
But there is still a debate about how and when exactly, in our planet’s 4.5 billion years of history, the plates formed. Estimates range from less than a billion to 4.3 billion years ago.
It’s also unclear exactly how fast plate tectonics evolved, says Dr. Hugo Moreira, a geologist at the University of Montpellier in France. Did the Earth’s crust abruptly split into multiple plates and begin to move in just tens of millions of years, or was the process much more gradual, over hundreds of millions of years or more?
Understanding this could be crucial to understanding not only how the planet has evolved on its own, but also how life may have started on Earth. The conditions created by plate tectonics are believed to have helped make Earth hospitable to life in the first place and also provided the vital nutrients necessary for complex multicellular life to thrive.
The crystals time capsules
Dr. Moreira and his colleagues are searching for answers to these questions within tiny zirconium crystals , which are time capsules from Earth’s distant past due to their great robustness. Often, they are preserved in the rock despite the action of continuous meteorological and geological events.
Many of these crystals have been previously dated by analyzing the radioactive decay of the isotopes (different forms of elements) they contain. Some date as far back as 4.4 billion years ago, the earliest known fragments of the earth’s crust.
“That’s why zircon is amazing, because even though the rocks that make up the continents were destroyed, the zircon survived in the sedimentary record ,” Dr. Moreira said. Scientists have previously used zircon crystals to study the history of the continental crust. from Earth, but it has not yet been enough to provide a definitive consensus on how plate tectonics began, he says.
“After analyzing hundreds of thousands of them, we still don’t have an agreement,” said Dr. Moreira, a member of the MILESTONE project, led by Dr. Bruno Dhuime, a geoscience researcher at the French National Center for Scientific Research, also from the University of Montpellier.
The researchers hope to use these crystals, which are typically one-tenth of a millimeter or about the thickness of a human hair, to improve our understanding of time and the evolution of plate tectonics.
The MILESTONE group will drill on an even smaller scale (around one hundredth of a millimeter) to examine traces of apatite and feldspar minerals trapped within the zirconium crystals. The strontium and lead isotopes in these “inclusions” can add unprecedented detail about the source of zircon formation and whether this occurred in the various types of magma beneath stagnant or moving plates, says Dr. Moreira.
“It will be a crucial step towards a better understanding of how our planet evolved ,” he said. If we can measure the isotopic composition of these tiny inclusions, we could tell what was the composition of the rock from which the zircon crystallized. Perhaps we can understand. how the crust evolved at that point and in what kind of tectonic environments the magma formed. “
This small-scale analysis has been possible thanks to the installation of a laboratory that contains a specialized and highly sensitive mass spectrometer, equipment that measures the characteristics of atoms.
The team hopes to begin analyzing the samples next month, ultimately investigating inclusions in more than 5,000 zircons of different ages from around the world to build a large-scale image. “What we want to do is determine precisely when plate tectonics globalized rather than when it was localized to isolated points here and there ,” Dr. Moreira said.
Underground structures
At the opposite end of the scale, other researchers have been looking for clues to the origins of plate tectonics in two huge continental-size structures found deep underground under the Pacific and African plates.
These “thermochemical stacks” , mysterious structures located about 2,900 kilometers below the surface at the boundary between the core and the Earth’s mantle, were discovered in the 1990s with the help of seismic tomography, that is, obtaining images from seismic waves produced by earthquakes or explosions. They were detected as potentially warmer zones of material in which seismic waves travel at different speeds than the surrounding mantle , but there is still much debate about what exactly they are, including their composition, longevity, shape and origins.
Over the past two decades, there has been a “fiery” debate about its proposed link to movements on the planet’s surface, and thus its possible role in the emergence of plate tectonics, explained Dr. Philip Heron, Geoscientist who studied structures as principal investigator in the TEROPLATE project at Durham University. “These stacks are thought to have an impact on how material moves within the planet and therefore how the surface behaves over time,” he said. Events on the surface can in turn drive its activity.
One theory is that these mounds are stable over long geological periods and their edges correspond to the position of key features involved in plate tectonics on the Earth’s surface, such as supervolcanoes. However, their extreme depth makes these piles difficult to observe directly. “Since these structures are 100 times higher than Mount Everest, they may be the biggest things on our planet that we know the least about,” said Dr. Heron.
Supercomputer power
The TEROPLATE project harnessed the power of the supercomputer to investigate. Using more than 1,000 computers working in tandem, the team developed 3D models of the Earth to show how the putative chemical composition of large hot regions deep underground could influence the formation and location of deep mantle plumes.
However, their models indicated that the piles could be more passive in plate tectonics than was initially thought and that the world would continue to form similar geological features without them. “By looking at the position of large plumes of material that form supervolcanoes, our numerical simulations indicated that chemical heaps were not the controlling factor,” said Dr. Heron. But he added that these findings were not entirely conclusive and have also opened the door to other interesting avenues of research, such as exploring the implications that these structures are constantly moving through the mantle rather than being largely stationary. “It lends weight to the theory that chemical piles may not be rigid and fixed on our planet, and that deep Earth can evolve as easily as the continents on our surface move around,” he said. “It’s an impulse to start looking deeper.”
Some of the TEROPLATE results also indicate that the stacks may have been robust enough to survive Earth’s early beginnings. That makes it possible that they existed for the start of plate tectonics, and therefore had roles in the process that we don’t yet know about, Dr. Heron adds.
All of this could also have implications for understanding our own place on Earth. If, for example, plate tectonics evolved rapidly in the early years of Earth’s history, it could raise questions like why complex life didn’t emerge earlier or how closely linked they are, says Dr. Moreira.
Artículo original
This article was originally published in Horizon, the EU Research and Innovation Magazine
