2023-06-28 06:38:37
Plate tectonics involves the horizontal movement and interaction of large plates on Earth’s surface. New research indicates that the movement of tectonic plates – believed to be necessary to create a habitable planet – has not occurred on Earth for 3.9 billion years. Credit: University of Rochester Illustration/Michael Osadcio A study from the University of Rochester revealed, using zircon crystals, that plate tectonics was inactive during the period when life first appeared on Earth. Instead, a “stagnant cap” mechanism was actuated, releasing heat through surface cracks. This discovery challenges the traditional belief that plate tectonics is essential to the origin of life, which may reshape our understanding of the conditions necessary for life on other planets. Scientists have traveled back in time to unravel the mysteries of Earth’s early history, using tiny mineral crystals called zircons to study plate tectonics billions of years ago. The research sheds light on the conditions that existed in the early Earth, revealing a complex interplay between Earth’s crust, life’s core, and the emergence of life. Plate tectonics allows heat from Earth’s interior to escape to the surface, forming the continents and other geological features needed for the emergence of life. As a result, “there was an assumption that plate tectonics is essential to life,” says John Tarduno, a professor in the University of Rochester’s Department of Earth and Environmental Sciences. But new research casts doubt on this assumption. Tarduno, Junior Professor of Geophysics William R. Keenan, who is the lead author of an article in the journal. Examining the nature of plate tectonics 3.9 billion years ago, when scientists believe the first traces of life appeared on Earth. The researchers found that tectonic plate movement did not occur during this time. Instead, they discovered, the Earth releases heat through what is known as a stagnant mantle system. The results indicate that although plate tectonics is a major factor in sustaining life on Earth, it is not necessary for life to arise on an Earth-like planet. “We discovered that there was no plate tectonics when life first appeared, and there was no plate tectonics for hundreds of millions of years after that,” Tarduno explains. “Our data suggests that when we look for exoplanets that harbor life, planets do not necessarily need plate tectonics.” An unexpected twist from a zircon study The researchers did not originally plan to study plate tectonics. “We study the magnetization of zircons because we study Earth’s magnetic field,” Tarduno says. Zircons are small crystals that contain magnetic particles that can lock in the Earth’s magnetism at the time of formation of the zircons. By dating the zircons, researchers can create a timeline to track the evolution of Earth’s magnetic field. The strength and direction of Earth’s magnetic field changes with latitude. For example, the current magnetic field is stronger at the poles and weaker at the equator. Armed with information about the magnetic properties of zircons, scientists can deduce the relative latitudes at which the zircons formed. That is, if the efficiency of the geodynamo – the process that generates the magnetic field – is constant and the field strength changes over a period of time, then the latitude in which the zircons were formed must also change. But Tarduno and his team discovered the opposite: The zircons they studied in South Africa indicated that, from about 3.9 to 3.4 billion years ago, the strength of the magnetic field did not change, which means that the latitudes did not change either. Since plate tectonics involves changes in the latitudes of many land masses, Tarduno says, “the movements of the tectonic plates may not have occurred during this time, and there must have been another way for the Earth to shed heat.” To reinforce their findings, the researchers found the same patterns in zircons that they studied in Western Australia. “We are not saying that the zircons formed on the same continent, but rather they seem to have formed at the same unchanging latitude, which strengthens our argument that there was no plate tectonics movement at this time,” Tarduno says. Stagnant Cap Tectonics: An Alternative to Plate Tectonics Earth is a heat engine and plate tectonics is ultimately the release of heat from the Earth. But mantle tectonic stagnation—which results in cracks in the Earth’s surface—is another way for heat to escape from the planet’s interior to form continents and other geological features. Plate tectonics involves the horizontal movement and interaction of large plates on Earth’s surface. Tarduno and colleagues report that, on average, over the past 600 million years, plates have moved at least 8,500 kilometers (5,280 miles) in latitude. In contrast, stagnant mantle tectonics describes how the Earth’s outermost layer behaves as a stagnant mantle, with no active horizontal plate motion. Instead, the outer layer stays in place while the planet’s interior cools. Large plumes of magma from deep within the Earth can cause the outer layer to crack. The stagnant mantle tectonics movement is not as efficient as the tectonic plate movement in releasing heat from the Earth’s mantle, but it still leads to the formation of the continents. “The primary Earth was not a planet where everything was dead on the surface,” Tarduno says. “There are still things happening on the surface of the Earth; Our research indicates that they simply did not occur through plate tectonics. We had at least enough of the geochemical cycles provided by stagnant damming to produce conditions for the origin of life. Maintain a habitable planet While Earth is the only planet known to experience plate tectonics, other planets such as[{” attribute=””>Venus, experience stagnant lid tectonics, Tarduno says. “People have tended to think that stagnant lid tectonics would not build a habitable planet because of what is happening on Venus,” he says. “Venus is not a very nice place to live: it has a crushing carbon dioxide atmosphere and sulfuric acid clouds. This is because heat is not being effectively removed from the planet’s surface. Without plate tectonics, Earth may have met a similar fate. While the researchers hint that plate tectonics may have started on Earth soon after 3.4 billion years, the geology community is divided on a specific date. “We think plate tectonics, in the long run, is important for removing heat, generating the magnetic field, and keeping things habitable on our planet,” Tarduno says. “But, in the beginning, and a billion years after, our data indicates that we didn’t need plate tectonics.” Reference: “Hadaean to Palaeoarchaean stagnant-lid tectonics revealed by zircon magnetism” by John A. Tarduno, Rory D. Cottrell, Richard K. Bono, Nicole Rayner, William J. Davis, Tinghong Zhou, Francis Nimmo, Axel Hofmann, Jaganmoy Jodder , Mauricio Ibañez-Mejia, Michael K. Watkeys, Hirokuni Oda and Gautam Mitra, 14 June 2023, Nature.DOI: 10.1038/s41586-023-06024-5 The team included researchers from four US institutions and institutions in Canada, Japan, South Africa, and the United Kingdom. The research was funded by the US National Science Foundation.
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