Scientists have taken a trip 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 interaction between the Earth’s crust, 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. Accordingly, “there was an assumption that plate tectonics is essential to life,” says John Tarduno, a professor in the Department of Earth and Environmental Sciences at the University of Rochester. But new research casts doubt on this assumption.
Tarduno, Professor of Geophysics at the William R. Keenan Jr., who is the lead author of a research paper published in nature Study plate tectonics 3.9 billion years ago, when scientists believe the first traces of life appeared on Earth.
The researchers found that no mobile tectonic plate movement was occurring 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 key factor for the continuation of life on Earth, it is not a condition for the emergence of life on an Earth-like planet.
“We found that there was no plate tectonics when life was first thought to arise, and there was no plate tectonics for hundreds of millions of years after that,” Tarduno says. “Our data suggests that when we’re looking for exoplanets that harbor life, planets don’t necessarily need plate tectonics.”
An unexpected turn from the zircon study
Researchers did not originally set out to study plate tectonics.
“We were studying the magnetization of zircons because we were studying the Earth’s magnetic field,” Tarduno says.
Zircons are tiny crystals that contain magnetic particles that can trap the Earth’s magnetization at the time the zircons were formed. 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 depending on 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, then the latitude at which the zircons formed must also change.
But Tarduno and his team discovered the opposite: The zircons they studied from South Africa indicated that from about 3.9 to 3.4 billion years ago, magnetic field strength did not change, which means latitudes did not change either.
Since plate tectonics involves changes in latitude for different land masses, Tarduno says, “it is likely that plate tectonic motions did not occur during this time, and there must have been another way to remove heat from the Earth.”
To reinforce their findings, the researchers found the same patterns in zircons that they studied in Western Australia.
“We’re not saying that the zircons formed on the same continent, but they seem to have formed at the same unchanging latitude, which strengthens our argument that there was no plate tectonics going on at this time,” Tarduno says.
Stagnant cap tectonics: an alternative to plate tectonics
The Earth is a heat engine, and plate tectonics is ultimately the heat release from the Earth. But mantle tectonic stagnation—which results in cracks in the Earth’s surface—is another way of allowing heat to escape from the planet’s interior to form continents and other geological features.
Plate tectonics involves the horizontal movement and interaction between large plates on Earth’s surface. Tarduno and his colleagues report that, on average, plates from the past 600 million years have moved at least 8,500 kilometers (5,280 miles) in latitude. In contrast, stagnant mantle tectonics describes how the outermost layer of Earth behaves as a stagnant mantle, without active horizontal plate motion.
Instead, the outer layer stays in place while the planet’s interior cools. Large plumes of molten material rising deep in the Earth’s interior can cause fracturing of the outer layer. 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 early Earth was not a planet where everything was dead on the surface,” Tarduno says. “Things were still happening on Earth’s surface; our research suggests they weren’t happening through plate tectonics. We had at least enough geochemical cycles provided by stagnant mantle processes to produce conditions suitable for the origin of life.”
Preserving a habitable planet
Tarduno says that while Earth is the only known planet to experience plate tectonics, other planets, such as Venus, experience the slump of mantle tectonics.
“People tend to think that stagnant cap tectonics will not build a habitable planet because of what happens on Venus,” he says. “Venus is not a pleasant place to live: it has an atmosphere of crushing carbon dioxide and clouds of sulfuric acid. This is because heat is not effectively removed from the surface of the planet.”
Without plate tectonics, Earth might have met a similar fate. While researchers hint that the movement of tectonic plates may have begun on Earth a little over 3.4 billion years later, the geological community is divided on a specific date.
“We think that, in the long term, plate tectonics is important for removing heat, generating magnetic field, and keeping things habitable on our planet,” Tarduno says. “But, first, a billion years later, our data suggests we don’t need plate tectonics.”
John A. Tarduno et al., Hadaean to Palaeoarchaean stagnant cap tectonics revealed by zircon magnetism, nature (2023). DOI: 10.1038/s41586-023-06024-5
the quote: Plate tectonics not required for the emergence of life, study argues (2023, June 14) Retrieved June 14, 2023 from https://phys.org/news/2023-06-plate-tectonics-required-emergence-life.html
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