Life arose from “stagnant mantle,” not from plate tectonics

A study from the University of Rochester, using zircon crystals, found that tectonic plates were inactive during the period when life first appeared on Earth. Instead, a “stagnant cap” mechanism was in operation, releasing heat through surface cracks. This discovery challenges the traditional belief that plate tectonics is essential for the origin of life, and potentially reshapes our understanding of the conditions required for life on other planets.

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 paper published in the journal. 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 latitude changes for different land masses, Tarduno says, “it is likely that plate tectonic movements did not occur during this time and there must be some other 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 the surface of the earth; Our research indicates that they were not occurring through plate tectonics. We had at least enough of the geochemical cycles provided by stagnant cap processes to produce conditions suitable for the origin of life.”

Preserving a habitable planet

While Earth is the only known planet 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 removed effectively 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.