Scientists have traveled back in time to unravel the mysteries of Earth’s early history by using zircons, tiny mineral crystals, to study plate tectonics billions of years ago. Study sheds light on the conditions that existed at the early stage of the development of the Earth, and reveals the complex relationship between the earth’s crust, core and the emergence of life.
Plate tectonics allows heat from the Earth’s interior to rise to the surface, forming continents and other geological features necessary for the origin of life. Accordingly, “there was an assumption that plate tectonics was necessary for the origin of life,” says John Tarduno, lecturer in the Department of Earth and Environmental Sciences at the University of Rochester. “However, a new study casts doubt on that assumption.”
John Tarduno, a professor of geophysics, is the lead author of a paper published in the journal Nature that examines plate tectonics that began 3.9 billion years ago, when scientists believe the first traces of life appeared on Earth. The researchers found that there were no moving plates at that time. Instead, they found, the Earth gave off heat in a so-called “stagnant lid mode”, due to the thermal conductivity of the crust. The results obtained indicate that although plate tectonics is one of the key factors supporting life on Earth, it is not a necessary condition for the emergence of life on a terrestrial planet.
“We found the absence of plate tectonics at the beginning of life, and also for hundreds of millions of years after that,” says Tarduno. “Our data suggests that when looking for exoplanets that could harbor life, planets don’t have to have plate tectonics.”
Initially, the researchers did not set themselves the task of studying plate tectonics.
“We studied the magnetization of zircons because we were studying the Earth’s magnetic field,” says Tarduno. Zircons are tiny crystals containing magnetic particles that can capture the Earth’s magnetization at the time the zircons form. By dating zircons, researchers can build a timeline for the development of the Earth’s magnetic field.
The strength and direction of the Earth’s magnetic field varies with latitude. For example, the current magnetic field is strongest at the poles and weakest at the equator. With information about the magnetic properties of zircons, scientists can infer the relative latitudes at which they formed. In other words, if the efficiency of the geodynamo, a process that generates a magnetic field, is constant, and the field intensity changes over a certain period, then the latitude at which zircons formed must also change.
However, Tarduno and his colleagues found the opposite: the zircons they studied from South Africa showed that in the period from about 3.9 to 3.4 billion years ago, the magnetic field strength did not change, which means that latitudes did not change either.
Because plate tectonics involves changing the latitudes of various land masses, Tarduno says, “Tectonic plate movements most likely did not occur at this time, and the Earth must have removed heat in a different way.” In support of their findings, the researchers found the same patterns in zircons they studied in Western Australia.
“We are not saying that the zircons were formed on the same continent, but it appears that they were formed at the same constant latitude, which strengthens our argument that there was no plate tectonic movement at this time,” says Tarduno.
Stagnant Lid Tectonics: An Alternative to Plate Tectonics
The Earth is a heat engine, and plate tectonics is ultimately the release of heat from the Earth. But stagnant lid tectonics, which cause cracks to form on the Earth’s surface, is another way for heat to escape from the planet’s interior and form continents and other geological features.
Plate tectonics involves the horizontal movement and interaction of large plates on the Earth’s surface. Tarduno and his colleagues report that, on average, over the past 600 million years, the plates have shifted at least 8500 km in the latitudinal direction. In contrast, stagnant cap tectonics describes the behavior of the Earth’s outer layer as a stagnant cap, without active horizontal plate movement. Instead, the outer layer stays in place while the inner part of the planet cools down. Large plumes of molten material originating in the depths of the Earth can lead to cracking of the outer layer. Stagnant cap tectonics is not as efficient as plate tectonics at releasing heat from the Earth’s mantle, but it can still lead to the formation of continents.
“The early Earth was not a planet where everything was dead on the surface,” says Tarduno. – Something was still happening on the surface of the Earth; our research shows that these processes did not proceed through plate tectonics. At the very least, the geochemical circulation provided by the processes in the stagnant lid was sufficient to create conditions suitable for the origin of life.
While Earth is the only planet known to have plate tectonics, other planets such as Venus have stagnant lid tectonics, Tarduno says.
“People tend to think that stagnant lid tectonics won’t allow for a habitable planet because of what’s going on on Venus,” he says. – Venus is not a very pleasant place to live: it has a destructive carbon dioxide atmosphere and sulfuric acid clouds. This is due to the fact that heat is removed from the surface of the planet inefficiently.
Without plate tectonics, the Earth might have suffered the same fate. While researchers suggest that plate tectonics may have begun on Earth shortly after 3.4 billion years, the geological community disagrees on specific numbers.
“We believe that plate tectonics is important in the long term for removing heat, creating a magnetic field and keeping our planet habitable,” says Tarduno. “But right from the start, and a billion years after that, our data shows that we don’t need plate tectonics.”
The group included researchers from four US institutions and institutions in Canada, Japan, South Africa and the UK. The study was funded by the US National Science Foundation.