The finding contradicts previous assumptions about the role of mobile plate tectonics in the development of life on Earth.
What is plate tectonics? And why do plates move?
Plate tectonics is the theory that describes how Earth鈥檚 rigid outer layer鈥攃alled the lithosphere, which is made up of the crust and upper mantle鈥攊s composed of a series of plates that move and interact with each another. These plates are like big pieces of a jigsaw puzzle that fit together to form Earth鈥檚 surface.
The ocean floor is made of plates that form where molten rock from Earth鈥檚 mantle鈥攖he layer of hot, semi-solid rock beneath the lithosphere鈥攆lows upward. Over hundreds of millions of years, these plates cool and thicken so much that they eventually sink back into the Earth鈥檚 mantle at subduction zones. The gravitational force of this movement makes the plates shift and move, driving the horizontal motion of plate tectonics.
While this movement is very slow鈥斺�渁 rapid plate moves as fast as fingernails grow, about 10 centimeters (approximately four inches) a year,鈥� Tarduno says鈥攐ver the course of millions of years, this can add up to thousands of kilometers.
Scientists have taken a journey back in time to unlock the mysteries of Earth鈥檚 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 early Earth, revealing a complex interplay between Earth鈥檚 crust, core, and the emergence of life.
Plate tectonics allows heat from Earth鈥檚 interior to escape to the surface, forming continents and other geological features necessary for life to emerge. Accordingly, 鈥渢here has been the assumption that plate tectonics is necessary for life,鈥� says , who teaches in the at the . But new research casts doubt on that assumption.
Tarduno, the William R. Kenan, Jr. Professor, is lead author of a 聽examining plate tectonics from a time 3.9 billion years ago, when scientists believe the first traces of life appeared on Earth. The researchers found that mobile plate tectonics was not occurring during this time. Instead, they discovered, Earth was releasing heat through what is known as a stagnant lid regime. The results indicate that although plate tectonics is a key factor for sustaining life on Earth, it is not a requirement for life to originate on a terrestrial-like planet.
鈥淲e found there wasn鈥檛 plate tectonics when life is first thought to originate, and that there wasn鈥檛 plate tectonics for hundreds of millions of years after,鈥� says Tarduno. 鈥淥ur data suggests that when we鈥檙e looking for exoplanets that harbor life, the planets do not necessarily need to have plate tectonics.鈥�
An unexpected detour from a study of zircons
The researchers did not originally set out to study plate tectonics.
鈥淲e were studying the magnetization of zircons because we were studying Earth鈥檚 magnetic field,鈥� Tarduno says.
Zircons are tiny crystals containing magnetic particles that can lock in the magnetization of Earth at the time the zircons were formed. By dating the zircons, researchers can construct a timeline tracing the development of Earth鈥檚 magnetic field.
The strength and direction of Earth鈥檚 magnetic field change depending on latitude. For example, the current magnetic field is strongest at the poles and weakest at the equator. Armed with information about zircons鈥� magnetic properties, scientists can infer the relative latitudes at which the zircons formed. That is, if the efficiency of the geodynamo鈥攖he process generating the magnetic field鈥攊s constant and the intensity of the field is changing over a period, the latitude at which the zircons formed must also be changing.
But Tarduno and his team discovered the opposite: the zircons they studied from South Africa indicated that during the period from about 3.9 to 3.4 billion years ago, the strength of the magnetic field did not change, which means the latitudes did not change either.
Because plate tectonics includes changes in latitudes of various land masses, Tarduno says, 鈥減late tectonic motions likely weren鈥檛 occurring during this time and there must have been another way Earth was removing heat.鈥�
Further reinforcing their findings, the researchers found the same patterns in zircons they studied from Western Australia.
鈥淲e aren鈥檛 saying the zircons formed on the same continent, but it looks like they formed at the same unchanging latitude, which strengthens our argument that there wasn鈥檛 plate tectonic motion occurring at this time,鈥� Tarduno says.
Stagnant lid tectonics: an alternative to plate tectonics
Earth is a heat engine, and plate tectonics is ultimately the release of heat from Earth. But stagnant lid tectonics鈥攚hich results in cracks in Earth鈥檚 surface鈥攁re another means allowing heat to escape from the interior of the planet to form continents and other geological features.
Plate tectonics involves the horizontal movement and interaction of large plates on Earth鈥檚 surface. Tarduno and his colleagues report that, on average, plates from the last 600 million years have moved at least 8,500 kilometers (5280 miles) in latitude. In contrast, stagnant lid tectonics describes how the outermost layer of Earth behaves like a stagnant lid, without active horizontal plate motion. Instead, the outer layer remains in place while the interior of the planet cools. Large plumes of molten material originating in Earth鈥檚 deep interior can cause the outer layer to crack. Stagnant lid tectonics is not as effective as plate tectonics at releasing heat from Earth鈥檚 mantle, but it can still lead to the formation of continents.
鈥淓arly Earth was not a planet where everything was dead on the surface,鈥� Tarduno says. 鈥淭hings were still happening on Earth鈥檚 surface; our research indicates they just weren鈥檛 happening through plate tectonics. We had at least enough geochemical cycling provided by the stagnant lid processes to produce conditions suitable for the origin of life.鈥�
Maintaining a habitable planet
While Earth is the only known planet to experience plate tectonics, other planets, such as Venus, experience stagnant lid tectonics, Tarduno says.
鈥淧eople have tended to think that stagnant lid tectonics would not build a habitable planet because of what is happening on Venus,鈥� he says. 鈥淰enus 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鈥檚 surface.鈥�
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.
鈥淲e think plate tectonics, in the long run, is important for removing heat, generating the magnetic field, and keeping things habitable on our planet,鈥� Tarduno says. 鈥淏ut, in the beginning, and a billion years after, our data indicates that we didn鈥檛 need plate tectonics.鈥�
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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