On Earth, the land moves. Over millions of years, continents shift and the entire surface of the planet reshapes itself. The driver of all this is plate tectonics: Earth鈥檚 surface is divided into several dozen plates, which move horizontally. Figuring out how this got started, however, has proved surprisingly challenging. Research has come up with dates ranging from 800 million years ago to 4 billion years ago, not long after the planet formed. Now, the reason for this huge discrepancy is finally becoming apparent.
Today, plate tectonics is a global process. Everywhere, plates are imperceptibly moving. At mid-ocean ridges, hot magma oozes up from inside Earth, forming new crust and pushing the plates apart. Where two collide, one is forced under the other, destroying it, in a process called subduction.
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Things were very different when Earth was new 鈥� notably, it was much hotter, which meant the rocks of the crust were softer. But what that crust was doing is unclear. Some researchers argue there was a stagnant lid: the crust barely moved, leaving the same rocks at the surface for hundreds of millions of years. Others think plates moved vertically rather than horizontally, as denser rocks sank and less dense ones rose. Somehow, the crust divided into plates and they started moving horizontally. But when?
A decade ago, many researchers argued that plate tectonics began between 3 billion and 3.2 billion years ago, says at Harvard University. Several lines of evidence pointed to big changes at that time, including the first evidence of minerals that only form in tectonic settings. One study also found that the crust began to accumulate rocks formed during subduction , reaching its modern state by 2.5 billion years ago.
Changing geodynamics on Earth
More recently, evidence has emerged of tectonic-like behaviour occurring much earlier. In 2022, Drabon and her colleagues identified . Before that, the crust seemed to be very long-lived, indicating it wasn鈥檛 being subducted. Afterwards, there are signs of rocks having been melted, suggesting the crust was less stable and more mobile. 鈥淚f not the onset of plate tectonics, at least there鈥檚 some kind of fundamental shift in the Earth鈥檚 geodynamics,鈥� says Drabon.
However, some hallmarks of plate tectonics are truly recent. Rocks called blueschists form when cold and dense rock sinks deep into Earth鈥檚 mantle. They only start to appear 800 million years ago, so some researchers say this was the onset of true plate tectonics.
In an attempt to explain these radically different dates, the solution geologists are alighting on is that plate tectonics emerged in that took hundreds of millions of years. As young Earth cooled, the crust hardened and cracked into plates. In some places, these were forced up against each other and one was subducted, but this was localised and short-lived at first. Only later did it become a continuous, global process.
In 2022, researchers led by at Monash University in Melbourne, Australia, set out for the development of plate tectonics, starting 4.45 billion years ago with the solidification of Earth鈥檚 crust. Global plate tectonics, they say, was operating by 2.5 billion years ago, at the start of the fifth stage. The exact details of the stages are 鈥渟till pretty much up in the air鈥�, says Drabon. 鈥淏ut there鈥檚 been an increasing recognition that it鈥檚 not just an 鈥榦n鈥� switch.鈥�
Understanding the timeline of plate tectonics matters because it almost certainly made Earth more habitable. It cycles water and essential nutrients, and by building continents, it creates new habitats. 鈥淚 don鈥檛 think you need tectonics for the origin of life,鈥� says Drabon. 鈥淏ut I do think it might be important to really proliferate life.鈥�
This article is part of a special series exploring seven of the biggest chronological conundrums of all time.
The science of deep time: Wales
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