Plate tectonics are a planet’s way of showing an observer it's “alive.” Without the formation and destruction of a continental and oceanic crust, our Earth would not have volcanic features, terrestrial land masses for life to colonize, or even a complex atmosphere. A pioneering study published in Science gives an estimate as to when this epic natural process began on our planet: 3 billion years ago.
In order for plate tectonics to operate, a planet requires a significant internal heat source. Liquid water is also necessary and essential to the process of subduction – without it, the descent of plates down through the mantle would be impossible. Fortunately, Earth has both of these things, and is the only planet we know of that has fully operational plate tectonics.
Mars perhaps did in the distant past, and Venus and Jupiter's moon Io have “failed” plate tectonics. However, without another example to compare it to, there is a lot about Earth’s tectonics that scientists simply do not know. In particular, there is a huge debate in the geological community as to when plate tectonics actually started.
Some suggest that it began 4.5 billion years ago, immediately following on from its fiery creation; others suggest that it began much later, around 800 million years ago, roughly coinciding with the rise of multicellular life. For this new study, researchers at the University of Maryland decided to investigate this problem by looking at the geochemistry of some of the oldest rocks in existence.
Compared to the other rocky planets, Earth’s continental crust lacks magnesium. An analysis of the oldest terrestrial rocks, dating way back to the Archean Eon – around 4 to 2.5 billion years ago – reveals that the crust once contained far higher magnesium levels. At some point, the Earth’s crust “evolved” through complex melting processes to contain far more granite, a rock lacking magnesium.
This unique geochemical fingerprint is generally agreed to have come about as a result of plate tectonic action. During subduction, a large amount of water is dragged down beneath the crust, which is a key part of the process that ultimately makes granite. The question is, when did this subduction begin?
How the Earth would have looked in the Early Archean, compared to today. The transition from “mafic” to “felsic” rocks making up the continental crust correlates with a decrease in magnesium content. Tang et al./Science
By looking through the geological record, in theory the research team could look for this sudden dip in magnesium, thereby allowing them to pinpoint the genesis of plate tectonics. However, magnesium easily erodes out of rock when exposed to water, meaning that it has been largely “removed” from the oldest rocks.
Fortunately, several other metallic elements are relatively insoluble in water. These elements change their concentrations in various rock types as magnesium is removed. As it turns out, higher ratios of nickel to cobalt correlate to a higher magnesium content, so these elements could be used to indirectly calculate original magnesium contents.
Using this approach, the researchers were able to determine that subduction across the world began roughly 3 billion years ago, about a billion years after life started on Earth.
“Without plate tectonics, you get no continents, a feature unique to our planet,” Tang told IFLScience. “And we owe our existence to continents, all of life does.” Zeroing in on when plate tectonics began, then, is as much about geology as it is about the story of life on Earth.
