The lithosphere is the rigid outermost shell of a planet, and on Earth, it is divided into tectonic plates. These plates travel around, grind against each other, get submerged underneath each other and provide answers for why we get earthquakes and how mountain ranges form.
However, the Earth may not have always been set up like this. New research from the University of Maryland, Curtin University and the Geological Survey of Western Australia suggests the Earth’s first outer layer was a single, solid but malleable shell — it wasn’t until later that this shell began to crack into tectonic plates. The study was published in the journal Nature.
It has been long debated in the geology community about whether plate tectonics were always around, and this new study suggests that probably wasn’t the case. Dr Michael Brown, a Geology professor at the University of Maryland and co-author of the paper likened the single shell to a “stagnant lid.”
To come to the conclusion that a single shell was more likely on early Earth, the team studied rocks collected from the East Pilbara Terrane — a large area of ancient crust in Western Australia, where some of the oldest rocks in the world are found, ranging from 3.5 to 2.5 billion years old.
Cai Tjeenk Willink / Wikimedia Commons
The Nyiragongo Volcano in Virunga National Park.
They selected basalt rocks (produced when volcanoes erupt) and granites specifically, which had the same chemical composition as rocks found at volcanic arcs, which are volcanoes that form on a subducting plate — one sinking underneath another. Volcanic arcs are a giveaway that tectonic activity is occurring.
They wanted to find out whether the basalts had melted into granite in a different way other than subduction of the basalt beneath Earth’s surface. If subduction wasn’t the answer, that would mean plate tectonics weren’t happening when the Pilbara granites formed.
To work it out, the team performed thermodynamic calculations to measure how the rocks behaved under various temperature and pressure conditions. They found through a series of modelling experiments that the Pilbara granites could in fact have formed from the Coucal basalts, and this transformation could have happened in the same pressure and temperature conditions associated with a “stagnant lid” or single shell.
When one plate sinks underneath another, the rock starts of relatively cool and then heats up over time. When it reaches a high temperature, it’s made it quite far down, and so it’s also subjected to high pressure. In comparison, if there was a single shell, rocks would be very hot at relatively shallow depths and low pressures, which is known as a “high thermal gradient.”
“Our results suggest the Pilbara granites were produced by melting of the Coucal basalts or similar materials in a high thermal gradient environment,” Brown said in a statement. “Additionally, the composition of the Coucal basalts indicates that they, too, came from an earlier generation of source rocks.
“We conclude that a multi-stage process produced Earth’s first continents in a ‘stagnant lid’ scenario before plate tectonics began.”
What would a ‘stagnant lid’ look like?
Brown told Business Insider that Earth with a single outer shell may have looked like a water world with volcanoes poking through, a bit like Hawaii or the Ontong Java Plateau.
You could assume that there wouldn’t be as many mountain ranges and rifts if the Earth had a single shell, because of the lack of tectonic movement, but Brown said this is unlikely.
“There might have been differences in crustal thickness, and therefore depth of water above,” he told Business Insider. “There would have been many, many more volcanic edifices because magmatism is the way the inner Earth loses heat.”
As for why the Earth formed tectonic plates rather than staying in this state forever, Brown said this is the million dollar question.
“It must relate to formation of a more evolved crust, weakening of the lithosphere due to downwelling and maybe accumulation of damage in the lithosphere over time,” he said. “This is the tough question that we have yet to answer.”