Scientists are constantly on a mission to untangle how Earth alone among the planets was able to evolve complex life.
Scientists know that Earth’s past internal movements of the tectonic plates under our feet make our planet one-of-a-kind — they trapped carbon dioxide, making our planet habitable.
A new study, published in the April 6 issue of the journal Nature, may have figured out the mystery of how these plates formed — and why only Earth has them.
Why plate tectonics is important
Plates cover the entire Earth, and their boundaries play an important role in geologic happenings. The movement of these plates atop a thick fluid “mantle” is known as plate tectonics, and is the source of earthquakes and volcanoes. Plates crash together to make mountains, such as the Himalayas. They leave trenches where one slips beneath the other. They make giant rift valleys and ridges when going their separate ways.
The process is actually very important to life on Earth — several billion years ago, the surface of our Earth began forming into puzzle pieces called plates. This process trapped our atmospheric carbon dioxide into rocks and stabilised our climate, making Earth habitable.
A Mylonite mystery
How this developed has been a mystery for centuries. But one feature present at all plate boundaries could be the clue needed to crack the mystery: a rock called mylonite.
Mylonite rocks show up at every plate boundary, and have puzzled scientists since at least the late 1800s.
“Their presence is a bit of a mystery,” study researcher David Bercovici, of Yale University, told Business Insider. “There are well known observations of mylonite at all different kinds of plate boundaries and there’s been a long debate about what causes them.”
Mylonite is a highly deformed rock, which gives it a small grain size. Small grain size equals a weaker rock. Since these rocks occur around all plate boundaries, their deformation and subsequent weakness peaked Bercovici’s interest.
“It was a big motivation for developing this theory in the first place,” said Bercovici who worked with Yankick Ricard of the Université de Lyon on the study.
How rocks define Earth’s plates
Using a set of laws to to describe how grains evolve and “many many pages of physics,” Bercovici and Ricard were able to calculate how the birth of plate tectonics may have come about. Bercovici began with the idea that, back when Earth was just a ball of hot goo, certain parts of the surface would become cooler than others and sink, called downwellings. “You can see this happen in a cup of coffee or soup,” he said.
Downwellings cause deformation of the sinking material as it bends downward. In the globe to the right, the blue is a sinking area on the Earth’s surface, the beginnings of a plate boundary:
“The idea of our model is that if I deform a rock, I’ll actually make the grains smaller.” Smaller grains equal weaker rock and a weaker rock means it will more easily succumb to future deformations.
This creates a feedback loop where Earth’s slowly cooling crust accumulates weak zones. “And you accumulate enough of these weak zones and you’ll eventually get plate boundaries,” he said.
In the globe to the right, the sinking from the red globe above has created a convergent boundary — in blue — where plates move together. This pulls on the rest of the Earth’s surface, eventually forming a divergent boundary — in red — where plates move away from each other. Once a piece of Earth’s surface is enclosed by boundaries, it becomes a moving plate.
On Venus, damaged areas of the crust were never able accumulate into boundaries because it was too hot. The weak zones healed relatively quickly and the planet was never able to develop plate tectonics.
Venus’s plates never trapped CO2, never cooled, and the impact on the planet’s atmosphere makes it uninhabitable.
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