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Earth, like so many of its human inhabitants, may have experienced a midlife crisis that resulted in baldness. But it wasn’t a hair loss our planet needed to worry about; it was a receding horizon line.
For nearly a billion years during the “middle ages” of our planet (1.8 billion to 0.8 billion years ago), EarthThe mountains literally stopped growing, while erosion reduced existing peaks to stumps, according to a study published in the journal Feb.11. Science.
This extreme mountain-building hiatus – which was the result of a persistent thinning of the Earth’s continental crust – coincided with a particularly dark aeon that geologists call the “boring billion“, Write the researchers. Just as Earth’s mountains failed to grow, the simple life forms of Earth’s oceans also haven’t evolved (or at least evolved incredibly slowly) for a billion years.
Related: Why don’t mountains grow forever?
According to lead author of the Ming Tang study, the mountain of problems on Earth’s continents may have been partly responsible for the slowness of Earth’s seas.
“The continents were painless in the Middle Ages,” Tang, an assistant professor at Peking University in Beijing, China, said in an email. “Flatter continents may have reduced nutrient supply [to the ocean] and prevented the emergence of a complex life. “
When the mountains disappear
At the converging borders where the Earth continental plate shock, mountains rise in a process called orogeny. The continental crust at these limits is on average thicker and supported by magma, lifting surface rocks to dizzying heights. Meanwhile, erosion and gravity push back the summits; when the tectonic and magmatic processes below the surface stop, erosion wins out, shrinking mountains.
Because even the most powerful mountains disappear over time, studying the thickness of the Earth’s crust of the old Earth may be the best way to measure the degree to which mountains have formed in the past. To do this, the study authors analyzed the changing composition of zircon minerals that crystallized in the crust billions of years ago.
Today, tiny grains of zircon are easily found in sedimentary rocks all over the planet’s surface. The precise elemental composition of each grain can reveal the conditions in the crust where these minerals first crystallized, centuries ago.
“Thicker crust forms higher mountains,” Tang said. The thickness of the crust controls the pressure at which the magma changes its composition, which is then registered by anomalies in the zircons crystallizing from that magma, he added.
In a previous study published in January in the journal Geology, Tang and his colleagues found that the amount of europium encrusted in zircon crystals could reveal the thickness of the crust at the time these crystals were formed. More europium means more pressure on the crystal, which means a thicker crust on top, the researchers found.
Now, in their new study in Science, the researchers analyzed the zircon crystals in each content and then used these europium anomalies to build a history of continental thickness stretching back billions of years. They found that “the average thickness of the active continental crust varied over time scales of a billion years,” the researchers wrote, with the thickest crust forming in the Archean eon (there is 4 billion to 2.5 billion years ago) and the Phanerozoic (540 million years ago to the present).
Just between these active mountain-forming eras, the crustal thickness plummeted across the Proterozoic Aeon (2.5 billion to 0.5 billion years ago), reaching a low during the “Middle Ages”. ” of the earth.
The eon of nothing
It might not be a coincidence that Earth’s flattest aeon on earth was also its “dullest” aeon at sea, Tang said.
“It is widely recognized by our community that the evolution of life was extremely slow between 1.8 billion and 0.8 billion years ago,” Tang told Live Science. “Even if eukaryotes appeared 1.7 billion years ago, they only became dominant about 0.8 billion years ago. “
In contrast, Tang said, the Cambrian explosion, which occurred only 300 million years later, introduced almost every major group of animals we see today. For some reason, life evolved slowly and painfully during the “Boring Billion” and then kicked off as the crust began to thicken.
What is the correlation? If no new mountains formed during this time, then no new nutrients were introduced to the Earth’s surface from the mantle below, the researchers wrote – and a nutrient shortage on earth also meant a shortage of nutrients entering the ocean through the water cycle. As the mountain’s formation stagnated for a billion years, a “famine” of phosphorus and other essentials could have starved Earth’s simple sea creatures, limited their productivity and slowed their evolution, the team suggests.
Life and mountains eventually flourished when the Nuna-Rodinia supercontinent split up at the end of the Proterozoic Aeon. But before that, this gargantuan continent was perhaps so massive that it actually altered the structure of the mantle below, stalling tectonic plates over the “boring billion” and resulting in an eternity of crustal thinning, the researchers wrote. But more research is needed to fully unravel the mystery of the disappearance of mountains from Earth.
Originally posted on Live Science.
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