Canada Might Be the Best Place to Learn About Ancient Earth
Parts of the Earth's original crust are still traceable there, a new study finds.
About 4.5 billion years ago, when the Earth was brand new, its molten rock cooled to form a solid crust, and water vapour condensed into liquid oceans where life eventually would take hold. It was a hellacious period on our planet often called the "Hadean," after the Greek god Hades.
Parts of that original crust are still traceable in Canada, the same country where the oldest water and the oldest fossils of life (both of them dating back billions of years) have also been preserved. It's easy for fishermen to ignore the rocks that cradle the Hudson's Bay and the upper Great Lakes—but these Archean cratons (the core crusts of continents, from 4 to 2.5 billion years ago) are some of the oldest in the world.
"It's all part of the Canadian Shield," the slab of rock that makes up the vast expanse of eastern Canada, Jonathan O'Neil, a scientist at the University of Ottawa, told me. In new research published in Science, O'Neil and his colleagues detail their discovery of a 2.7-billion-year-old rock—that was formed through the recycling of a more than 4-billion-year-old parent rock, which is still traceable in the sample.
The discovery tells us that the parent rocks were produced at the bottom of an oceanic floor, a strange thought if you've ever visited the region, which is lake-dotted land today.
Most of our planet's original crust is long gone, recycled into younger rocks, O'Neil explained. "These [2.7-billion-year-old] rocks are not primary crust. They had to come from the melting of an older father or mother rock—we call it a precursor rock," he said.
The rocks they sampled were typical of the region north of the Great Lakes; they're what's called tonalite-trondhjemite-granodiorites (or TTGs), a non-oceanic crust in the granite family.
O'Neil and his colleagues knew that to get to these TTGs, a parent rock of oceanic-type crust would have to be melted down—and they could figure out the age of the parent rock by looking at the baby rock's isotopes (basically, atoms of the same family that have a differing number of neutrons) to use as tracers.
So why does so much evidence of ancient Earth keep turning up in Canada? It was only a couple of weeks ago that another team of scientists announced they'd found 4-billion-year-old microfossils there (this time in the Canadian Arctic). The oldest known water on our planet, which dates back 2 billion years, was found in Ontario.
The country is huge, for starters, so there's a lot of land to explore. But there's also something special about the Canadian Shield, where the ancient water and rocks were found. "We believe that all sections of the Canadian Shield were formed by recycling this much older primitive crust," O'Neil said.
That rock—trapped underwater on a toxic planet billions of years ago that we wouldn't even recognize as our home—melted, solidified, rose in mountains, sunk into lava over billions of years and formed the Earth we know.
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