geochronology

Some perspective on (geologic) time [w/ the Acasta gneiss!]

Due in large part to the lasting legacy of Christopher Columbus’s expeditions, North and South America will forever bear the mark of the “New World.” Whereas Europe has cathedrals centuries older than the United States itself, North America appears to not be able to offer much in terms of relics of antiquity. The earliest humans did not even begin to cross the Bering Strait (Beringia) into the western hemisphere until 12,500 years ago, 20,000 years ago at the earliest, while Jericho had already become the earliest large settlement in the world 10,000 years ago. However, just because the human occupation record in the western hemisphere doesn’t date back as far as it does in the eastern hemisphere, it doesn’t mean that there’s anything “newer” about this “New World.” One only has to look right beneath their feet to the rocks below to find the oldest earthly material on the entire planet.

In the Northwest Territories of Canada lies the Acasta Gneiss Complex (gneiss, pronounced like “nice”), which has been dated to an astounding 4 billion years old! These rocks are so old, that if you were to imagine the whole 4.6 billion year history of the earth as a 24-hour day, the Acasta gneiss would have formed at 3:08 a.m., while the dinosaurs would have reigned from 10:47 p.m. to 11:39 p.m., the first humans ancestors (Australopithecus afarensis, “Lucy”) would have emerged at 11:59 p.m., and the Pyramids of Giza would have been built just 49 milliseconds before the day ended.

Although a few tiny crystals, minerals called zircons, found in younger sedimentary rock in Australia have been dated to 4.4 billion years old, the Acasta gneiss are the oldest existing piece of Earth’s earliest continental crust—the rocks that compose the continents and continental shelves, which are the shallow seabed areas that surround the continents.That means that these rocks have been able to survive melting, erosion, and continents shifting and colliding for 4 billion years, which is no small feat!

On a geologic time scale, the Acasta gneiss can help provide insight into the Hadean Eon, with its name aptly derived from Hades the Greek god of the underworld to describe the “hellish” conditions on Earth during this time. Lasting from the formation of the earth 4.6 billion years ago to around 4.0 billion years ago, there have been no rocks found from the Hadean, save for the tiny zircons in Australia. As the Acasta gneiss are the earliest substantial pieces of continental crust, they mark the beginning of the Archean Eon, lasting from 4.0 billion to 2.5 billion years ago, and the start of the observable rock record. However, there’s a whole 600 million years of Earth’s history seemingly unaccounted for in the rock record.

This 600 million years is quite a long time! 600 million years ago from current day, enough oxygen had just accumulated into the atmosphere to form an ozone layer. (The first complex multicellular organisms didn’t even begin appearing until ~580 million years ago.) To put this time frame into some perspective, 600 million years is about 13% of the entire Earth’s history. The Earth really is super old (and super cool), and the Acasta gneiss can help us understand some of the earliest parts of Earth’s life a little more!

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Image: A 3.96 billion year old sample of Acasta gneiss on display at the Denver Museum of Nature and Science; photo by author

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