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When geologist Hanika Rizo goes into the field she spends most of her time looking at her feet. Simply by trekking across the stark Arctic landscape in Northern Quebec, and paying close attention to the ground, she can glean clues about deep time in the color, texture, and arrangement of the solid outcrops of ancient rock that lie exposed here. Large, bauble-like shapes in the magma—“pillow lava”—speak of contact with water. Finding vestiges of water is always exciting, Rizo says, given that it is one of the requirements for the evolution of life. But the ultimate prize lies in the search for Earth’s oldest rock, which can serve as a window into the embryonic stages of Earth’s formation.
In a publication this summer, together with colleagues, Rizo, a scientist at Carleton University in Ottawa, described rocks at one study site, the Nuvvuagittuq Greenstone Belt, as dating to 4.16 billion years ago, making them some of the oldest rocks ever collected on Earth, possibly the oldest ever to have formed on the planet.
“The chemistry of the rocks tells us that they look very similar to the oceanic crust today,” Rizo says. Their surface is hard and crystalline, made of basalt. Rizo calls them the “first rocks” or the “oldest rocks” interchangeably. Geologists believe that to qualify as a “rock,” a piece of the Earth must be taken from the crust, which first formed once our planet’s mantle stabilized, the upper layer of hot, churned-up magma cooling into a solid shell on which continents and oceans rest.
“If they are confirmed, then it would mean that life actually began very early on, when these rocks were formed.”
On a geological time scale the formation of the first or the oldest rock marks the end of the so-called Hadean period, Rizo says. At the moment, this boundary is set by rocks found at the Acasta Gneiss Complex in Yellowknife Canada, which date to 4.05 billion years ago. But the new rocks are clearly older than that, Rizo says. The finding pushes the timeline for the formation of Earth’s first crust back by 130 million years. It also accelerates the timeframe for how quickly life may have emerged after the Earth formed, which Rizo hopes to study next in collaboration with paleobiologists who will look for microfossils of cells in the area.
“The goal is to see if [we] can find traces of life in those rocks. If they are confirmed, then it would mean that life actually began very early on, when these rocks were formed,” Rizo says.
The Nuvvuagittuq Greenstone Belt is a chunk of a craton—the durable cores of the modern continents. The Inuit of Inukjuak, on whose homeland the site falls, describe the landscape as dating to “time immemorial,” an era that stretches beyond living memory, oral tradition and recorded history, going back to the beginning of existence.
When Rizo and her colleagues travel to Nuvvuagittuq, they require permission to work on the land and rely on local chaperones to help navigate the landscape. The Nuvvuagittuq Greenstone Belt is flat and treeless, so the Inuit use Inukshuk landmarks to help with navigation. These Inukshuks are sculptures made of stone—a sophisticated and enduring form of communication, navigation, and cultural expression that has been integral to survival in one of the world’s most challenging environments.
To collect samples of rock, Rizo uses a small hammer she carries in her backpack. “We collect just what we need,” she says, rocks that are no larger than the size of “a grapefruit.” Once she is back in the lab, the rocks are cleaned of lichen, and the exterior part is removed to get to the core, “the most preserved piece of the rock,” Rizo says. The heart of the rock is crushed and dissolved in powerful acids. When all the elements are in solution Rizo can look for relationships between them that predict their age.
The finding accelerates the timeframe for how quickly life may have emerged after the Earth formed.
The undisputed consensus amongst geologists is that parts of the Nuvvuagittuq Greenstone Belt are at least 3.7 billion years old, but findings of even older rocks have proliferated since 2008, when the age of a rock from the region was estimated to be 4.28 billion years. That research was led by Rizo’s husband, Jonathan O’ Neil. The pair have worked together for more than a decade and are co-authors of the most recent study that arrived at the 4.16-billion-year date.
But some scholars remain skeptical. They were never convinced by O’Neil’s startling findings from 2008 and have similar doubts about the new finding. The gold standard for aging rocks is to look at zirconium crystals, which contain traces of radioactive uranium, whose decay sets the rhythm for measuring time. But the rocks that Rizo and O’Neil work with contain no zirconium. Out of necessity, they chose the rare Earth element Neodymium, instead, which is traditionally used to age asteroids. “If you have a zircon age, no one’s going to dispute it,” Rizo says, but other techniques leave room for doubt.
“It’s plausible” says Jörg Elis Hoffmann, a geologist from the University of Berlin, who specializes in ancient rocks and who was not involved in the studies at the Nuvvuagittuq Greenstone Belt. Hoffmann is especially convinced by the latest analysis. There were two separate methods used to age the rocks, and they both arrived at the same age estimate, Hoffman says. “The data is coherent,” he says. But there is a snag. A separate study concerned with subduction processes published earlier this year suggests that rocks at the Nuvvuagittuq Greenstone Belt could have been fused with pieces of ancient mantle that is 4.4 billion years old. This could have scrambled the age signature.
While much scientific progress is driven by debate and the reinterpretation of data, in this case, “the problem is, with these rocks, no other group can take any samples there anymore,” Hoffmann says. Hoffmann believes that what is missing here, crucially, is the work of being at the field site together, on foot, gathering the observational data that goes along with lab analyses. “When people have different opinions, the best is actually to go together in the field,” Hoffmann says. The context on the ground is invaluable, Rizo agrees.
But shortly after the COVID-19 pandemic subsided and borders re-opened, the Inuit closed off their land to all scientific expeditions. Some visiting scientists had vandalized Inukshuks, Rizo says, while other groups extracted whole slabs of the landscape for purported museum curations. The Inuit decided the unethical behavior had to stop. “They have been there for longer than we have ever had, and I totally understand that they decided to close access,” Rizo says.
The resolution to the scientific debate may have to wait, but Rizo is hopeful about patching local relationships. “At the end of next week, we’ll be there for an outreach event to give back to the community. They know the oldest rocks are on their land, but we want to talk to them about why studying these rocks is so important to scientists.” 
Lead image: Wead / Shutterstock
