Oldest Rock Ever Found Shines Light on the Earth's Early Days

By Eliza Strickland | September 26, 2008 8:38 am

bedrock CanadaA slab of bedrock on the shore of Canada’s Hudson Bay may be the oldest piece of the planet ever discovered: Researchers believe the rock is 4.28 billion years old, which would mean that it formed less than 300 million years after the earth itself came together. However, geologists say that considerable controversy remains over the research team’s method of dating the rocks.

Study author Richard Carlson says that if his team is right about the rock’s extraordinary old age, it will change conceptions of how the planet developed its current form, with solid tectonic plates and a stable crust. Says Carlson: “These rocks paint this picture of an early earth that looked pretty much like the modern earth.” … [T]he existence of solid rock 4.28 billion years ago would run counter to the traditional image of the young earth as a roiling cauldron of magma oceans, a view that is falling by the wayside among researchers as more geological data is unearthed [The New York Times].

In the study, published in Science [subscription required], researchers determined the age of the rock by measuring its amount of the isotope neodymium-142, which is produced by the radioactive decay of samarium-146, an isotope that was common in the earth’s earliest epoch, but that had largely disappeared by 4.2 billion years ago. Any rocks that formed while samarium-146 was still around would today contain larger than usual quantities of neodymium-142…. However, the neodymium-142 levels may not be an indicator of the rock’s age. O’Neil himself admits his team may instead be measuring the age of the magma from which the rocks formed. “All rocks have precursor, something that came before they formed,” says [geologist Martin] Whitehouse [New Scientist].

But if the dating proves correct, the Canadian slab will take the title of the oldest complete rock ever found. Previously, scientists have found isolated mineral grains called zircons that date back to 4.36 billion years ago, but the rock that was originally around these grains has eroded away. So until now, the oldest-known rock has been the Acasta Gneiss, an outcropping in Canada’s Northwest Territories that’s thought to be 4.03 billion years old [NPR].

In 2007, researchers found evidence of the earliest plate tectonics; read about it in the DISCOVER article “Crust Formed Early in Earth’s History.”

Image: Science/AAAS

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