Standing among patches of muddy snow on the outskirts of Baltimore, Maryland, I bent down to pick up a piece of the planet that should have been hidden miles below my feet.
On that chilly February day, I was out with a pair of geologists to see an exposed section of Earth's mantle. While this layer of rock is usually found between the planet's crust and core, a segment peeks out of the scrubby Maryland forest, offering scientists a rare chance to study Earth's innards up close.
Even more intriguing, the rock's unusual chemical makeup suggests that this piece of mantle, along with chunks of lower crust scattered around Baltimore, was once part of the seafloor of a now-vanished ocean.
Over the roughly 490 million years since their formation, these hunks of Earth were smashed by shifting tectonic plates and broiled by searing hot fluids rushing through cracks, altering both their composition and sheen. Mantle rock is generally full of sparkly green crystals of the mineral olivine, but the rock in my hand was surprisingly unremarkable to look at: mottled yellow-brown stone occasionally flecked with black.
“Those rocks have had a tough life,” says George Guice, a mineralogist at the Smithsonian’s National Museum of Natural History.
Because of this geologic clobbering, scientists have struggled for more than a century to determine the precise origins of this series of rocks. Now, Guice and his colleagues have applied a fresh eye and state-of-the-art chemical analyses to the set of rocky exposures in Baltimore. Their work shows that the seemingly bland series of stones once lurked underneath the ancient Iapetus Ocean.
More than half a billion years ago, this ocean spanned some 3,000 to 5,000 miles, cutting through what is now the United States’ eastern seaboard. Much of the land where the Appalachian mountains now stand was on one side of the ocean, and parts of the modern East Coast were on the other.
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