from National Geographic
Rare chunks of Earth’s mantle found exposed in Maryland
The set of rocks strewn throughout Baltimore likely represent a slice of prehistoric seafloor from a now-vanished ocean.
BY MAYA WEI-HAAS
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.