Rare chunks of Earth’s mantle found exposed in Maryland

April 28, 2021

Set of rocks strewn throughout Baltimore likely represent a slice of prehistoric seafloor from 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.

“Those rocks have had a tough life,” says George Guice, a mineralogist at the Smithsonian’s National Museum of Natural History.

Because of this geological 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.

While the origin story of the curious Baltimore rocks has long been suspected, the new study provides the best data yet to support the tale.

“We’ve been visiting some of these locations for decades for our petrology field trips,” says Richard Walker, a geochemist at the University of Maryland who was not part of the study team. “It was really nice to see a paper come out to provide some geochemical evidence for what we’ve been assuming all along.”