Rare chunks of Earth’s mantle found exposed in Maryland

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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.

“It’s a huge ocean between them, and we’ve got a little bit of that ocean smooshed in Baltimore,” says Guice, lead author of a recent study describing the find in the journal Geosphere.

Throughout our planet’s history, oceans have proven to be impermanent features, opening and closing through the ages as their seafloors are recycled back into the depths of the planet in subduction zones, where one tectonic plate plunges beneath another. But occasionally chunks of the ocean floor, like the series of rocks in Baltimore, are tossed up onto the surface. These rocks provide a rare window into ancient oceanic processes, and they could help scientists better understand the future of the Atlantic Ocean. Someday it, too, will likely close.

“It will happen to the Atlantic as it did to the Iapetus,” says study co-author Daniel Viete, a geologist who specializes in tectonic processes at Johns Hopkins University in Baltimore. “It’s this long-term dance of the continental fragments, which remain at the Earth’s surface.”

The lure of green sparkle

Guice has long chased after sparkly green rocks, known to geologists as ultramafic. They’re rich in magnesium and make up the majority of our planet as the mantle. But pieces of the mantle are rare at the surface, and ultramafic rocks can form in several different ways, including in large crystallizing magma chambers. They’re also devilishly difficult to study.

Ultramafic rocks form deep underground at high temperatures and pressures, so their minerals are not stable near Earth’s surface. In this shallow environment, they’re often exposed to hot fluids rushing through cracks, transforming their mineral makeup, as seen in the rocks strewn around Baltimore. Understanding the rock’s history through these changes, Guice says, is like looking through thick fog.

“I’m so exhausted by these guys,” he says with a mix of dejection and humor.

When he heard of the curious ultramafic rocks in Baltimore, he was eager to learn more. Soon after he moved to Washington, D.C. in August 2019, he hopped on a train to meet up with Viete and other Johns Hopkins researchers. They all piled in a minibus to head out to a site known as Soldiers Delight.

Guice took me to this very spot. As I followed his rented silver SUV out to the site, away from Baltimore’s dense city center, I watched the landscape undergo a remarkable change. The greenery shifted from a variety of leafy trees and plants to barren grassy fields rimed with stunted oak and pine. The difference in vegetation reflected a change in the underlying geology, evincing rocks with excess magnesium and too little calcium for most plants to thrive. We had driven onto the mantle.

Many have suggested before that these seemingly unremarkable stones once resided under an ancient seafloor, yet convincingly demonstrating that has eluded generations of scientists. Slices of the ocean crust and underlying mantle thrust onto land are known as ophiolites. They’ve been found in other places around the world, such as a sequence in Oman, where yo
u can walk from the mantle up through the crust.

Other ophiolites have also been identified in the northern reaches of the Appalachian mountain range, which stretches far beyond U.S. borders into Canada, and then reappears in Ireland, the United Kingdom, and even Norway—the same original mountain range, now known by different names. All these now-disparate terrains piled up between 300 and 500 million years ago as multiple continents crashed together in what would become the supercontinent of Pangaea. Today, these mountainous landscapes extend more than 3,100 miles, but the original mountain belt may have been even longer, Viete says.

Unlike other known ophiolites, however, the series of rocks in Baltimore are dismembered and jumbled, with a sprawling metropolis plunked down on top of them. “We can’t see the sequence, we’ve just got little snippets,” Guice says. 

A half-billion-year journey

Starting with their initial trip out to Soldiers Delight, Guice and his colleagues collected a string of rock samples throughout Baltimore. They arranged to work with a team that was excavating a yawning basin in the region slated to become a water reservoir, which they later discovered once sat at the boundary between the mantle and crust. Excavators pulled chunks of the gray rocks from the ground, making the scientists’ job much easier.

“It’s everywhere, there are just piles of it,” Viete said, gesturing to the rocky mounds as we walked around the parking lot of the site about a year later.

The researchers also spent a sunny afternoon in November 2019 sampling rocks in the Forest Park Golf Course, which also once sat at the boundary between the mantle and crust. The rocks seemed artfully arranged among the wild grasses next to the manicured green of the ninth hole. The team hammered away at the exposure, pausing out of courtesy whenever a group of golfers passed through to putt. 

In total, the team collected 19 samples of ultramafic rock from five locations and then returned to the lab for a closer look. The key, Guice says, is in the chemistry of the mantle. The upper mantle is frequently melted a little bit at a time, but different minerals melt at different temperatures. So when the mantle partially melts, it becomes increasingly devoid of a predictable series of elements, which creates a specific chemical fingerprint.

“That is the thing that had not been identified in this part of the Appalachians before,” Guice says.

By sussing out these and other chemical clues, the scientists arrived at a story of the system’s formation. Nearly half a billion years ago, the Iapetus Ocean began to shrink thanks to a newborn subduction zone off the coast of the ancient continent Laurentia, which contained the core of modern North America. This created a continental pile-up that warped the surface and raised the mighty Appalachians, which some scientists estimate once rivaled the Himalaya

The violent shifts, according to the new research, also ripped up a chunk of the seafloor, strewing its dismembered bits across what is now Baltimore—one…

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