Oregon State University

College of Earth, Ocean, and Atmospheric Sciences

Going on a Treasure Hunt

South Atlantic Map

How rock samples from the 1872 Challenger expedition are changing what we know about volcano hotspots

Susan Schnur couldn't believe she was handling rock samples from the 1872 Challenger expedition, the one that launched modern oceanography during its 80,000-mile voyage of the world's oceans.

"When I got the rocks, I kept thinking, if I mess this up, is this it? Is there any more of it?" says Schnur, a Ph.D. student with the College of Earth, Ocean, and Atmospheric Sciences at Oregon State. "It was a treasure hunt to try and find these samples."

Schnur chased rumors and hearsay, faded memories and near-legends, hitting dead ends and empty archives. But then, the British Museum gave her good news. They had the samples, neatly catalogued, and would be happy to lend them out.

"I think it's wonderful that the British Museum had such a well-designed system. The fact that they had not lost these rocks along the way is remarkable," she says.

From the outside, Schnur's pursuit might look like the scientific equivalent of an American Pickers episode where fanatics clamor for collectibles. But a closer look reveals that her quest was an important step toward solving a perplexing geologic puzzle — the little-understood hotspot trail called Walvis Ridge in the South Atlantic. Because the region is often inaccessible and difficult to survey, her Challenger samples allowed her to properly date the young, active end of the ridge and better understand its geologic history. And perhaps in spirit of the ship's namesake, Schnur's findings are challenging the status quo of Walvis' geology while unraveling how hotspots originate and where they may trigger future volcanic activity.

Susan Schnur Ph.D. student Susan Schnur tracked down rocks from the 1872 Challenger expedition to learn about volcanic hotspots.

Hot on the hotspot trail

Walvis Ridge is a rocky spine that curves off the coast of Namibia before scattering into a series of seamounts, like vertebrae. As it spreads south and west, the ridge progresses from old to young. The young, active end is a hotspot, fueled by a plume of molten rock deep from within the Earth's mantle. Scientific models depict these plumes almost like a lava lamp. The volcanic trail itself forms as tectonic plates slide over the plume-fed hotspot.

Normally seamount trails have a single active hotspot at the young end. But the Walvis Ridge has two potential hotspots that are unusually far apart — Gough Island and Tristan da Cunha, the most remote inhabited island in the world. Tristan is so remote, that it can only be reached by sea. Fishing boats from South Africa service the island and its 260 inhabitants just a handful of times a year.

Schnur began to wonder: could a single plume be the source of two hotspots? Could Tristan da Cunha, an island relatively isolated from the rest of the chain, really be the young, active end?

Schnur went to sea for seven weeks to collect dredge rock samples and survey the area. Because she was unable to actually step foot on Tristan, her Challenger samples provided much-needed data. Back in the lab, she used argon isotopes to age date the islands and put together a history of the ridge's young end.

The results were surprising. Tristan da Cunha wasn't 17 million years old, as reported in previous studies. Schnur's more precise dating method put it closer to 1.4 million years, a veritable babe in the woods (sea?) when compared to its neighbors.

"All the other seamounts that are around it are 10 to 27 million years old. And there is supposed to be an age progression, going old to young. So, Tristan doesn't fit within this progression," Schnur says.

Orphaned island

Like the fate of former planet Pluto, Tristan da Cunha may find itself demoted from long-believed hotspot of the Walvis Ridge to an orphaned island. The findings re-write history and change the trajectory of where the next seamount will form.

rock sample One of the Challenger samples from Inaccessible Island. Photo courtesy the British Natural History Museum (NHM).

"Everyone was using Tristan as the focus point of the Walvis Ridge. But we are arguing that it's not. It's pretty serious to get rid of a hotspot location and move it," Schnur says. "It means we've been looking in the wrong place. I believe the next volcano will appear elsewhere."

Besides untangling large-scale geologic processes, Schnur's research shows the importance of archived geologic samples. She notes Oregon State's recent efforts to expand its sediment core collection to one of the largest in the world. Scientists will be able to sift through decades of collected sediment cores, which provide evidence of the Earth's climate over the past millions of years, oceanic conditions, the history of the magnetic field, plate tectonics, seismic and volcanic events, ice ages and interglacial periods, and even the origin of life.

"I think it's worth investing in something, even if the payout comes a hundred years later," she says. "Without the Challenger rocks, I would have never been able to uncover that Tristan da Cunha might not be one of the Walvis Ridge hotspots."

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