Picking up pieces of tectonic plates
In the ship’s Wet Lab, Ken Rubin holds a chunk of rock that Frieder Klein has just chopped in half using a rock saw. Its coloration looks otherworldly—striations of deep forest green twist in and around bands of blood red.
But even more captivating than its streaks of color is where this rock came from—Rubin is holding a piece of oceanic crust from the North American Plate. A few hours ago, it was over 5,000 meters deep in the Puerto Rico Trench. Now, thanks to Alvin, it’s in our lab.
“It’s beautiful but it’s a bit of a metamorphic mess,” says Rubin, a geologist and geochemist from the University of Hawaii. “It’s the most confusing rock we’ve collected during this expedition.”
The Puerto Rico Trench has long been an area of interest for earth scientists due to several unique geologic features. This is where the Caribbean Plate and the North American Plate meet, forming transform faults in certain areas and a subduction zone elsewhere.
During their Alvin dive, WHOI deep-sea biologist Tim Shank and geochemist Klein had the rare opportunity to view what most people only see depicted in geology textbooks: the interior of a tectonic plate. Peering through the Alvin viewports, they were able to see the rocky innards of the oceanic portion of the North American Plate.
Along with pilot Danik Forsman, Shank and Klein began their dive at 6,310 meters, then ascended up the north wall of the Puerto Rico Trench to a depth of 5,800 meters—with stunning views the whole way up.
“We saw what looked like evidence of dyke injections,” Shank says. “We saw remnants of magma migration through the crust that took place just over 100 million years ago.”
As they traversed 500 meters of near-vertical terrain, Forsman used Alvin’s mechanical arm to collect four different rock samples, all of which were mafic (magnesium-rich) except the colorful, “confusing” one. At the top of the wall, they found a sediment-covered slope, where they took push core samples.
“We had great fun checking out all these rock formations and plucking a few out of the wall,” Klein says. “In one case, the rock just fell away—because everything is so faulted down there.”
For Shank, the landscape reminded him of the area around a hydrothermal vent field he saw from Alvin in 2002: the Lost City of the Mid-Atlantic Ridge. Where they dove today has similar white carbonate on some of its rocks, but no hydrothermal vents. Still, it’s a fascinating and understudied area.
The last time scientists took samples from this location was in the 1960s and 70s—when the theory of plate tectonics was still in its infancy and the collection methods were much more rudimentary than the precision sampling Alvin enables today. The last systematic study was conducted in 1980.
“This opens a big door that I didn’t know existed,” Klein says.
To ensure they had enough time to make it to the top of the feature, the team didn’t make many stops for biological samples, but they did observe and document a variety of animal life: about two dozen isopods, two different kinds of arthropods, one red jelly, an octopus, a couple small glass sponges, two rattail fish, and a cusk eel.
Shank has spent decades diving in Alvin, but this dive—2,400 meters deeper than any other of his career—was special.
“It was definitely one of my best dives ever,” he says. “The morphology of the seafloor was awesome, and it was so exciting to see a vibrant diversity of life at those depths.”