Geologist Digs Deep – The Pito Deep, 6,000 Meters Beneath The Ocean

Late last January, while most people were battling winter’s cold and snow, University of Illinois structural geologist

Stephen Hurst left for a monthlong cruise in the South Pacific. It was no vacation, though. Hurst joined a team of scientists, engineers and technicians who set sail from Easter Island to explore the Pito Deep, a rift in Earth’s crust nearly 6,000 meters deep.

Funded by the National Science Foundation, the expedition had as its goal to probe the ocean crust, and gain a better understanding of how it was created. “Pito Deep is one of the few locations where such investigations can be made,” Hurst said. “The rift is on the boundary between the Easter Island microplate and the Nazca plate, in an area where tectonic movement is pulling the crust apart.”

Unlike rifts caused by sea-floor spreading, at Pito Deep there is no fresh magma obscuring the chasm. As a result, the crust is exposed like a split watermelon. The naturally occurring cross-section offers scientists an opportunity to study the structure of the ocean crust and how it formed. Hurst rendezvoused with the rest of the scientific team on Easter Island. While awaiting final preparations, he had an opportunity to explore the quarry where most of the island’s famous stone heads, or Moai, were carved. “The quarry is spectacular,” Hurst said. “There are approximately 300 Moai scattered throughout the quarry area, in various stages of completion. Some are 40 feet long. While the workmanship may not equal that of Michelangelo, the Moai are still very impressive.”

When all was made ready, Hurst and the others boarded the Atlantis (host ship for the deep-sea submersible Alvin) and began the 24-hour cruise to Pito Deep, which is about 350 miles north and slightly east of Easter Island. Having participated in six similar cruises, Hurst was involved with many technical operations of the expedition, from preparing bathymetric maps to analyzing photographs to diving in Alvin.

The floor of Pito Deep lies about 1,500 meters deeper than Alvin can safely dive, but this was not a problem for the researchers. “The bottoms of these canyons are usually filled with sediment and debris from rockslides,” Hurst said. “For our studies, we wanted to collect rocks from the steepest, not the deepest, part of the chasm.” The descent takes nearly two hours. The pilot and two “observers” spend the time talking, listening to music or rechecking the equipment. “You can’t sit back and enjoy the view,” Hurst said, “because there is no view. Sunlight doesn’t penetrate the ocean much past the first 100 meters, so for nearly the entire ride down it’s dark as night outside.”

To conserve battery power, Alvin’s powerful floodlights are rarely used during the descent. Because of the cramped space and limited view, each prospective observer is tested for claustrophobia before being allowed on a dive. During the dive, the water temperature falls from about 80 degrees Fahrenheit at the surface to close to freezing at depth. Separating the sub’s occupants from the cold water is 2 inches of titanium hull, which also offers protection from the crushing pressure. “Because of the enormous pressures we experience, it’s not uncommon to find Alvin’s hull festooned with net bags filled with Styrofoam cups and mannequin heads at the beginning of a dive,” Hurst said. “The water pressure squeezes them to a tiny fraction of their original size, making neat souvenirs of the dive.”

The researchers have about five hours to explore the abyss and collect rock samples from the cliff face before Alvin’s power runs low and they must float to the surface. A veteran of 19 dives, Hurst said each dive is unique. “In addition to some spectacular outcrops, this time I saw a deep-water holothurian for the first time. A very unusual type of sea cucumber, the animal was deep purple, translucent, with a bright pink fluorescent patch and a fringe around the top that waved back and forth. The creature was peacefully swimming at a depth of 3,700 meters.”

There was also a moment of consternation on one of his Pito Deep dives, when a pilot-in-training accidentally shut off Alvin’s power. “In an instant, all was dark and quiet,” Hurst said. “Fortunately, the master pilot quickly restored power.” Although data analysis will take many months, Hurst said the expedition’s preliminary results are positive. The researchers found the types of rocks they were looking for – rocks from the base of the ocean crust.

“We discovered that Pito Deep has a sort of layer cake geology,” Hurst said. “Like frosting on a cake, the top layer consists of horizontal lava flows. Beneath that is a layer of vertical dikes – the conduits through which the lava flowed. Beneath that is the now solid magma chamber at the base of the ocean crust. And beneath that lies the mantle.” It may be years before Alvin is scheduled to return to Pito Deep. But when it is, Hurst said he will be ready for another cruise to the South Pacific.