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Science on the sea floor.

Believe it or not, scientists know more about the surface of Venus than they do about Earth's surface. That's because 70 percent of our home planet is covered with water. Scientists haven't been able to explore huge expanses of the seafloor, and focus on many of its intricate detail -- until now.

With recent advances in underwater research vessels and satellite technology, scientists are getting the sharpest views yet of the seafloor's nooks and crannies. They're exploring an enormous deep-sea chasm that rivals the Grand Canyon, and peering at blind worms that live near belching volcanic vents. Here's your chance to explore these ocean-floor wonders -- without getting your feet wet!

VENT ADVENTURES

While drilling for minerals on the ocean floor last fall, 25 scientists aboard a research vessel were shocked to find fountains of superhot water bursting through the holes they were drilling. Temperature sensors connected to the drills read 288 [degrees] C (550 [degrees] F) -- as hot as an oven broiler! Where was the hot water coming from?

The scientists had uncorked a system of hydrothermal vents -- natural hot springs gushing through the seafloor, 2.7 kilometers (1.7 miles) below the water's surface. Hot springs like these occur elsewhere on the seafloor, and on land -- at Yellowstone National Park, for instance. But this is the first time scientists were able to witness the creation of a new vent environment.

In 1977, scientists first discovered hydrothermal vents near the Galapagos Islands. The vents typically deposit valuable minerals like iron, zinc, and gold on the seafloor. Today scientists are still trying to solve the mystery of how vents form. "Knowing how the vents form can provide a guide to finding new mineral deposits on land," says Bruce Malfait, a National Science Foundation marine geologist.

The vents tend to emerge where tectonic plates (slabs of Earth's crust) meet. At some points along these fissures, Malfait says, molten rock wells up to form new ocean floor. When the edges of the plates grind past each other, they produce many smaller, branching cracks. Scientists presume that cold ocean water seeps down through the cracks, hits hot rock, heats up, and rises. On its way back to the surface, the water pulls minerals from the rocky crust. Finally, it simmers up or bursts through the seafloor, with a mineral treasure trove in tow.

This hot mineral "soup" attracts a motley crew of deep-sea creatures: giant tube worms and clams, tiny white crabs, clumps of bacteria, and mussels. All have adapted to the vents' blistering heat, and the intense pressure and darkness of the deep.

Pitch darkness means food is scarce. So vent creatures survive on a meager diet. The bacteria thrive on hydrogen sulfide and carbon dioxide, which spew from the vents. The worms -- which have no eyes, mouths, or obvious means of movement -- live in symbiosis (a cooperative relationship) with the bacteria. The bacteria live sheltered inside the worms, where their wastes provide a steady stream of nutrients for their hosts. When the bacteria and worms die, scavengers like crabs scour the seafloor for remains.

For some reason, the vents often stop spewing after about six years. And scientists have wondered how vent critters travel to find a new home. At the newly discovered hot springs, says Melanie Summit, a microbiologist at the University of Washington, "we can start from time zero and watch how these sites become colonized." In September, scientists hope to send submarine robots to the vents to study these steamy sites some more.

GRAND

CANYON

OF THE SEA

Not too far from the camera-snapping tourists at Arizona's Grand Canyon, lies an even deeper gorge that only a few scientists have ever seen. It's called the Monterey Canyon and it's hidden under thousands of meters of ocean water in California's Monterey Bay. The canyon walls gradually slope down 2,300 meters (7,360 feet) to the seafloor. That's a quarter mile deeper than the deepest part of the Grand Canyon. Now, using an undersea robot called Ventana, scientists are closing in on explanations for how this canyon formed.

Any geologist can tell you how the U.S. Grand Canyon came to be: The mighty Colorado River carved its way down through layers of rock over millions of years (see SW 9/2/94, p. 16). But the formation of Monterey's undersea canyon has left scientists puzzled. According to Dan Orange, a geologist at the Monterey Bay Aquarium Research Institute (MBARI), there are no large rivers flowing into Monterey Bay. But underwater turbidity currents may have played a role in carving the canyon's deepest chasms.

Turbidity currents are powerful landslides of underwater debris. The debris is composed of sediments, tiny bits of mud, rocks, and minerals that settle on the seafloor and pile up. "After a while the mud becomes unstable," Orange says. It slides down the continental slope, the land sloping from the continent to the seafloor, like a mudslide rushing down a hill. The high-speed slurry erodes (scours away) rock and other sediment, carving out the canyon walls.

A process known as sapping may also have helped carve the branching passageways of the canyon.

"In Monterey Bay we have water leaking out of the seafloor from the bottom up," Orange says. No one's sure where the water comes from. But Orange suspects that it's the product of sedimentary rock formation.

When layers of sediment pile up on the seafloor, he explains, they squeeze out water and compress to form rock. "It's like squeezing water out of a sponge," he says.

When the water seeps up through the seafloor, it can carry sediments away, grain by grain. Orange speculates that over 10 million years, this process could have moved enough sediment to carve branches in the canyon.

This spring, MBARI scientists will launch a new undersea robot, Tiburon (Spanish for "shark"), to further explore the canyon's mysteries. Orange hopes the robot will help fill in the great gulch of knowledge about the seafloor.

BIRTH OF

AN ISLAND

Seventeen miles south of Hawaii, scientists are monitoring the slow growth of Loihi, an active undersea volcano. "Loihi is an island in the womb," says Alexander Malahoff, a geophysicist and director of the Hawaii Undersea Research Laboratory. When another 1,000 meters (3,280 feet) of lava pile up on the mound, Loihi will break through the ocean surface and become the newest island in the Hawaiian chain.

"I estimate it will take another 50,000 years," Malahoff says. He's been observing Loihi for nearly two decades, taking regular dives inside a submersible, a compact research submarine. He and others believe that Loihi is growing the same way Hawaii's other volcanic islands formed about 100 million years ago.

Unlike most volcanoes, which rise at the boundaries of tectonic plates (see SW 2/7/97, p. 14), the Hawaiian islands are in the middle of the Pacific plate. Scientists theorize that each island formed as rising magma (molten rock from deep within Earth) burned a hole in the seafloor crust (see diagram, above). Gradually, the molten rock poured out, cooled, and hardened to form the mound of a volcano.

Over time, the plate shifted northwest, carrying the new volcanic island with it. But the hot spot-the area of rising magma beneath the crust -- stayed in place and started to form a new volcanic island. Today, Malahoff says, Loihi is growing over the same hot spot.

Last August, however, Loihi suffered a setback. In a three-day period, 3,000 earthquakes shook the undersea volcano. "We couldn't dive down because it was too danger-ous," Malahoff says. A month later, though, he returned to the site. Instead of finding Loihi's tall cone, he found a 5.6-kilometer (3.5-mile) wide, 300-meter (1,000-foot) deep crater. "We estimate that 300 million tons of rock suddenly disappeared down the hole," he says. Still, the cave-in lowered Loihi's height by only 45 meters (150 feet).

When Loihi does cut through the waves, "there will be spectacular volcanism," Malahoff says. Unfortunately, none of us will be around to witness the big event.

BIG

MAP

ATTACK

Seeing the seafloor is easier than ever thanks to satellites orbiting Earth. Last year, scientists used satellite data to create this new map exposing the entire ocean floor.

"This is the first time anybody has seen the ocean floor at this level of detail," says Charles De Met, a professor of geophysics at the University of Wisconsin at Madison. Among other features, the map reveals twice the previously known number of undersea volcanoes. "It's like being able to drain the oceans," says David Sandwell, one of the map makers and a geophysicist at the Scripps Institution of Oceanography in La Jolla, California.

Like humans, satellites can't "see" through ocean water down to the mountains and trenches on the seafloor. Instead, the satellites measured variations in gravity (Earth's downward pull) at different points on the ocean surface.

The overall idea, says Sandwell, is that the swells and dips on the ocean water's surface reflect the peaks and trenches on the seafloor. For example, the mass of a large undersea volcano creates a strong gravitational pull. The volcano's gravity draws enough water toward it from all directions to create a matching "twin" peak on the water's surface. Unlike the sudden peak of a breaking wave, though, the watery peak forms gradually over a wide area (so you can't see it from a boat).

On the map, the continents appear black. The tallest undersea mountains -- those with the strongest gravitational pulls -- are shown in red and orange. The deepest chasms -- with the weakest pulls -- are blue and purple. Green areas are in between. How do you think this info helps scientists study the seafloor?

Surf the Web

to learn more

about the

seafloor:

For more info about the seafloor map above, visit:

http://www.ngdc.noaa.gov/ mgg/announcements/ announce_predict.html

To learn more about the Monterey Canyon and Tiburon, the newest undersea robot that will explore the canyon this spring, visit:

http://www.mbari.org and http://www.ifremer.fr/ anglais/

Find out about Shinkai 6500, the newest piloted deep-sea submersible:

http://www.jamstec.go.ip/
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Title Annotation:includes related articles and a list of Web resources
Author:Stiefel, Chana Freiman
Publication:Science World
Date:Mar 7, 1997
Words:1688
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