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Down to the sea in a ship.

The ocean bottom, its shape, its interior, its activity, and its origins stimulate the curiosity of geological and geophysical oceanographers. We are drawn to a subject that is remote and multifaceted, and that we investigate with a variety of technologies. We explore from oceanographic ships, satellites, airplanes, drilling platforms, and submersibles, and with robotic cameras and towed sensors. In moments of frustration, some of us would eagerly dispose of the overlying ocean once and for all. It gets in our way, is unforgiving to our instruments, and makes us miserably seasick!

The vastness of the ocean and the secrets of its submerged landscape contribute to its mystery. Seawater is opaque to light beyond a few hundred meters' penetration. Hence, there are no mountain tops one can scale to directly gaze at vast expanses of the abyssal seafloor. Instead, we visualize the hidden seascape with digital data sets, picture element by picture element, as tiles of a growing quilt, each stitched in the course of month-long expeditions.

As an initiation to ocean-floor geology and geophysics, we join one of these expeditions as the guest of Bill Haxby, Steve Cande, and Carol Raymond, our co-chief scientists. Bill is a Senior Research Scientist at Lamont-Doherty Geological Observatory specializing in geodetic studies and plate tectonics. Steven has joined the faculty at the Scripps Institution of Oceanography, continuing with a career of using marine magnetics to study the shifting of earth's lithospheric plates. Carol is a Research Scientist at the Jet Propulsion Laboratory of the California Institute of Technology. She undertook graduate studies at Columbia University where she investigated the nature of the magnetization of oceanic crust and lithosphere using sensors from ships, airplanes, and satellites.

The expedition, funded by the National Science Foundation, occurs during the Southern Hemisphere summer of 1991 to 1992. Its objective is to survey a corridor across the limb of the world-circling Mid-Ocean Ridge that separates New Zealand from Antarctica. We are aboard the research vessel Maurice Ewing, a new member of the US oceanographic fleet equipped with specialized instrumentation for marine geology and geophysics. Prior to departure, Bill exploits satellite altimetry to generate an overview of the Pacific-Antarctic spreading center.

Satellite-borne radar measures sea-surface shape with a resolution of a few centimeters. Elevated seafloor displaces seawater, and the greater density of the crustal layer exerts a gravitational attraction to the adjacent ocean. In response, water is tugged toward the elevation to form a slight bulge in the sea. Over deeps, the ocean surface is slackened into a trough. Superior detail of a sea-surface shape materializes at high latitudes where satellite orbits converge. Bill has devised a method to convert the altimetry to a representation of seafloor shape. The pseudomap below portrays more than a dozen giant, deep scars that cut long trajectories through the broad Pacific-Antarctic Ridge. These scars are fracture zones, fossil traces of active faults that offset the spreading center at the Mid-Ocean Ridge axis.

Steve chooses one of these scars as the focus of our survey. His plan is to employ Ewing's Hydrosweep multibeam sonar to image the seafloor to much finer detail than the altimetry provides (Beginning on page 74, Deborah Smith describes how this type of sonar tool advances our understanding of ocean-floor morphology.) The changing orientation of the scars is evidence of past shifts in the separation direction of the Pacific and Antarctic Plates. Some plate edges, most notably around the margin of the Pacific, sink back into Earth's interior. A major objective of our survey is to learn if fracture-zone bends correspond to a rearrangement of global plate motions caused by a large plateau clogging a trench. Ewing is equipped to measure the strength of Earth's gravity for a check against the gravity that can be calculated from the altimetry.

Carol is keen to probe the three-dimensional distribution of crustal magnetism. The many kilometers of water that float Ewing high above the oceanic crust act as a filter that removes the short-wavelength noise of Earth's past magnetic field intensity that is frozen into the volcanic bedrock when erupting lava chills into solid rock. Repeated flip-flops of Earth's magnetic field polarity back over a hundred million years are recorded in stripes of alternating strong and weak magnetism that strike parallel to the ridge axis. If it were not for the fortuitous filter provided by the 3 to 4 kilometers depth of the ocean ridge, it is possible that the symmetry of magnetic stripes on either side of the Mid-Ocean Ridge would still not be recognized. The discovery of this symmetry in the early 1960s confirmed the theory of seafloor spreading and ignited the powder keg of the plate-tectonic revolution. The scar we map is named the Pitman Fracture Zone in honor of Walter C. Pitman III who realized in 1966 that these magnetic stripes could be used to predict the age of oceanic crust.

Benefiting from precise navigation based on the Global Positioning System's constellations of orbiting radio beacons the chief scientists directed Ewing back and forth across the spreading center on 650-kilometer-long adjacent tracks separated by 6 to 8 kilometers. We tow an array of air-driven guns that fire an explosive discharge every ten seconds. The sound travels into the seabed where it reverberates and reflects from layers of sediment and from discontinuities in the volcanic crust. The returned echoes arrive at hundreds of sensors in a long streamer towed through quiet water far behind the ship.

The Pacific and Antarctic Plates are separating today at 60 millimeters per year. This is a rate midway between the slow Mid-Atlantic and Southwest Indian ocean ridges and the much faster East Pacific Rise. The magnetic stripes recorded by previous surveys show that the speed of separation and the rate of formation of new oceanic crust (a process the marine petrologist calls crustal accretion--page 74) has varied in the past, sometimes speeding up, sometimes slowing down. The spacing of the stripes on the two opposite flanks also indicates that at some periods more crust is accreted to one plate than to the other, so that the spreading is not symmetrical. Fast-spreading ridges generally exhibit smoother, more gentle relief and axial elevation, while slow-spreading ridges have rougher terrain and axial rift valleys.

Our co-chief scientists have strategically located our survey of the Pacific-Antarctic Ridge at a threshold between the fast and slow personalities. North of the Pitman Fracture Zone the spreading center seems to behave as if it is fast, and to the south it has some of the facade of a slow ridge. My contribution is to quantify the reflective strength of the sonar echoes bouncing back from the seabed. Using the reflectors, we generate a map that locates the most recently created crust and tells us how fixed the site of crustal formation has been though time.

What we don't understand with any confidence is the interior-earth physical process responsible for the changes in speed and the perceived asymmetry of the spreading. We employ theoretical modeling to simulate the ascent of Earth's ductile mantle as it floats upward to fill the void created by the diverging plates. As the hot mantle rises, it decompresses and some of it melts. The liquid melt is buoyant, and it percolates rapidly upwards into the shallow crust. Then it erupts in an astonishingly narrow belt along the axis of the ridge. This young strip of neovolcanic seafloor is glassy, and its bumpy lava surface reflects a multitude of points of light from the strobes and incandescent lamps of cameras towed overhead, or the illumination of robots and submersibles such as Jason and Alvin. Rock chemists, mineralogists, and those studying the mechanics of fluid flow are indeed impressed by the narrow focus of the upward flow of melt. Most of it collects into a thin magma lens no wider than a kilometer and no thicker than a few hundred meters. Melt that escapes channeling to the ridge axis erupts only a few kilometers away where it is responsible for the short-lived growth of seamounts found in a range of heights and sizes that is probably linked to the depth of the magma conduits. The flow paths or "plumbing" of the melt as it moves upward through focused pipes and horizontally through the crust along fissures that bisect the ridge axis is one of the "hot" topics of current research (pun intended).

Magma reaches the seafloor in discrete events, one in 1991 perhaps only hours before a serendipitous visit by the submersible Alvin. The chemistry of the lava densely sampled by new rock-coring techniques is beginning to show some systematic patterns of spatial variation, both along the axis and away from it. These patterns represent changes in melt properties and melt delivery through time. Boreholes drilled deep into the crust pass though thick residues of multiple eruptions that sometimes occur in cycles, beginning with voluminous high-effusion-rate flows of hot, primitive, low-viscosity lava and terminating in small eruptions of slightly cooler, more evolved, viscous lava. This cyclic layering of upper oceanic crust is imaged with logging tools lowered into the boreholes.

Our cruise is the first to image what I call the "schizophrenic" ridge axis, where the visual appearance of the ocean bottom takes on different characteristics in response to small but measurable differences in the seafloor spreading rate. Being the first to address this issue, this cruise is carrying out the exploratory phase of research designed, in this case, to capture the full range of personality change. In the past, the problem has been defined by a reconnaissance cruise directed to cover a large area. Instead we have decided to limit the study area so that we might obtain full, 100-percent coverage of the ocean floor. Satellite altimetry is an invaluable asset, because it immediately saves us the enormous task of locating a suitable fracture zone. With its trace already fully delineated, we proceed to "mow the lawn," using adjacent swaths of the Hydrosweep sonar to fully cover the whole corridor so that we might return with a 20-million-year history of the rates, styles, and directions of crustal accretion.

With complete coverage, our survey tracks are sufficiently close to one another that Carol's continuous sampling of Earth's magnetic field facilitates the creation of a high-fidelity image of the crustal magnetic intensity. Instead of picking inflection points along individual profiles (the traditional method for analyzing spreading rate), we can visualize the crustal growth with a time resolution improved by an order of magnitude. For the first time it becomes practical to superimpose lines that represent isochrones, that is, precise crustal ages, on the shaded relief of the seafloor. It quickly becomes apparent that between 3 and 4 million years ago the amount of new crust accreted to the Antarctic Plate was almost twice as wide as that accreted to the Pacific Plate, whereas the opposite is true for the period between 5 and 6 million years ago.

Bill displays on his computer screen a composite image of the multibeam bathymetry with a horizontal sampling space of only 50 meters and a vertical resolution of better than 5 meters. Remarkably delicate abyssal hill lineations appear. Regions of densely spaced, high-relief hills are interspersed with regions of wider spacing and lower relief. The boundaries seem to correspond to the quickening and slowing of the spreading rate. I note bulbous shapes on the outward flanks of many of the hills that are characteristic of volcanic outpourings, whereas the inward (ridge-axis facing) slopes are strikingly linear and much steeper, as if they are faces of faults that vertically displace the crust.

The fracture zone itself surprises each of us with its remarkably narrow and delicate-looking appearance. There are localities where the length of a single football field separates crust with an age difference of 4 million years! The shipboard-measured gravity field image shows the physical width of the fracture zone scar to be less by two orders of magnitude than its gravimetric counterpart. The gravity trough is not the zone of shearing between the two plates as they slide past each other, as some of us had mistakenly anticipated, but it is a representation of ridge-parallel crustal thickness and flexure. The variable width of the gravity trough becomes our first semi-quantitative and accurately dated record of the amount of melt delivered to the ridge axis. It appears that a large supply of melt produces long, robust, curved ridge tips and a narrowing of the gravity trough; a starved supply accounts for wispy, straight tips and a deep, wide gravity low.

Our graduate students will undertake much more thorough analysis in the course of their dissertation research. This brief introduction gives you only a flavor of our science; I have not even touched on the continental margins. However, the common thread to nearly all of the last decade's marine geology and geophysics research is the bold and successful implementation of new technology. This eagerness to take risks with sometimes untried tools has accelerated our understanding of the inner workings of Earth's seafloor and below. Although seagoing observationalists outnumber theorists by a hefty margin, a reading of this issue of Oceanus confirms that more attention is being given to computer simulation and numerical modeling than ever before. Data-sets are growing to enormous size--Ewing now processes up to 10 gigabytes of digital data per survey day. We expect that in the near future, this data will bring exciting new understanding to our field.

William B.F. Ryan is a Doherty Senior Scientist at Lamont-Doherty Geological Observatory of Columbia University. He began his oceanographic career in 1961 as a WHOI marine technician aboard R/V Chain in an expedition across the Mid-Atlantic Ridge into the Mediterranean and back. He obtained his Ph.D. in geology from Columbia University in 1970. He has been a user of Alvin to provide ground truth to ocean-floor imagery obtained with new generations of sidelooking sonars and deep-sea cameras developed by his research group. His joys are expeditions with friends and colleagues at sea with tools that give transparency to the ocean and reveal the seafloor in its splendor, and the classroom teaching of marine geology and plate tectonics. "I find it a thrill that the generation of ocean crust at a spreading center continues to defy simple intuitive logic."
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Title Annotation:includes related article on the research vessel Maurice Ewing
Author:Ryan, William B.F.
Date:Dec 22, 1992
Previous Article:The legal odyssey of the continental shelf.
Next Article:Island arcs, deep-sea trenches, and back-arc basins.

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