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Illuminating the seafloor.

If we could empty the ocean basins and obtain a clear view of the seafloor, we would learn a great deal about Earth's interior processes. We would see volcanoes erupting and building, and be able to map their distribution along the Mid-Ocean Ridge axis and off its axis. We would also see new oceanic crust forming at the ridges, being chopped up by faulting, and moving away from the ridge. Such observations would provide insight into magmatic and tectonic processes. However, the seafloor remains hidden from our direct view, except for brief excursions to the ocean bottom in manned submersibles or through the "eyes" of remotely operated vehicles. Its topography currently must be inferred from data collected by remote sensing instruments deployed from ships on the ocean surface.

Because ship tracks are sparse, most seafloor topography is still poorly known. This is especially obvious in small-scale bathymetric maps of South Pacific regions where water-depth contours resolve to circles that are centered on widely spaced tracks. Nevertheless, statistical interpretations of measurements collected during ocean-basin crossings reveal many things. For example, submarine volcanoes far outnumber their terrestrial counterparts. Because of their inaccessibility for study, however, the factors that control the formation of submarine volcanoes are largely unknown, and several first-order scientific questions remain to be answered, such as How do their abundances and size distributions vary geographically within and between ocean basins? and, How do volcano morphologies differ, and what do they indicate about eruption processes? The ocean is so effective at concealing the bottom that today we know more about the topography of Venus than we do about Earth's ocean floor.

Recent advances in instrument capabilities are beginning to allow marine geologists to investigate submarine topography in much the same way terrestrial geologists study land features. Using new high-resolution sonar systems, we can observe seafloor features in three-dimensional detail. Historically, technological advances in surveying tools mark stages in the advancement of our understanding of the seafloor and the underlying processes controlling its formation and evolution.

Early Views of Seafloor Topography

Laying of the first transatlantic submarine telegraph cable in the 19th century sparked interest in seafloor morphology. Before then, the ocean basins were thought to be vast featureless plains covered with huge amounts of sediment shed from the continents. The seafloor's shape was only known in the shallower areas of the continental rise where it was necessary for navigation.

In 1872, H.M.S. Challenger sailed from England on what is considered to be the first scientific oceanographic expedition. In three and a half years, Challenger circumnavigated the globe stopping every few thousand kilometers to collect samples and measure water depth by unreeling a sounding line. These widely spaced soundings showed that the seafloor was not featureless, but instead contained large topographic features. The nature and frequency of such features remained unknown, however, for quite a while.

Little more was discovered about the seafloor until the echosounder was developed in the 1920s. These instruments illuminated deep-ocean-basin mountain ranges, valleys, and other morphologic features similar to those observed on land. An echosounder measures the time it takes a sound pulse generated at the sea surface to return to the surface after reflecting off the seafloor; its travel time is then converted to water depth using a velocity-depth curve for sound through the water column. As the ship moves across the ocean, a profile of the bottom is assembled from the "echoes." The area of the seafloor reflecting the sound pulse is proportional to the echosounder's beam-width and the water depth: The larger the beam width and the deeper the water, the larger the resulting area. A traditional widebeam echosounder sends out a pulse that spreads outward as a sphere so that in water depths of 4,000 meters (an average ocean depth), a widebeam profiler samples a piece of seafloor with diameter of about 4,600 meters. A single depth represents this large patch of seafloor, and because the depth recorded is from the nearest acoustic reflector in this patch, an echosounder typically records the depth of the tallest feature, even if that feature is off the ship's track.

Because the first echosounders had low spatial resolution and poor vertical depth resolution, only the gross character of features was recorded. Therefore, emphasis was placed on charting features rather than determining their shapes, but seafloor mapping had begun.

Systematic coverage of the Pacific Ocean increased dramatically in 1935 when the US Navy, following a National Academy of Sciences recommendation, began rerouting ships to areas that had never been sounded. The need for oceanographic data in World War II's Pacific theater brought an exponential increase in ocean soundings as Navy ships stationed in the Pacific routinely collected depth data. A turning point for marine geology occurred when Harry Hess, a Princeton University professor and commanding officer of USS Cape Johnson, studied bathymetric profiles collected during transits between Hawaii and the Mariana Islands. He discovered curious fiat-topped peaks that he named "guyots" for the 19th century Princeton geologist Arnold Guyot. In 1946 Hess suggested that these peaks are drowned ancient islands whose tops were planed off at sea level by wave action and erosion. The idea of vertical island motion was not new. In 1842 Darwin had proposed that atolls are formed from islands as Earth's crust subsides. However, the discovery of fiat-topped features in the middle of the Pacific, 1 to 2 kilometers below the sea surface, was exciting. This discovery, and the hypothesis for their origin (which turned out to be correct), stimulated marine geologic exploration for some time to come.

New Tools Improve Seafloor Topography Definition

The 1950s brought the introduction of the precision depth recorder, capable of measuring water depths with a resolution of approximately 1 fathom (1.8 meters). Collecting bathymetric profiles then became standard procedure on most oceanographic expeditions. By the late 1960s marine geologists had accumulated broad knowledge of the North Pacific and North Atlantic seafloor morphology. Topography maps constructed from these data became an integral part of synthesizing ideas that led to the theory of plate tectonics.

Also during these years, the concept for collecting bathymetric data changed from reconnaissance to detailed surveying, and interest focused on major bathymetric features such as plate boundaries. These included spreading ridges where new oceanic crust is generated, fracture zones that offset the spreading ridges, and subduction zones, where oceanic crust is consumed back into Earth's mantle. Again, an advance in instrument capabilities preceded this new focus. High-resolution multibeam and sidescan sonars began to provide a far more detailed picture of the seafloor. The Sea Beam multibeam echosounder joined the suite of oceanographic research tools in the early 1970s. This system covers a seafloor swath whose width is approximately 75 percent of the water depth, and returns up to 16 cross-track depths: Each depth represents a patch of seafloor approximately two and two-thirds degrees on a side. In 4,000-meter water depths, the "footprint" is about 200 meters on a side, much more focused than the larger area (4,600 meters in diameter) sampled by a widebeam echosounder. Another important early 1970s advance occurred when ships began to use satellites for navigation--accurate navigation is essential in correlating bathymetry between ship tracks and in developing coherent maps.

Multi-narrowbeam echosounders such as Sea Beam, and more recent systems that cover wider swaths, revolutionized marine geology. In a 30-day cruise, tens of thousands of square kilometers of the seafloor can be mapped. Such high-resolution, two-dimensional swath maps reveal seafloor details not readily obtained from bathymetric profiles, such as the shapes of features. With a one-dimensional bathymetric profile, for example, it is impossible to discriminate between a ridge and a circular volcano because both appear simply as peaks. The first Sea Beam cruises confirmed that most ocean-basin topography is highly lineated. For the first time, the orientation and the length of these abyssal hills could be accurately measured. Abyssal hills are generated by faulting near the spreading ridge, and their orientation parallels that of the ridge axis at the time of their formation. Thus, changes in the abyssal-hill orientation provide us with information about how processes at the spreading ridge change through time.

Deeply towed sidescan systems such as Deep-Tow, operated by the Scripps Institution of Oceanography, were also available for use in the early 1970s. These types of sonar systems are towed about 100 meters above the seafloor, typically cover a 1-kilometer-wide swath, and produce high-resolution (spatial resolution of less then 10 meters) seafloor images that allow investigation of small-size topography. For example, several faults imaged by Deep-Tow may appear as one large fault on Sea Beam data, because they cannot be resolved by the Sea Beam system. The finer resolution information is vital for understanding such things as how the oceanic crust responds to stresses induced by sea floor spreading.

In addition to these instruments, the new 1970s submersible capability to dive to mid-ocean depths led to firsthand observations of the deep-ocean bottom. Submersible dives offer a number of significant advantages including the ability to distinguish different styles of eruptions by observing and mapping the morphology of individual lava flows, the ability to assess the importance of processes such as mass wasting (that is, debris flows and landslides) in shaping topography, and to collect samples from specifically targeted locations.

Many studies of the seafloor have now taken advantage of these mapping and sampling capabilities to further expand our knowledge of spreading ridges and their distinctive topography. Since the 1960s when marine geologists and geophysicists accepted the theory of plate tectonics, and, in particular, that new crust is generated at oceanic spreading centers, it has been known that topographic signatures change from spreading ridge to spreading ridge. This variation seems to be most strongly associated with spreading rate. Our current view is that fast-spreading ridges such as the East Pacific Rise (which opens at a rate of about 80 millimeters a year) are distinguished by an axial high or ridge 1 to 2 kilometers wide. Tectonic faults caused by extension are on the order of tens of meters high, and low-relief lava flows are common. By contrast, most slow-spreading ridges, such as the northern Mid-Atlantic Ridge (which spreads apart at about 25 millimeters a year) contain a major axial rift valley 30 to 45 kilometers wide. The median valley has an inner-valley floor 5 to 12 kilometers wide that is the primary site of volcanism and crustal construction. The valley floor is bordered by walls where numerous faults chop up the crust as it moves vertically 1 to 2 kilometers into the bounding mountains. Typically, eruptions on the inner-valley floor build small (1 to 2 kilometers in extent) volcanic edifices that pile upon one another to form prominent axial volcanic ridges. These ridges can be several hundred meters high, 1 to 5 kilometers wide, and tens of kilometers long.

In the late 1960s scientists recognized that the data density and coverage of mid-ocean ridge axial zones was insufficient to understand the processes that create new oceanic crust, and Project FAMOUS (French-American Mid-Ocean Undersea Study) was organized. The Mid-Atlantic Ridge near 37 |degrees~ N was chosen as the study site because it was thought to be typical of slow-spreading ridges. Investigations began with regional-scale surveys of the ridge axis using multibeam echosounders and surface-towed sidescan sonars. Then selected subregions were surveyed at increasingly greater detail with deep-towed sidescan sonars and cameras. The final component was submersible dives on particular targets. Project FAMOUS included collecting bathymetry, magnetic, gravity, and seismic data, as well as camera runs and dredge samples. There were more than 25 cruises and dives in the FAMOUS area between 1971 and 1974.

From the FAMOUS investigations, a consistent model for the creation and evolution of the rift axis of the Mid-Atlantic Ridge was advanced. Studies of inner-valley floor volcanic structures led marine scientists Robert Ballard (Woods Hole Oceanographic Institution, WHOI) and Tjeerd van Andel (then at Oregon State University) in 1977 to suggest that the inner-valley floor is produced by volcanic activity that forms axial volcanoes. They hypothesized that major volcano-building episodes alternate with periods of reduced volcano construction. Entire axial volcanoes migrate laterally away from the rift axis, and are lifted out of the inner-valley floor by faults. The goal of surveying the entire width of the FAMOUS area's broad axial zone, however, limited coverage along the axis of the spreading ridge. Therefore, the resulting models for crustal generation were mainly two-dimensional, focusing on the evolution of the topography as it moved away from the crustal accretion zone. The next step was to incorporate the third-dimension, along-axis variations into our thinking.

After Project FAMOUS, intense investigative attention turned to the East Pacific Rise because of its simpler structure. The narrow axial zone of the East Pacific Rise can be completely mapped in a single multibeam bathymetry swath. Based on these investigations our view of crustal construction and evolution changed from a two-dimensional to a three-dimensional perspective, and the wide variability in ridge-crest processes along a spreading axis was first understood. Extensive surveys along the axis revealed that the ridge axis is partitioned into segments that are tens of kilometers long. It was also realized that there are intrasegment signals within segments: For example, segments shallow near their midpoints and deepen at their ends. This trend has recently been related to discrete regions of mantle upwelling that concentrate at a segment's center. Ultimately, from this new view of mid-ocean ridges, marine scientists recognized the three-dimensional dynamic variability of spreading ridges.

Recent Work Brings Sharper Focus

Until recently there was no equivalent high-resolution along-axis mapping of the Mid-Atlantic Ridge, and the nature of ridge-axis segmentation at the Mid-Atlantic Ridge remained poorly known. The axial zone at this slow-spreading ridge is so wide that a major mapping effort was needed to obtain the necessary along-axis coverage. During 1989 and 1990, scientists from WHOI's Department of Geology and Geophysics spent two months mapping over 900 kilometers of the Mid-Atlantic Ridge between about 24 |degrees~ and 32 |degrees~ N. These surveys are the first to clearly reveal the segmentation at the northern Mid-Atlantic Ridge, and the along-axis variability in topographic characteristics similar to that at the East Pacific Rise. These data are yielding much information about the processes of faulting and volcanism at a slow-spreading ridge.

One of the most important capabilities of swath-mapping sonars is that large areas of the seafloor can be surveyed during one cruise. It is clear, however, that to fully understand the processes that shape the seafloor, we must obtain detailed surveys at many different scales. Systems such as Sea Beam are now considered to be the lower resolution instruments that provide valuable overviews of a region. This was apparent when Joe Cann (University of Leeds) and I recently used Sea Beam data to make inferences about small-scale volcanic edifices. We suggested that the inner-valley floor of the Mid-Atlantic Ridge was littered with small volcanoes, and the question was raised whether such interpretations made from Sea Beam maps are valid. For example, it was suggested that some of the circular highs on the bathymetric map that we identified as volcanoes may instead be circular flow fronts. To address this and other questions, a joint British-American cruise went back to the ridge axis in February 1992. We used the newly developed British deep-towed sidescan system TOBI (Towed Ocean Bottom Instrument) to survey large areas within the Sea Beam coverage. TOBI has an approximate resolution of 2 meters across-track and 12 meters along-track, providing a much higher resolution seafloor image than can be obtained from the Sea Beam echosounder. It is towed 400 to 600 meters above the seafloor, covers a total swath of 6 kilometers, and so provides detailed images of large sections of the topography.

The results of this cruise corroborate our hypothesis that small volcanoes are widespread at the axis of the Mid-Atlantic Ridge. In addition, we find that small volcanic features pile up to form the axial volcanic ridges that dominate the larger topography of the spreading segments. This style of volcanism is very different from that observed at the fast-spreading East Pacific Rise, where we observe sheet flows that spread out from fissures, and has important implications for magmatic processes at a slow-spreading ridge. The next step in our investigation is to return to the Mid-Atlantic Ridge to survey and sample individual volcanic features in an effort to understand the relationship between small volcanic features built on the seafloor and the magma bodies that feed them.

Our "view" of the seafloor has changed radically with our capability to image it. One hundred years ago we believed that the oceans were featureless plains drowned in sediment. Thirty years ago we began to accept the new theory of plate tectonics. Today, as we obtain higher resolution images of seafloor topography, we are discovering the complexity of the system that creates it. This complexity has much to tell us about the processes occurring within Earth. Our goal in the 1990s is to understand the processes that combine to generate the oceanic crust at spreading ridges. We have yet to answer many basic questions about this system, including how magma is supplied from the deep mantle, how and where magma is stored in the crust, and what controls the eruption of lavas at the seafloor. The technology that brings us our new and ever-changing view of the seafloor will help us answer these fundamental questions.

Deborah K. Smith is an Associate Scientist in the Department of Geology and Geophysics at the Woods Hole Oceanographic Institution. She developed her interest in the oceans while sailing a small boat from San Francisco to New England, and has continued to go to sea ever since.
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Copyright 1992 Gale, Cengage Learning. All rights reserved.

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Title Annotation:studies on seafloor topography
Author:Smith, Deborah K.
Date:Dec 22, 1992
Previous Article:Global seismic tomography.
Next Article:Micro-magnetic field measurements near the ocean floor.

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