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50 Years of Ocean Discovery: National Science Foundation 1950—2000
cated by "bathtub" rings around the perimeter of the lake. Clearly, the faster spreading rates associated with the East Pacific Rise and Galapagos Rift are commonly characterized by high volumes of sheet flows that flood large areas of the inner rift valley.
Another strange feature found within these lava lakes is lava pillars standing within the lake that resemble "tree molds," which are common in active volcanic areas on land such as those in Hawaii. When lava flows into a forested area on land, the molten rock is quenched when it comes into contact with the moist surface of the tree. Although the tree is consumed by fire, leaving only remnants of charcoal, a hollow cylindrical column of rock is formed; whose interior lining commonly preserves an imprint of the tree's bark. When the eruptive cycle ends and lava flows back into the magma chamber, a forest of tree molds is left standing as mute evidence of the forest that once stood there.
The lava pillars discovered within the lava lakes of the East Pacific Rise and Galapagos Rift are formed in a similar way. Prior to an eruptive cycle along a given spreading segment of the rift axis, the older lava terrain is characterized by a complex and dense network of fissures that are thoroughly permeated with seawater. When the eruptive cycle begins, sheet flows issue from only a few of the fissures within the fractured floor and spread out laterally covering a much larger area. As a result, the remaining water-filled fissures are capped by the flows, trapping large volumes of water beneath them. This seawater becomes heated and seeks to escape upward. Passing through the layer of molten lava contained in the lakes within the rift, this superheated water rapidly quenches the lava through which it passes. Hollow vertical chimneys of solidified rock form within the lava lake. As the level of lake drops, these chimneys remain as pillars of rock commonly supporting a thin canopy or crust of quenched lava running around the perimeter of the once-liquid lava lake. In appearance, it resembles "Yorkshire pudding."
Although the French dive series in 1978 did not result in the discovery of any active hydrothermal vents, it did locate one inactive site characterized by an accumulation of large white clam shells that were badly dissolved. During another dive, the scientist aboard Cyana came across some unusual chimneys on the older flanking volcanic terrain, which were sampled. After later analysis onshore, this sample was found to be 100% sphalerite or zinc sulfide containing 10 percent iron, 50 percent zinc, and 1 percent copper, with trace concentrations of lead and silver. The French had discovered an ore deposit on the East Pacific Rise that must have been formed under very high temperature conditions.
Although the highest temperature measured at the vent sites along the Galapagos Rift in 1977 was 23°C, laboratory analyses of the collected water samples suggested that the initial starting temperature of the hydrothermal fluids as they left the reactive zone around the magma chamber was between 350 and 400°C. Clearly, the discovery in 1978 of high-temperature mineral deposits by the French indicated that high exiting temperatures for hydrothermal vents might actually be possible.
The American phase of the joint U.S.-French investigation of the East Pacific Rise took place in late 1979. At the time, we were just beginning to understand how narrowly confined the central volcanic axis of the Mid-Ocean Ridge truly was, given the significant lateral dimensions of the crustal plates it was creating.
Some of the scientists participating in the expedition, particularly those from Scripps, were convinced that the zone of volcanic extrusion was wide and that there were significant areas of off-axis eruptions taking place several kilometers from the central rift valley. These scientists, headed up by Dr. Fred Spiess, were interested in the seismic velocity, density, porosity, and permeability of the upper oceanic crust. They wanted to know about the fine-scale motions of the seafloor on a time scale of months to years. To this end, they also wanted to use Alvin to carry out scientific experiments and instrument deployments, rather than serve as a vehicle for qualitative observation. Previous Deep Tow lowerings had located a reasonably flat area to the west of the central axis known as "Tortilla Flats," where they hoped to locate fresh lava flows using their Deep Tow system and then visit the site with the submersible Alvin.
Deep Tow's primary sensors were a side-scan sonar, temperature probe, and magnetometer: indirect geophysical devices designed to paint a broad regional picture of the sea-floor. Although it did have a black-and-white slow scan television camera and a black-and-white still camera, it spent little time in close visual contact with the bottom. Day after day passed as Deep Tow surveyed the area, but no active venting was located as the so-called Tortilla Flats proved to be old in age, covered by a thick blanket of sediments.
Another team aboard the Melville, including Jean Francheteau and me, was convinced that the zone of volcanic activity was narrowly confined along the central axis and that it was within this narrow zone of recent volcanism that active venting would be found. During our 1977 and 1979 expeditions to the Galapagos Rift, we had discovered that the active hydrothermal vents lay along a straight line apparently associated with the eruptive fissure responsible for the youngest flows within the rift valley. Once a vent was found, it became relatively easy to find additional vents along any particular fissure system by simply driving along the fissure, parallel to the rift axis.
Our tool for this search effort was Angus, and we patiently awaited our chance to go into the water. Angus, unlike the Deep Tow system, was designed by geologists to remain in constant contact with the bottom. It was designed to take a head-on collision with the rugged volcanic terrain and survive, making it possible to enter the narrow axial graben bound on either side by steep fault scarps.
After extensive Deep Tow coverage failed to locate any hydrothermal activity, Angus was finally permitted to enter