Exploring the Global Mid-Ocean Ridge: A Quarter-Century of Discovery
Macdonald, Ken C., Oceanus
Professor of Marine Geophysics, University of California, Santa Barbara
There is a natural tendency in scientific investigations for increased specialization. Most important advances are made by narrowing focus and building on the broad foundation of earlier, more general research. This was certainly the case for the French-American Mid-Ocean Undersea Study submersible expedition launched 25 years ago. The mid-ocean target was the rift valley of the Mid-Atlantic Ridge spreading center. In the 1950s, Bruce Heezen of Columbia University's Lamont Geological Observatory collected wide-beam echo sounder cross sections of the rift valley and correctly surmised that it is part of a global rift system that wraps around the earth like the seam of a baseball. British and Canadian marine geologists took the next step and mounted a series of ambitious expeditions to study the Mid-Atlantic Ridge near 45 [degrees] N using every geophysical and geological tool available at the time. An American group focused its attention on the rift valley near 22 [degrees] N. However, the floor of the rift valley itself, where new oceanic crust intrudes and erupts, remained as obscure and enigmatic as ever. The hundreds of active volcanoes that occupy the floor of the rift valley were hidden from depth recorders by booming side-echoes of sound reverberating from the steep, 1,000 meter high cliffs of the valley.
Then, in 1972, three years after Neil Armstrong left the first human footprint on the moon, an international group of marine geologists initiated a bold advance: to explore the rift valley with the only vehicles that could take them there - submersibles. Despite a decade or so of deep-sea submersible experience, there was still considerable skepticism about their usefulness as scientific tools. However, those who believed prevailed, the French made the bathyscaphe Archimede and the submersible Cyana available, and the US offered the reliable underwater workhorse Alvin. The French-American Mid-Ocean Undersea Study (Project FAMOUS) was underway.
Precise base maps for the dive expedition were assembled using a US Navy classified multi-beam echo sounder, a French narrow beam echo-sounding system, and a deeply towed instrument package from the Marine Physical Laboratory at Scripps Institution of Oceanography (University of California, San Diego). I recall the hushed amazement aboard the research vessel Knorr when we first saw high-resolution, deep-tow depth profiles slowly burned into the paper of our malodorous precision depth recorders. The rift's center shape finally was revealed clearly as a deep trough nested within a wider rift valley, which contained many hills that appeared to be volcanic cones. These sonar records were the base maps for the dive expedition, and a team of geologists was assembled to be the first mid-ocean ridge divers using Archimede in the summer of 1973.
Alvin and the other submersibles certainly proved their worth as scientific tools during FAMOUS, and they have been heavily used ever since. Indeed, the French and also the Japanese have replaced their original subs with vehicles that can dive twice as deep, to depths exceeding 6,000 meters. The FAMOUS geological work showed that the rift valley is created by large faults that break through the newly formed oceanic crust and that active volcanoes are abundant along the rift valley floor. The youngest volcanoes form a narrow zone of oceanic crustal creation only 1 to 2 kilometers Wide, remarkable when compared to the dimensions of the plates, which are thousands of kilometers across. FAMOUS magnetic, geochemical, gravitational, and seismic studies resulted in the most detailed and comprehensive investigation of a spreading center up to that time. So much was learned that in 1977 two entire issues of the Bulletin of the Geological Society of America were dedicated to the results of this unprecedented expedition.
But the age of discovery on mid-ocean ridges was only beginning. …