Sea, from space
MARINE geologists in the us are currently being deluged with sea-floor data collected by two satellites -the us Geosat and the European Space Agency's ERS-l. These satellite images have revealed hitherto unknown features of the ocean floor, which could question the basic premise of existing geological theories.
The two satellites studied the sea- floor by taking accurate measurements of the height of the ocean surface. These measurements combined with precise tracking of the satellite positions, allowed the scientists to deduce the local strength of gravity. Features like broad hills and valleys of the ocean surface reflect the gravitational pull of the surrounding rocks. A submerged high mountain, for instance, attracts water from surrounding areas, creating a slight bulge over the peak, which are easily detected by satellites. From the satellite gravity images, seabed features such as tall ridges and deep troughs are clearly detailed (Science News, Vol148).
The Geosat first began its operations in 1985, during the Cold War. The us Navy had then deemed measurements of the sea-floor necessary to help navigate their submarine-launched missiles, which were otherwise diverted by a slight increase in the ocean's gravity. This data had been kept classified until recently by the Navy. Following petitions for the release of the valuable data, bits and pieces of information on features of the sea-floor around Antarctica and later, all data south of 30" latitude, were released to marine geologists. The mask of secrecy was taken off completely in April 1995, when the ERS-1 began collecting unclassified data on the same features.
This data from the two space-borne sensors helps to fill in vast gaps on sea- floor charts collected during ship surveys. However, as ships were unable to scan entire oceans, large tracts were left uncovered and as a result, unexplored. David T Sandwell of the Scripps Institution of Oceano- graphy in La Jolla, California, US, along with colleagues -studying bits of unclassified data -has come across some unprecedented discoveries. In the east of Tahiti, 1for instance, the team found a thousands of kilometre long volcanic chain, the presence of which was previously unknown.
! Closer inspection of this long , ridge named Pukapuka Ridges, led Sandwell and colleagues to hypothesise that it has originated due to the welling up of hot molten lava onto the rift zone created by the pulling apart of the Pacific plate in the region. Conventional maps would have led sci- entists to believe the ridge to be a trail of hot spots -a linear scar, the result of the dragging of an oceanic plate over a stationary hot spot in the earth's mantle. Sandwell contends that a single hot spot could not have given .rise to such a long chain of volcanoes. The possibility that a number of hot spots belched out lava at the same time is also quite remote. Commenting on other mountain chains, Sandwell says, " A lot of so-ca1led hot spot traces might not be." This is just one of the well researched theoretical models whose credibility has been put into doubt by this new data; chances are that many others may be questioned.
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