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Minerals Report - International Seabed Authority

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Figure.8: Plot of incidence of hydrothermal plume activity versus full spreading rate (cf. Fig.3)<br />

but now also including the Reykjanes Ridge (RR) and the mid-Atlantic Ridge near the Azores<br />

Triple Junction (ATJ), both coloured red, and the SW Indian Ridge (SWIR) and the SE Indian<br />

Ridge (SEIR), both coloured green.<br />

Further, we do not believe this to be a purely random distribution.<br />

Recently, Cannat et al. (1999) have shown that the Azores Triple Junction was<br />

extremely volcanically active approximately 10 million years (Ma) ago. At<br />

that time, the ridge topography around the Rainbow area would have been<br />

very similar to that seen along the Reykjanes Ridge today. It seems highly<br />

plausible; therefore, that it is precisely because this “swollen” ridge crest has<br />

subsequently cooled, subsided, and become so extensively cracked/faulted,<br />

that the abundant ridge-crest hydrothermal circulation now observed has<br />

been developed. Along the Reykjanes Ridge by contrast, continuing high<br />

levels of melt generation, subsurface, continue to warm and buoy up the<br />

overlying crust which remains ductile, not brittle, thereby reducing the<br />

efficiency of hydrothermal cooling and circulation through this section of<br />

ridge crest (German & Parson, 1998). If this hypothesis is correct then it may<br />

INTERNATIONAL SEABED AUTHORITY 386

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