Minerals Report - International Seabed Authority
Minerals Report - International Seabed Authority Minerals Report - International Seabed Authority
In relation to spreading rates at various ridges, Dr German pointed out that based on data obtained from the North American and Japanese collaboration on the south eastern Pacific rise in the 1990s, it has been determined that this is the fastest spreading ridge presently known. The next fastest spreading ridge he said is the northeastern Pacific Rise, followed by the Juan de Fuca Ridge, and then the Mid-Atlantic Ridge. Based on the hypothesis, he said that as a first approximation, if you knew what the spreading rate of a given ridge is, then you could predict whether you would have hydrothermal activity on it. From another perspective, if you could only have abundant hydrothermal activity on the fastest ridges, then large parts of the world’s ocean basins are places that you could immediately then turn around and decide were uninteresting with respect to hydrothermal activity. Because of their studies, it has been revealed that this is not necessarily the case. With slides, Dr German illustrated their findings with respect to vent sites and spreading rates in three areas. In an area of the mid-Atlantic ridge between 10 degrees and 30 degrees north, there was evidence of hydrothermal activity at a spacing of 150 kilometres. In an area just south of Iceland on the Reykjanes Ridge with exactly the same spreading rate as at the mid-Atlantic ridge above, Dr German stated that very little hydrothermal activity was found. Finally, in a third area just south of the Azores amidst significant faulting of the seafloor, Dr German showed many sites of hydrothermal venting. He pointed out that in the central Atlantic a vent site was found about every 150 kilometres, that on the Reykjanes Ridge one vent site was found every 600 or 700 kilometres and that near the Azores the frequency was every 20 kilometres or so. Dr German said that the Working Group was of the opinion that the incidence of higher hydrothermal activity on the mid-Atlantic ridge was because of fracture zones found along it that are not characteristic of fast spreading ridges. He described these fractures as deep-seated cracks that cause breaks in the ocean crust, which is still quite hot. As the seawater, INTERNATIONAL SEABED AUTHORITY 414
percolates through these cracks Dr German said that it strips minerals back out of the rocks and causes hydrothermal circulation. He therefore described these areas on the mid-Atlantic ridge as new settings away from volcanic activity that had not been previously anticipated as areas of hydrothermal activity. As a result of this discovery, he said that there are now two ways known for seafloor hydrothermal venting to occur: one related to fresh volcanic reaction (volcanically-hosted hydrothermal systems), and the second related to ridges cooling down, cracking up and forming fractures following volcanic activity. He also said that the fractures enable seawater to re-enter the earth’s crust, stripping minerals from rocks and causing hydrothermal circulation (fault-hosted hydrothermal activity). Noting that the basic rock type in this reaction is very similar to the original rock type and that the seawater is the same, he pointed out that it would be the same kind of compositional material arriving at the sea floor and producing sulphides deposits. The process he further noted is very interesting because at these places even larger sulphides deposits may evolve. As support for this theory, Dr German offered as examples the Rainbow field and TAG hydrothermal deposits on the mid-Atlantic ridge that are both products of fault controlled systems. He said that to his knowledge, both deposits are substantially larger than most of the other sulphides deposits found elsewhere on the world’s mid-ocean ridges. Dr. German reemphasised the findings of the Working Group firstly: that at faster spreading ridges the more hydrothermal signals and individual hydrothermal vent sites are to be found along the ridge crest. In this regard, he used as an example the southeastern Pacific rise. Secondly, he emphasised the effect of secondary controls such as fracturing that occurs at some of the vents in the mid-Atlantic ridge, in particular places like the Azores. He said that what appears to be happening there is that the ridge never cools down long enough to start cracking and the venting is on one single line, but that as one moves through the rest of the world’s ridge crests this line basically broadens out into some big fan through the fractures formed at the slower spreading ridges. He postulated that it could be that all slower spreading ridges have quite a lot of hydrothermal activity. He noted however that most of the slower spreading ridges are in ridiculously remote places, which require some international INTERNATIONAL SEABED AUTHORITY 415
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In relation to spreading rates at various ridges, Dr German pointed out<br />
that based on data obtained from the North American and Japanese<br />
collaboration on the south eastern Pacific rise in the 1990s, it has been<br />
determined that this is the fastest spreading ridge presently known. The next<br />
fastest spreading ridge he said is the northeastern Pacific Rise, followed by the<br />
Juan de Fuca Ridge, and then the Mid-Atlantic Ridge. Based on the<br />
hypothesis, he said that as a first approximation, if you knew what the<br />
spreading rate of a given ridge is, then you could predict whether you would<br />
have hydrothermal activity on it. From another perspective, if you could only<br />
have abundant hydrothermal activity on the fastest ridges, then large parts of<br />
the world’s ocean basins are places that you could immediately then turn<br />
around and decide were uninteresting with respect to hydrothermal activity.<br />
Because of their studies, it has been revealed that this is not necessarily the<br />
case.<br />
With slides, Dr German illustrated their findings with respect to vent<br />
sites and spreading rates in three areas. In an area of the mid-Atlantic ridge<br />
between 10 degrees and 30 degrees north, there was evidence of hydrothermal<br />
activity at a spacing of 150 kilometres. In an area just south of Iceland on the<br />
Reykjanes Ridge with exactly the same spreading rate as at the mid-Atlantic<br />
ridge above, Dr German stated that very little hydrothermal activity was<br />
found. Finally, in a third area just south of the Azores amidst significant<br />
faulting of the seafloor, Dr German showed many sites of hydrothermal<br />
venting.<br />
He pointed out that in the central Atlantic a vent site was found about<br />
every 150 kilometres, that on the Reykjanes Ridge one vent site was found<br />
every 600 or 700 kilometres and that near the Azores the frequency was every<br />
20 kilometres or so.<br />
Dr German said that the Working Group was of the opinion that the<br />
incidence of higher hydrothermal activity on the mid-Atlantic ridge was<br />
because of fracture zones found along it that are not characteristic of fast<br />
spreading ridges. He described these fractures as deep-seated cracks that<br />
cause breaks in the ocean crust, which is still quite hot. As the seawater,<br />
INTERNATIONAL SEABED AUTHORITY 414
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