during a geological survey of the East Pacific Rise near 13°N under the leadership
of Harold Bäcker from Preussag (Germany). It was at the same time, during the
Cyatherm cruise with the RV LE SUROIT, that we met the German scientists on
site. I invited Harold Bäcker on board the R.V. LE SUROIT to participate in a
diving operation with Cyana. After his dive, we were invited on board the Sonne
where we gathered for drinks on the evening of February 9, 1982. It was then that
Harold, Jean Francheteau and I had a look at the first multichannel (Hydrosweep)
map that the Sonne had made of the area at 13850
0 N with its curving bathymetric
contour line. At the time, we did not understand what this ‘‘curving structure’’
meant. Could it have been an artifact of the multichannel acoustic signal? We did
not give the problem any further thought, but this was a mistake. We should have
tried to better understand that the structure we saw would later be identified as an
OSC. In fact, in the same year during July, the US Oceanographic Vessel Thomas
Washington sailed from San Diego to explore another area of the East Pacific Rise
using its newly acquired multichannel system called ‘‘SeaBeam’’. A few years
later, in an article published by Ken Macdonald and others from the University of
California in Santa Barbara, we noticed that the features described by the authors
were familiar and we realized we had missed this while we were on board the RV
SONNE in 1982 (Macdonald et al. 1988).
The Mid-Atlantic Ridge
The Mid-Atlantic Ridge (MAR) consists of slow spreading (\3 cm/year total rate)
ridge segments and has the particularity of having a variable and more irregular
axial topography than that of faster spreading ridges. The MAR segments show
marked discontinuities such as well-defined transform and non-transform offsets.
The segments have variable and heterogeneous lava morphology. They are the
sites of tall seamounts as well as island formations.
Going from the Arctic Sea to the Bouvet triple junction (Bouvet island) in the
south Atlantic where it branches off towards the Indian Ocean, the MAR is about
14,000 km in length. The topographic differences are related to the changes in
volcanic activity along the ridge segments. The presence of several islands
piercing the sea surface, such as Jan Mayen, Iceland, the Azores, Ascension and
Tristan de Cunha indicates localized volcanism of hotspot origin.
Two distinct structural provinces are recognized in the Atlantic:
(1) The northern MAR province is related to the long-lasting, highly magmatic
and volcanically active ridge segments located north of the Azores including
the Reykjanes Ridge near Iceland and extending down to the Oceanographer
fracture zone at 35°N. In the south Atlantic, the MAR continues below the
Romanche fracture zone at 15°S down to the triple junction of Bouvet and
Gough islands near 40°16
0 S–10°00
0 W. These southern regions of the MAR are
also characterized by an elevated topography of their ridge segments with
170
7 Oceanic Spreading Ridges and Sea Floor Creation
of Harold Bäcker from Preussag (Germany). It was at the same time, during the
Cyatherm cruise with the RV LE SUROIT, that we met the German scientists on
site. I invited Harold Bäcker on board the R.V. LE SUROIT to participate in a
diving operation with Cyana. After his dive, we were invited on board the Sonne
where we gathered for drinks on the evening of February 9, 1982. It was then that
Harold, Jean Francheteau and I had a look at the first multichannel (Hydrosweep)
map that the Sonne had made of the area at 13850
0 N with its curving bathymetric
contour line. At the time, we did not understand what this ‘‘curving structure’’
meant. Could it have been an artifact of the multichannel acoustic signal? We did
not give the problem any further thought, but this was a mistake. We should have
tried to better understand that the structure we saw would later be identified as an
OSC. In fact, in the same year during July, the US Oceanographic Vessel Thomas
Washington sailed from San Diego to explore another area of the East Pacific Rise
using its newly acquired multichannel system called ‘‘SeaBeam’’. A few years
later, in an article published by Ken Macdonald and others from the University of
California in Santa Barbara, we noticed that the features described by the authors
were familiar and we realized we had missed this while we were on board the RV
SONNE in 1982 (Macdonald et al. 1988).
The Mid-Atlantic Ridge
The Mid-Atlantic Ridge (MAR) consists of slow spreading (\3 cm/year total rate)
ridge segments and has the particularity of having a variable and more irregular
axial topography than that of faster spreading ridges. The MAR segments show
marked discontinuities such as well-defined transform and non-transform offsets.
The segments have variable and heterogeneous lava morphology. They are the
sites of tall seamounts as well as island formations.
Going from the Arctic Sea to the Bouvet triple junction (Bouvet island) in the
south Atlantic where it branches off towards the Indian Ocean, the MAR is about
14,000 km in length. The topographic differences are related to the changes in
volcanic activity along the ridge segments. The presence of several islands
piercing the sea surface, such as Jan Mayen, Iceland, the Azores, Ascension and
Tristan de Cunha indicates localized volcanism of hotspot origin.
Two distinct structural provinces are recognized in the Atlantic:
(1) The northern MAR province is related to the long-lasting, highly magmatic
and volcanically active ridge segments located north of the Azores including
the Reykjanes Ridge near Iceland and extending down to the Oceanographer
fracture zone at 35°N. In the south Atlantic, the MAR continues below the
Romanche fracture zone at 15°S down to the triple junction of Bouvet and
Gough islands near 40°16
0 S–10°00
0 W. These southern regions of the MAR are
also characterized by an elevated topography of their ridge segments with
170
7 Oceanic Spreading Ridges and Sea Floor Creation