axis show hydrothermal activities and sulfide mineralization. Up to now, only a
few cases of extensive hydrothermal fields have been reported in association with
seamounts on the ocean floor. Some of these consist of massive sulfide deposits
capped by a mixture of Fe-oxyhydroxide and manganese crusts. It has been shown
that part of this capped material is the result of low temperature fluid precipitation
giving rise to a laminated crust of Fe-oxide-hydroxide, and a green and reddishyellow clay called nontronite.
The off-axis Clipperton Seamount is the tallest volcano in the area of study. It
is about 1000 m high above the surrounding floor and located 18 km west of the
EPR axis near 12°36.507
0 N-104°02.02
0 W. The seamount was found on May 15,
1981 during the CLIPPERTON cruise of the NO. JEAN CHARCOT, and its name
was given after the cruise of the same name. This seamount, about 4 km in
diameter, has a summit that culminates at 2,200 m depth. It is formed by four
volcanic cones, which are located on top of a plateau that is shallower than
2,200 m. Three of the cones are less than 60 m tall and the major cone is about
150 m high above the plateau. This seamount is of particular interest because it
was constructed about 200,000 yr ago near the ridge axis, and since then, it has
been moved to its present location. It is not known whether or not the Clipperton
seamount it is still volcanically active.
I decided to do a dive on this seamount and the goal was to make a survey by
reaching the flank of the seamount near the summit and then explore the top to
look for recent volcanism. The dive with Cyana (CY82-29) took place on February
23, 1982, with Nivaggioli as pilot and Normand as co-pilot. We started our dive at
8:42 and reached the sea floor 11:06 at 2182 meters depth. When looking through
the porthole, I saw a few scattered pillow lavas that were partially buried by
sediment. I had the track of the dive by my side and told Niva to take the course to
point ‘‘B’’ towards one of the transponders that served as a reference for navigation. This location was near one of the small volcanic cones located on the
southeast corner of the seamount and we had to travel for about 2 km in order to
reach the top. The sediment blanket on top of what seemed to me to be a flat flow
surprised me. The sediment cover was thin (millimeter scale) and thus the flow
must have been relatively young (\1000 yr). At 11h52 we climbed on the flank of
the volcanic cone and stopped to collect a sample in a sheet flow type of terrain in
the form of ‘‘drapery’’. It was then, while the pilot had extended the starboard
mechanical arm for grabbing the rock, that suddenly we noticed droplets of oil
leaking through the fore-arm branch. Niva did not stop the sampling. He took the
rock and put it into the basket. However the leak had now extended to the level of
the claw. But this did not stop Niva, who had not yet reported the anomaly to the
surface ship.
We continued our ascension towards the top of the volcanic cone following a
heading of 345°N. Some organic material (gorgonians) covered the 5-m high scarp
at 2,158 m depth. We reached the summit of the volcanic cone at 2,162 m depth at
12h11 and stopped again to sample. When the arm was pulled out we still could
see the droplets of the oil leak. This meant that water must be coming into the
system. There was no rush because this type of situation had happened before, so
Off-Axial Seamounts on the EPR at 12°50
0 N and 11°20
0 N
229
few cases of extensive hydrothermal fields have been reported in association with
seamounts on the ocean floor. Some of these consist of massive sulfide deposits
capped by a mixture of Fe-oxyhydroxide and manganese crusts. It has been shown
that part of this capped material is the result of low temperature fluid precipitation
giving rise to a laminated crust of Fe-oxide-hydroxide, and a green and reddishyellow clay called nontronite.
The off-axis Clipperton Seamount is the tallest volcano in the area of study. It
is about 1000 m high above the surrounding floor and located 18 km west of the
EPR axis near 12°36.507
0 N-104°02.02
0 W. The seamount was found on May 15,
1981 during the CLIPPERTON cruise of the NO. JEAN CHARCOT, and its name
was given after the cruise of the same name. This seamount, about 4 km in
diameter, has a summit that culminates at 2,200 m depth. It is formed by four
volcanic cones, which are located on top of a plateau that is shallower than
2,200 m. Three of the cones are less than 60 m tall and the major cone is about
150 m high above the plateau. This seamount is of particular interest because it
was constructed about 200,000 yr ago near the ridge axis, and since then, it has
been moved to its present location. It is not known whether or not the Clipperton
seamount it is still volcanically active.
I decided to do a dive on this seamount and the goal was to make a survey by
reaching the flank of the seamount near the summit and then explore the top to
look for recent volcanism. The dive with Cyana (CY82-29) took place on February
23, 1982, with Nivaggioli as pilot and Normand as co-pilot. We started our dive at
8:42 and reached the sea floor 11:06 at 2182 meters depth. When looking through
the porthole, I saw a few scattered pillow lavas that were partially buried by
sediment. I had the track of the dive by my side and told Niva to take the course to
point ‘‘B’’ towards one of the transponders that served as a reference for navigation. This location was near one of the small volcanic cones located on the
southeast corner of the seamount and we had to travel for about 2 km in order to
reach the top. The sediment blanket on top of what seemed to me to be a flat flow
surprised me. The sediment cover was thin (millimeter scale) and thus the flow
must have been relatively young (\1000 yr). At 11h52 we climbed on the flank of
the volcanic cone and stopped to collect a sample in a sheet flow type of terrain in
the form of ‘‘drapery’’. It was then, while the pilot had extended the starboard
mechanical arm for grabbing the rock, that suddenly we noticed droplets of oil
leaking through the fore-arm branch. Niva did not stop the sampling. He took the
rock and put it into the basket. However the leak had now extended to the level of
the claw. But this did not stop Niva, who had not yet reported the anomaly to the
surface ship.
We continued our ascension towards the top of the volcanic cone following a
heading of 345°N. Some organic material (gorgonians) covered the 5-m high scarp
at 2,158 m depth. We reached the summit of the volcanic cone at 2,162 m depth at
12h11 and stopped again to sample. When the arm was pulled out we still could
see the droplets of the oil leak. This meant that water must be coming into the
system. There was no rush because this type of situation had happened before, so
Off-Axial Seamounts on the EPR at 12°50
0 N and 11°20
0 N
229