volcanism were detected during the Oceanaut diving cruise in the axial rift valley.
The cones are well preserved in the southern portion of the ridge where a collapsed
crater, due to explosive volcanism, was noticed during dives OT02 and -03
(Fig. 7.9).
Highlights of the OCEANAUT Project Dives
Dive OT01 was on Sunday, August 27, 1995, and I was scheduled to be on this
first dive. The dive’s objective was to find the most recent lava flows, which would
help us pinpoint the location of the rift valley axis. As usual the dive was to begin
at exactly 8:00 AM. Pilot Jean-Michel Nivaggioli and Co-pilot Patrick Cheilan
were waiting for me to enter the sphere of the submersible. I went up the stairs and
climbed up the side of the submarine. Once again, I cleaned off my shoes before
entering the sphere where Patrick was waiting to give the signal ‘‘go’’. Diving
again on a Sunday meant that we were missing a good meal on board the Nadir. It
was hoped that on our return, the chief steward, Michel Squiban, also called
‘‘Aldo’’, would have kept some leftovers for our supper.
We arrived at contact with the sea floor at 10:10 and found ourselves in a field
of sheet flow, with ropy and ‘‘scoria-like’’ lava. The first dive is usually critical for
the rest of the cruise. We did not have any clear ideas about where the most recent
volcanic activity would occur. When we reached the sea floor, we tried to relocate
ourselves and asked the command post on the surface about our position. Looking
through the porthole, I could tell that we were not on an active part of the ridge,
but instead we had landed on the first faulted step of the eastern wall where there is
abundant rock debris forming talus. However, as soon as we got our position fix
from the surface and started to progress towards the west, we hit fresh lava flow.
This indicated the presence of a recent flow located on the eastern side of the rift
valley floor near the eastern wall. As we proceeded further west, we encountered
several mounds up to 20 m high and 100 m wide with older looking (dull
appearance) pillow lava in the middle of the fresher sheet lava. Suddenly, there
was a sharp contrast in the shiny nature of the lava flow that we were crossing. I
asked the pilot to follow the boundary line of the two flows that we had crossed. It
was then that we found a recently formed sheet flow terrain of 1 km long by
0.2 km wide that had invaded an older pillow lava terrain. It became clear that the
most recent flow had covered the pillow lava, leaving only a few older and taller
volcanic constructions. We were heading towards the eastern slope of the Median
Ridge, which became more distinct as a major feature on our radar screen. We
traveled with continuous visual contact with the sea floor for about 2 km before we
arrived at the foot of a fractured slope with abundant talus piles due to major
slumping.
The next day’s dive (OT02) with Daniel Bideau accompanied by Pilot Pierre
Triger and co-pilot Olivier Cipriani also took place in the rift valley. However,
instead of crossing the valley from the East to the West, the plan was to start at the
190
7 Oceanic Spreading Ridges and Sea Floor Creation
The cones are well preserved in the southern portion of the ridge where a collapsed
crater, due to explosive volcanism, was noticed during dives OT02 and -03
(Fig. 7.9).
Highlights of the OCEANAUT Project Dives
Dive OT01 was on Sunday, August 27, 1995, and I was scheduled to be on this
first dive. The dive’s objective was to find the most recent lava flows, which would
help us pinpoint the location of the rift valley axis. As usual the dive was to begin
at exactly 8:00 AM. Pilot Jean-Michel Nivaggioli and Co-pilot Patrick Cheilan
were waiting for me to enter the sphere of the submersible. I went up the stairs and
climbed up the side of the submarine. Once again, I cleaned off my shoes before
entering the sphere where Patrick was waiting to give the signal ‘‘go’’. Diving
again on a Sunday meant that we were missing a good meal on board the Nadir. It
was hoped that on our return, the chief steward, Michel Squiban, also called
‘‘Aldo’’, would have kept some leftovers for our supper.
We arrived at contact with the sea floor at 10:10 and found ourselves in a field
of sheet flow, with ropy and ‘‘scoria-like’’ lava. The first dive is usually critical for
the rest of the cruise. We did not have any clear ideas about where the most recent
volcanic activity would occur. When we reached the sea floor, we tried to relocate
ourselves and asked the command post on the surface about our position. Looking
through the porthole, I could tell that we were not on an active part of the ridge,
but instead we had landed on the first faulted step of the eastern wall where there is
abundant rock debris forming talus. However, as soon as we got our position fix
from the surface and started to progress towards the west, we hit fresh lava flow.
This indicated the presence of a recent flow located on the eastern side of the rift
valley floor near the eastern wall. As we proceeded further west, we encountered
several mounds up to 20 m high and 100 m wide with older looking (dull
appearance) pillow lava in the middle of the fresher sheet lava. Suddenly, there
was a sharp contrast in the shiny nature of the lava flow that we were crossing. I
asked the pilot to follow the boundary line of the two flows that we had crossed. It
was then that we found a recently formed sheet flow terrain of 1 km long by
0.2 km wide that had invaded an older pillow lava terrain. It became clear that the
most recent flow had covered the pillow lava, leaving only a few older and taller
volcanic constructions. We were heading towards the eastern slope of the Median
Ridge, which became more distinct as a major feature on our radar screen. We
traveled with continuous visual contact with the sea floor for about 2 km before we
arrived at the foot of a fractured slope with abundant talus piles due to major
slumping.
The next day’s dive (OT02) with Daniel Bideau accompanied by Pilot Pierre
Triger and co-pilot Olivier Cipriani also took place in the rift valley. However,
instead of crossing the valley from the East to the West, the plan was to start at the
190
7 Oceanic Spreading Ridges and Sea Floor Creation