level of the first dive OT01 where the freshest lava flows had been seen, and then
proceed southward in order to follow the track of the most recent eruption. During
the dive, Daniel identified several types of freshly extruded lava. He found
‘‘scoria’’-looking lobated lava, plus flat sheet flows and ropy lava along a gentle,
south oriented slope between 2,243 and 2,252 m depth. At about 2,230–2,242 m
depth in a small hilly area, the morphology of the lava flow changed. Indeed at
sample site OT02-05 older terrain of dull looking pillows covered with thin
manganese crusts and interstitial pockets of sediment between the pillows were
observed. Then, as the depth increased to 2,252–2,257 m, another area of fresh
sheet flow appeared until arriving at another shallower area (at 2,242–2,200 meters
depth). This along strike exploration permitted us to define the extent, 1 km in
length, of at least two freshly erupted lava flows covering an older, sediment
covered pillow lava terrain. From the previous dive (OT01) with its east west
passage across the rift valley, we had defined the width of the freshly extruded lava
to be about 1 km wide. When assuming a minimum thickness of 1 m it was
inferred that each eruptive event extruded about 0.001 km
3 of lava (see Chap. 5).
The third dive (OT03) took place on the east flank of the Median Ridge (MR)
where it was previously inferred that loose material was covering up a large area of
the edifice’s summit (Fig. 7.9). This was suggested from the bottom imagery
obtained from acoustic sonar during a previous cruise with the N.O. L’ATALANTE
and we wanted to have ground true observations. If this loose material were related
to an explosive event then we should be able to find a crater.
Marc Constantin, a geologist from Canada, University of Quebec, who was
doing his PhD with me at IFREMER-UBO, was assigned to dive as the scientific
observer to explore the volcanic edifice built on top of the Median Ridge. Pilot
Jean-Michel Nivaggioli and Co-pilot Patrick Cheilan accompanied Marc. The dive
started on the morning of August 29, 1995, at the usual time of 8 AM and the party
arrived on the sea floor at 10:42 at 2,330 m depth. After taking the first sample of a
fresh ropy lava fragment they started westward, and moved upward along the
slope. Marc described the presence of sheet flows and small lava ponds of 20 m in
diameter showing the pillars left after the drainage of the molten lava. They
traveled about 500 m westward before reaching the foot of the Median Ridge
(MR) at 2,330 m depths near the intersection of the rift valley floor, which is
marked by a North–South oriented fault. At 12:00, a little more than one hour after
landing, they started their ascension of the Median Ridge. The terrain had changed
its aspect and was more chaotic with abundant pillow lava fragments and coarsegrained debris forming large, 90 m-large talus piles up-slope. Several step-faulted
scarps made up of avalanche debris of pillows as well as pillow-lava tubes were
observed up to 1,800 m depth. Then suddenly, to their surprise, the rock debris
became more abundant and unstable along the slope. The pilot stared to slow down
and get back away from the slope to avoid any unexpected avalanches.
At 14h51 the divers observed the presence of gravel and heterogeneous scorialike debris mixed with large blocks. This is comparable to what is encountered on
subaerial volcanoes after an explosive eruption. Indeed, the material consisted of
accidental rock debris also called a ‘‘pyroclastic’’ flow, which was generated
Axial Rift Valley
191
proceed southward in order to follow the track of the most recent eruption. During
the dive, Daniel identified several types of freshly extruded lava. He found
‘‘scoria’’-looking lobated lava, plus flat sheet flows and ropy lava along a gentle,
south oriented slope between 2,243 and 2,252 m depth. At about 2,230–2,242 m
depth in a small hilly area, the morphology of the lava flow changed. Indeed at
sample site OT02-05 older terrain of dull looking pillows covered with thin
manganese crusts and interstitial pockets of sediment between the pillows were
observed. Then, as the depth increased to 2,252–2,257 m, another area of fresh
sheet flow appeared until arriving at another shallower area (at 2,242–2,200 meters
depth). This along strike exploration permitted us to define the extent, 1 km in
length, of at least two freshly erupted lava flows covering an older, sediment
covered pillow lava terrain. From the previous dive (OT01) with its east west
passage across the rift valley, we had defined the width of the freshly extruded lava
to be about 1 km wide. When assuming a minimum thickness of 1 m it was
inferred that each eruptive event extruded about 0.001 km
3 of lava (see Chap. 5).
The third dive (OT03) took place on the east flank of the Median Ridge (MR)
where it was previously inferred that loose material was covering up a large area of
the edifice’s summit (Fig. 7.9). This was suggested from the bottom imagery
obtained from acoustic sonar during a previous cruise with the N.O. L’ATALANTE
and we wanted to have ground true observations. If this loose material were related
to an explosive event then we should be able to find a crater.
Marc Constantin, a geologist from Canada, University of Quebec, who was
doing his PhD with me at IFREMER-UBO, was assigned to dive as the scientific
observer to explore the volcanic edifice built on top of the Median Ridge. Pilot
Jean-Michel Nivaggioli and Co-pilot Patrick Cheilan accompanied Marc. The dive
started on the morning of August 29, 1995, at the usual time of 8 AM and the party
arrived on the sea floor at 10:42 at 2,330 m depth. After taking the first sample of a
fresh ropy lava fragment they started westward, and moved upward along the
slope. Marc described the presence of sheet flows and small lava ponds of 20 m in
diameter showing the pillars left after the drainage of the molten lava. They
traveled about 500 m westward before reaching the foot of the Median Ridge
(MR) at 2,330 m depths near the intersection of the rift valley floor, which is
marked by a North–South oriented fault. At 12:00, a little more than one hour after
landing, they started their ascension of the Median Ridge. The terrain had changed
its aspect and was more chaotic with abundant pillow lava fragments and coarsegrained debris forming large, 90 m-large talus piles up-slope. Several step-faulted
scarps made up of avalanche debris of pillows as well as pillow-lava tubes were
observed up to 1,800 m depth. Then suddenly, to their surprise, the rock debris
became more abundant and unstable along the slope. The pilot stared to slow down
and get back away from the slope to avoid any unexpected avalanches.
At 14h51 the divers observed the presence of gravel and heterogeneous scorialike debris mixed with large blocks. This is comparable to what is encountered on
subaerial volcanoes after an explosive eruption. Indeed, the material consisted of
accidental rock debris also called a ‘‘pyroclastic’’ flow, which was generated
Axial Rift Valley
191