The morphological interpretation from side-scan sonar images obtained with
the N.O. L’ATALANTE in 1999 during the Polynaut cruise shows lava channels
and dike propagation within the explored Pitcairn hotspot region. It was found that
small volcanic cones (500 m high) are formed during episodic eruptions from a
main magma reservoir, which is the same reservoir that fed the large volcanoes. In
the Pitcairn hotspot, the only evidence of eventual feeder channels and eruptions
are observed by the distribution of volcanic cones along preferential directions.
Another indirect proof is suggested by the preferential occurrence of evolved
(silica-enriched) lava on the small conical edifices surrounding the larger edifices.
Diving on the Pitcairn Hotspot
During the Polynaut cruise (1999), fourteen dives were performed on the Pitcairn
hotspot and seven took place in the Society hotspot. In the Pitcairn area, 13
dredges and 14 hydrological stations (CTD = Current, Temperature and Density
measurements) were also performed in addition to the dives. Our exploration was
focused primarily on the geomorphology of the seafloor and on the composition of
the erupted lava on the various, different-sized edifices.
From the surface ship observations, such as the bathymetry and deep-towed
imaging systems, we were able to define the limit of the hotspots by the relative
abundance of the sediment cover and the sampling carried out from previous
cruises which gave us, by looking at the degree of rock alteration, the relative age
of the sea floor. It was found that some seamounts were indeed ancient edifices
probably having an age close to that of the sea floor on ancient spreading centers.
Our estimate on the limits for the hotspot corresponded roughly to that of the
3500 m contour line observed on the bathymetry and the side-scan imagery.
One of the major objectives of the dives was to verify our surface observations
and locate the boundary of the Pitcairn hotspot activity by direct observation and
in situ sampling. This would be important in order for us to infer the volcanic
budget associated with this hotspot. Also, due to the difficulty in acquiring continuous observations of oceanic outcrops, very little is known about the stratigraphy of submarine volcanoes. Thus, during the Polynaut cruise, a special effort
was made to conduct detailed geological observations along several dive profiles
from the bottom to the top of the volcanoes. Continuous sea floor observation of
the morphology and direct sampling enabled us to determine the cyclic nature of
the volcanic events that are responsible for building the tall edifices. The questions
that we tried to answer were: What is the extent of the hotspot activity? Is there
any magmatic and morpho-structural relationship between the various types of
volcanic constructions?
Another objective was to obtain a measure of the gravity anomalies on the
bottom. This was performed with a portable gravity instrument that Jacques
Dubois (Fig. 9.17) had brought with him to be placed inside the submersible
during each dive. The gravimetric data would serve to infer the flexure of the
Bathymetry and Structural Setting
331
the N.O. L’ATALANTE in 1999 during the Polynaut cruise shows lava channels
and dike propagation within the explored Pitcairn hotspot region. It was found that
small volcanic cones (500 m high) are formed during episodic eruptions from a
main magma reservoir, which is the same reservoir that fed the large volcanoes. In
the Pitcairn hotspot, the only evidence of eventual feeder channels and eruptions
are observed by the distribution of volcanic cones along preferential directions.
Another indirect proof is suggested by the preferential occurrence of evolved
(silica-enriched) lava on the small conical edifices surrounding the larger edifices.
Diving on the Pitcairn Hotspot
During the Polynaut cruise (1999), fourteen dives were performed on the Pitcairn
hotspot and seven took place in the Society hotspot. In the Pitcairn area, 13
dredges and 14 hydrological stations (CTD = Current, Temperature and Density
measurements) were also performed in addition to the dives. Our exploration was
focused primarily on the geomorphology of the seafloor and on the composition of
the erupted lava on the various, different-sized edifices.
From the surface ship observations, such as the bathymetry and deep-towed
imaging systems, we were able to define the limit of the hotspots by the relative
abundance of the sediment cover and the sampling carried out from previous
cruises which gave us, by looking at the degree of rock alteration, the relative age
of the sea floor. It was found that some seamounts were indeed ancient edifices
probably having an age close to that of the sea floor on ancient spreading centers.
Our estimate on the limits for the hotspot corresponded roughly to that of the
3500 m contour line observed on the bathymetry and the side-scan imagery.
One of the major objectives of the dives was to verify our surface observations
and locate the boundary of the Pitcairn hotspot activity by direct observation and
in situ sampling. This would be important in order for us to infer the volcanic
budget associated with this hotspot. Also, due to the difficulty in acquiring continuous observations of oceanic outcrops, very little is known about the stratigraphy of submarine volcanoes. Thus, during the Polynaut cruise, a special effort
was made to conduct detailed geological observations along several dive profiles
from the bottom to the top of the volcanoes. Continuous sea floor observation of
the morphology and direct sampling enabled us to determine the cyclic nature of
the volcanic events that are responsible for building the tall edifices. The questions
that we tried to answer were: What is the extent of the hotspot activity? Is there
any magmatic and morpho-structural relationship between the various types of
volcanic constructions?
Another objective was to obtain a measure of the gravity anomalies on the
bottom. This was performed with a portable gravity instrument that Jacques
Dubois (Fig. 9.17) had brought with him to be placed inside the submersible
during each dive. The gravimetric data would serve to infer the flexure of the
Bathymetry and Structural Setting
331