cruise was to study the Pitcairn hotspot, but we also concentrated a few of our
dives in the Society region.
Two dives were made at the boundary marked by the 3200 m contour line
limiting the hotspot. This area is also called the ‘‘bulge’’ (Fig. 9.4). The bulge is
the ancient oceanic crust on which the hotspot volcanoes are formed. It is now
covered with volcanic ash and fragmented debris (pyroclasts) that have been
scattered from the eruptions of the edifices.
Three dives were made on Teahitia and two dives were done on the Rocard
volcano of the Society hotspot. Dives PN17 (PN = Polynaut, N = Nautile),
PN18 and PN19 were made on September 21, 22, and 23,1999, on the summit of
the Teahitia Volcano to sample and observe the field of hydrothermal vents. Peter
Stoffers (Kiel University in Germany), Steve Scott (University of Toronto,
Canada) and Gindi French (Jet Propulsion Laboratory in Pasadena, California,
USA) were the scientific observers on board the Nautile. Peter already knew the
area because ten years before he had made his first dive here with the submersible Cyana. For Steve and Gindi, it was a new target and they were both
curious to see if active hydrothermal activity still existed. The task was to
observe the evolution of the hydrothermal field since the previous dives in 89.
When comparing the data, it was found that hydrothermalism in the deposits
located on the three summit cones was still active, however the presence of
pyroclastic deposits was more prominent than before, and these deposits were
now covering the flanks of the parasite volcanic cones.
Two other two dives, Dives PN20 and PN21, were dedicated to exploring
another recent volcano called Rocard, which we believed was the site of abundant
silica-enriched lava (Fig. 9.8a). We wanted to visualize the field of these types of
lava flows with respect to the more commonly found basaltic flows. Very little is
known about the mode and extent of silicic lava eruption on the seafloor.
Colin Devey, a geochemist from the University of Kiel, and I were scheduled to
dive on Rocard on the 24th and the 25th of September 1999. Rocard is a ‘‘multiple
volcano’’ made up of several (at least four) volcanic cones. Since we could not
explore all of them, we decided to look at the two located on the hotspot
extremities (Fig. 9.4): the most southern cone (dive PN20, with Colin Devey) and
the most northern one (dive PN21, with Roger Hekinian). During our exploration
of Rocard’s volcanic cones, we found that the edifice consisted of silicic lava
associated with hyaloclastites and iron manganese crust. The silica-enriched flows
had given rise to multiple edifices less than 400 m in height that covered a surface
of about 30 km on the sea floor.
Summary of Results
The important observations made during these expeditions on the Society hotspot
volcanoes were that the ‘‘bulge’’ (or newly-formed sea floor on top of ancient,
60-million year old, oceanic crust) differs in its lava morphology, its composition
318
9 Hotspots
dives in the Society region.
Two dives were made at the boundary marked by the 3200 m contour line
limiting the hotspot. This area is also called the ‘‘bulge’’ (Fig. 9.4). The bulge is
the ancient oceanic crust on which the hotspot volcanoes are formed. It is now
covered with volcanic ash and fragmented debris (pyroclasts) that have been
scattered from the eruptions of the edifices.
Three dives were made on Teahitia and two dives were done on the Rocard
volcano of the Society hotspot. Dives PN17 (PN = Polynaut, N = Nautile),
PN18 and PN19 were made on September 21, 22, and 23,1999, on the summit of
the Teahitia Volcano to sample and observe the field of hydrothermal vents. Peter
Stoffers (Kiel University in Germany), Steve Scott (University of Toronto,
Canada) and Gindi French (Jet Propulsion Laboratory in Pasadena, California,
USA) were the scientific observers on board the Nautile. Peter already knew the
area because ten years before he had made his first dive here with the submersible Cyana. For Steve and Gindi, it was a new target and they were both
curious to see if active hydrothermal activity still existed. The task was to
observe the evolution of the hydrothermal field since the previous dives in 89.
When comparing the data, it was found that hydrothermalism in the deposits
located on the three summit cones was still active, however the presence of
pyroclastic deposits was more prominent than before, and these deposits were
now covering the flanks of the parasite volcanic cones.
Two other two dives, Dives PN20 and PN21, were dedicated to exploring
another recent volcano called Rocard, which we believed was the site of abundant
silica-enriched lava (Fig. 9.8a). We wanted to visualize the field of these types of
lava flows with respect to the more commonly found basaltic flows. Very little is
known about the mode and extent of silicic lava eruption on the seafloor.
Colin Devey, a geochemist from the University of Kiel, and I were scheduled to
dive on Rocard on the 24th and the 25th of September 1999. Rocard is a ‘‘multiple
volcano’’ made up of several (at least four) volcanic cones. Since we could not
explore all of them, we decided to look at the two located on the hotspot
extremities (Fig. 9.4): the most southern cone (dive PN20, with Colin Devey) and
the most northern one (dive PN21, with Roger Hekinian). During our exploration
of Rocard’s volcanic cones, we found that the edifice consisted of silicic lava
associated with hyaloclastites and iron manganese crust. The silica-enriched flows
had given rise to multiple edifices less than 400 m in height that covered a surface
of about 30 km on the sea floor.
Summary of Results
The important observations made during these expeditions on the Society hotspot
volcanoes were that the ‘‘bulge’’ (or newly-formed sea floor on top of ancient,
60-million year old, oceanic crust) differs in its lava morphology, its composition
318
9 Hotspots