and in the style of eruption. The volcanoes that were built on the ancient seafloor
inherited from the EPR spreading plate boundary consist of conical edifices, which
have erupted different types of lava than what is found on modern spreading
ridges.
Petrological studies on samples collected from the Society hotspot volcanoes
have revealed the presence of two major magma types: (1) A low-K tholeiitic
basalt associated with ancient edifices, probably erupted during seafloor spreading at the ridge axis, and (2) an alkali-rich suite of basalt, trachybasalt and
trachyte related to the more recent volcanic events of the Society hotspot (Hekinian et al. 1991). Other geochemical studies (Devey et al. 2003) have also
shown the heterogeneous nature of the magma sources and their relationship to
the alkali-basalt suites characterizing hotspot volcanoes. It was found that
hydrothermal fluid circulation in hotspot volcanoes gives rise to precipitates on
the sea floor that are deprived in metallic sulfides.
Austral Volcanic Chain
The Austral volcanic chain extends for about 1700 km in length and runs parallel
to the Society Island Chain located 400 km to the south. As was true for the
Society volcanic chain, the ages of the islands and atolls increase westward suggesting that they were originated by hotspot volcanism.
The Macdonald and Ra Seamount
In 1987, during the R.V. SONNE Leg 65 cruise, we had been able to entirely map
the volcanically active Macdonald seamount for the first time. During the same
cruise, when making a transect to the first island of the chain called Marotiri, a new
volcanic edifice, about 50 km west of Macdonald and rising to 1000 m above the
sea floor was discovered. This new edifice was named the ‘‘Ra’’ seamount. The
name ‘‘Ra’’ means ‘‘Sun’’ in the Polynesian language as well as in ancient
Egyptian.
Since our main goal was to explore the Macdonald seamount, we spent very
little time on the Ra seamount. We only explored long enough to make a rock
dredge and perform a deep-towed camera station on its summit. The deep-towed
camera station revealed the presence of layered ash deposits covered by thin
manganese crust and bulbous pillow lava suggesting that the seamount was an
ancient, explosive volcano.
The Macdonald seamount located on the southeastern extension of the Austral
volcanic chain at about 100 km away from the Ra seamount is one of the most
active submarine volcanoes in the World (Fig. 9.9). The Macdonald seamount near
28°49°S-140°26°W was discovered in May 1967, following a strong seismic swarm
Summary of Results
319
inherited from the EPR spreading plate boundary consist of conical edifices, which
have erupted different types of lava than what is found on modern spreading
ridges.
Petrological studies on samples collected from the Society hotspot volcanoes
have revealed the presence of two major magma types: (1) A low-K tholeiitic
basalt associated with ancient edifices, probably erupted during seafloor spreading at the ridge axis, and (2) an alkali-rich suite of basalt, trachybasalt and
trachyte related to the more recent volcanic events of the Society hotspot (Hekinian et al. 1991). Other geochemical studies (Devey et al. 2003) have also
shown the heterogeneous nature of the magma sources and their relationship to
the alkali-basalt suites characterizing hotspot volcanoes. It was found that
hydrothermal fluid circulation in hotspot volcanoes gives rise to precipitates on
the sea floor that are deprived in metallic sulfides.
Austral Volcanic Chain
The Austral volcanic chain extends for about 1700 km in length and runs parallel
to the Society Island Chain located 400 km to the south. As was true for the
Society volcanic chain, the ages of the islands and atolls increase westward suggesting that they were originated by hotspot volcanism.
The Macdonald and Ra Seamount
In 1987, during the R.V. SONNE Leg 65 cruise, we had been able to entirely map
the volcanically active Macdonald seamount for the first time. During the same
cruise, when making a transect to the first island of the chain called Marotiri, a new
volcanic edifice, about 50 km west of Macdonald and rising to 1000 m above the
sea floor was discovered. This new edifice was named the ‘‘Ra’’ seamount. The
name ‘‘Ra’’ means ‘‘Sun’’ in the Polynesian language as well as in ancient
Egyptian.
Since our main goal was to explore the Macdonald seamount, we spent very
little time on the Ra seamount. We only explored long enough to make a rock
dredge and perform a deep-towed camera station on its summit. The deep-towed
camera station revealed the presence of layered ash deposits covered by thin
manganese crust and bulbous pillow lava suggesting that the seamount was an
ancient, explosive volcano.
The Macdonald seamount located on the southeastern extension of the Austral
volcanic chain at about 100 km away from the Ra seamount is one of the most
active submarine volcanoes in the World (Fig. 9.9). The Macdonald seamount near
28°49°S-140°26°W was discovered in May 1967, following a strong seismic swarm
Summary of Results
319