More viscous evolved lava (trachy-andesite and trachyte) forms flattened tubes
and tabular blocks with a thick (up to 10–20 cm thick) glassy crust. These flows
commonly have a scoria-like surface due to the cracking of the outer crust followed by melt spill and quenching during drainage of the lava tubes. Also they do
not show radial joints but, rather, reveal a layered internal structure with elongated
cavities (Fig. 9.18a).
The explosive events marked by the extrusion of pyroclasts in the form of dark
colored ash mixed with rock debris (pyroclast) and hyaloclasts are associated with
the evolved flows of trachybasalt and trachy-andesite at various depths (i.e.
500–2691 m depth). Sometimes, these flows are overlaid by hyaloclastites, which
were seen up to about 3000 m depth during dives PN14 and PN13 (Fig. 9.18a, b).
In summary, the observed sequential diversities of the exposed lava are related
to individualized volcanic pulses taking place during the construction of the volcanic edifices. Each individual flow extruded during each cycle has flowed over a
relatively short distance (\300 m) when compared to subaerial volcanism. This is
due to the smaller magma delivery as well as to the sea floor environment (seawater pressure and rapidly quenched lava cooling).
Hydrothermal Activity of Intraplate Hotspot Volcanoes
Since hydrothermal activity was abundantly found in most spreading ridge segments in the World’s oceans, it was important to look for such phenomena in
intraplate regions associated with hotspots. Cyana dive Cy83-62 in 1983 during
the Cyacite cruise of the N.O. JEAN CHARCOT found the first hydrothermal field
discovered on the south Pacific intraplate region of the Society hotspot. Jean Louis
Cheminée was the first scientist to see an active hydrothermal field on top of the
Teahitia volcano at 1454 meters depth.
The recently active hotspot seamounts contain hydrothermal material made up
of red and yellow Fe-oxyhydroxide, plus chimneys and slabs forming layered
sequences of semi-consolidated deposits on top of lava flows and/or volcanic ash
(lapilli) formations. Hydrothermal chimneys varying in size from less than 20 cm
up to 15 m in height were seen, mainly on the Teahitia and Turoi volcanoes. On
the Turoi seamount, which consists of extinct fields and older looking lava flows,
the chimneys are coated by Fe–Mn crust. The active fields on Teahitia consist of
smaller (\20 m) chimneys, which show no signs of Fe–Mn surface coatings.
Deep towed camera and submersible surveys showed that the summit
(1400–1700 m depth) of the Teahitia volcano is made up of reddish-orange
hydrothermal chimneys associated with flat-lying, powdery, ochreous, discontinuous Fe-rich deposits covering an area of more than 1 km
2 . The hydrothermal
fluids are charged with Fe, Si and Mn and minor amounts of other transitional
metals (Hoffert et al. 1987; Puteanus et al. 1991; Michard et al. 1989). They have a
similar morphology and composition to those encountered on the off-axial seamounts near the EPR at 12°43
0 N and 11°30
0 N. Low temperature (\30–60 °C)
340
9 Hotspots
and tabular blocks with a thick (up to 10–20 cm thick) glassy crust. These flows
commonly have a scoria-like surface due to the cracking of the outer crust followed by melt spill and quenching during drainage of the lava tubes. Also they do
not show radial joints but, rather, reveal a layered internal structure with elongated
cavities (Fig. 9.18a).
The explosive events marked by the extrusion of pyroclasts in the form of dark
colored ash mixed with rock debris (pyroclast) and hyaloclasts are associated with
the evolved flows of trachybasalt and trachy-andesite at various depths (i.e.
500–2691 m depth). Sometimes, these flows are overlaid by hyaloclastites, which
were seen up to about 3000 m depth during dives PN14 and PN13 (Fig. 9.18a, b).
In summary, the observed sequential diversities of the exposed lava are related
to individualized volcanic pulses taking place during the construction of the volcanic edifices. Each individual flow extruded during each cycle has flowed over a
relatively short distance (\300 m) when compared to subaerial volcanism. This is
due to the smaller magma delivery as well as to the sea floor environment (seawater pressure and rapidly quenched lava cooling).
Hydrothermal Activity of Intraplate Hotspot Volcanoes
Since hydrothermal activity was abundantly found in most spreading ridge segments in the World’s oceans, it was important to look for such phenomena in
intraplate regions associated with hotspots. Cyana dive Cy83-62 in 1983 during
the Cyacite cruise of the N.O. JEAN CHARCOT found the first hydrothermal field
discovered on the south Pacific intraplate region of the Society hotspot. Jean Louis
Cheminée was the first scientist to see an active hydrothermal field on top of the
Teahitia volcano at 1454 meters depth.
The recently active hotspot seamounts contain hydrothermal material made up
of red and yellow Fe-oxyhydroxide, plus chimneys and slabs forming layered
sequences of semi-consolidated deposits on top of lava flows and/or volcanic ash
(lapilli) formations. Hydrothermal chimneys varying in size from less than 20 cm
up to 15 m in height were seen, mainly on the Teahitia and Turoi volcanoes. On
the Turoi seamount, which consists of extinct fields and older looking lava flows,
the chimneys are coated by Fe–Mn crust. The active fields on Teahitia consist of
smaller (\20 m) chimneys, which show no signs of Fe–Mn surface coatings.
Deep towed camera and submersible surveys showed that the summit
(1400–1700 m depth) of the Teahitia volcano is made up of reddish-orange
hydrothermal chimneys associated with flat-lying, powdery, ochreous, discontinuous Fe-rich deposits covering an area of more than 1 km
2 . The hydrothermal
fluids are charged with Fe, Si and Mn and minor amounts of other transitional
metals (Hoffert et al. 1987; Puteanus et al. 1991; Michard et al. 1989). They have a
similar morphology and composition to those encountered on the off-axial seamounts near the EPR at 12°43
0 N and 11°30
0 N. Low temperature (\30–60 °C)
340
9 Hotspots