However, there is compelling evidence that explosive volcanism can and does
exist up to 3000 m depth. It is of common knowledge that vesicular lavas with gas
exsolution forming submarine flows are prevalent. The fact that highly vesicular
lavas and fragmented scoriaceous blocks (pyroclasts) are also found on the ocean
floor at relatively great depths ([1500–3000 m) has changed scientists’ point of
view concerning the occurrence of under water explosive activity.
The main difference between submarine and sub-aerial explosive volcanism is
that when we approach atmospheric pressure, water is the main volatile to degas
from magma, whereas in a deep-sea environment ([1000 m depths) about 85
volume percent of the volatile which degases is CO 2 . This is because H 2 O dissolves more easily in molten magma than CO 2 . The formation of CO 2 bubbles
(nucleation) can start to take place at depths of about 60 km under pressure of
20 kbar (Blank and Brooker 1994). This was extrapolated by measuring the
amount of bubbles trapped in magma (bubble density per total volume of gas)
(Burton et al. 2007).
The ‘‘quiet type’’ of volcanic eruptions are the commonest form of volcanism
taking place on the sea floor giving rise to basaltic pillow lava and sheet flows with
minor gas exsolution and very little vesicle formation (\5 %). These types of
samples are found along spreading ridge segments. However, the increase in the
degree of vesicularity ([15 %) and the occurrence of fragmented debris and ash
products at great depth ([1500 m depth) suggests that explosive types of volcanism must also have taken place. Intra-plate volcanism giving rise to highly
vesicular lava containing up to 15–30 % vesicles is commonly found in the
Pitcairn, Austral and Society hotspot areas, as reported by Binard et al. (1991).
Proof that submarine explosive events took place at depths greater than 1000 m
was found on several sea floor structures located at a significant distance from
emerged landmasses. The presence of hyaloclastites on off-axial volcanic edifices
in the northern Pacific were reported by several authors (Batiza et al. 1984; Bonatti
and Harrison 1988; Fornari et al. 1984; Smith and Batiza 1989; Hekinian et al.
1989). During two IFREMER cruises on slow spreading ridge segments of the
Mid-Atlantic ridge (DIVA 1 cruise in 1994 and OCEANAUT cruise in 1997),
evidence of explosive activity was collected by submersible. Accidental rock
debris of pyroclasts and hyaloclastites were observed and sampled by the submersible Nautile at more than 1000 m below the summit of volcanic edifices in the
rift valley of the Mid-Atlantic Ridge.
Submarine explosive deposits of pyroclastic debris are often associated with
hyaloclastites. The first pyroclast-hyaloclastite deposit was observed on-site in
1997 on the MAR (Mid-Atlantic Ridge) at 38°20
0 N (site called ‘‘Menez Gwen’’,
meaning ‘‘White Mountain’’ in Breton) and at 37°18
0 N (Lucky Strike site)
southwest of the Azores on the central volcano of the rift valley’s inner wall. These
two sites had layered volcaniclastic deposits at least 400 m thick (Ondreas et al.
1997; Eissen et al. 2004). The nature of the clasts found in these deposits consists
of highly vesicular altered shards of scoriaceous glass associated with other rock
fragments. In the Menez Gwen ridge segment (38°20
0 N), the pyroclastic deposits
extended over a large area of centrally located volcanoes in the rift valley of the
Submarine Volcanic Explosion
131
exist up to 3000 m depth. It is of common knowledge that vesicular lavas with gas
exsolution forming submarine flows are prevalent. The fact that highly vesicular
lavas and fragmented scoriaceous blocks (pyroclasts) are also found on the ocean
floor at relatively great depths ([1500–3000 m) has changed scientists’ point of
view concerning the occurrence of under water explosive activity.
The main difference between submarine and sub-aerial explosive volcanism is
that when we approach atmospheric pressure, water is the main volatile to degas
from magma, whereas in a deep-sea environment ([1000 m depths) about 85
volume percent of the volatile which degases is CO 2 . This is because H 2 O dissolves more easily in molten magma than CO 2 . The formation of CO 2 bubbles
(nucleation) can start to take place at depths of about 60 km under pressure of
20 kbar (Blank and Brooker 1994). This was extrapolated by measuring the
amount of bubbles trapped in magma (bubble density per total volume of gas)
(Burton et al. 2007).
The ‘‘quiet type’’ of volcanic eruptions are the commonest form of volcanism
taking place on the sea floor giving rise to basaltic pillow lava and sheet flows with
minor gas exsolution and very little vesicle formation (\5 %). These types of
samples are found along spreading ridge segments. However, the increase in the
degree of vesicularity ([15 %) and the occurrence of fragmented debris and ash
products at great depth ([1500 m depth) suggests that explosive types of volcanism must also have taken place. Intra-plate volcanism giving rise to highly
vesicular lava containing up to 15–30 % vesicles is commonly found in the
Pitcairn, Austral and Society hotspot areas, as reported by Binard et al. (1991).
Proof that submarine explosive events took place at depths greater than 1000 m
was found on several sea floor structures located at a significant distance from
emerged landmasses. The presence of hyaloclastites on off-axial volcanic edifices
in the northern Pacific were reported by several authors (Batiza et al. 1984; Bonatti
and Harrison 1988; Fornari et al. 1984; Smith and Batiza 1989; Hekinian et al.
1989). During two IFREMER cruises on slow spreading ridge segments of the
Mid-Atlantic ridge (DIVA 1 cruise in 1994 and OCEANAUT cruise in 1997),
evidence of explosive activity was collected by submersible. Accidental rock
debris of pyroclasts and hyaloclastites were observed and sampled by the submersible Nautile at more than 1000 m below the summit of volcanic edifices in the
rift valley of the Mid-Atlantic Ridge.
Submarine explosive deposits of pyroclastic debris are often associated with
hyaloclastites. The first pyroclast-hyaloclastite deposit was observed on-site in
1997 on the MAR (Mid-Atlantic Ridge) at 38°20
0 N (site called ‘‘Menez Gwen’’,
meaning ‘‘White Mountain’’ in Breton) and at 37°18
0 N (Lucky Strike site)
southwest of the Azores on the central volcano of the rift valley’s inner wall. These
two sites had layered volcaniclastic deposits at least 400 m thick (Ondreas et al.
1997; Eissen et al. 2004). The nature of the clasts found in these deposits consists
of highly vesicular altered shards of scoriaceous glass associated with other rock
fragments. In the Menez Gwen ridge segment (38°20
0 N), the pyroclastic deposits
extended over a large area of centrally located volcanoes in the rift valley of the
Submarine Volcanic Explosion
131