Island, Italy). It is a term used to describe an eruption of fluid, basaltic lava. An
explosive eruption in which fragmented magma and gas is released from a volcano
at high velocity is called a Plinian style of event. The etymology comes from Pliny
the Younger who first described this type of eruption taking place on Mount
Vesuvius in 79 A.D. Small-scale sub-aerial explosive volcanic eruptions inject
more than a million cubic meters of ash into Earth’s atmosphere every month
(Simkin and Siebert 2000).
Back in the sixties, scientists (such as McBirney 1963) thought that submarine
explosive volcanism could not take place at depths greater than a few hundred
meters. It was postulated that depths of no more than 500 m for depleted melt and
1000 m for alkali enriched melts, would be the limits for this type of volcanism.
Thus, it was believed that the fragmented debris (pyroclasts and hyaloclasts)
resulting from explosive eruptive events would be absent because the pressure of
the overlying water column would be sufficient to suppress juvenile gas exsolution.
In fact, during previous recovery of deep-seated ash and pyroclasts, the presence of
these products under the sea were often attributed to sub-aerial eruptions whose
explosive clasts and ash could have been subsequently transported either in the
atmosphere or by ocean currents.
Although fine ash is likely to be transported in the atmosphere as has been
observed during modern sub-aerial eruptions, it is evidenced that in situ deep-sea
explosion can also give rise to this type of deposit. Most of the records of oceanic
explosive events come from Archean ancient pyroclastic deposits found in continental areas. Others are from back-arc basins surrounded by island volcanism
such as the Okinawa Trough, the Lau Basin and the Tonga-Kermadec Ridge
submarine volcanoes. The pyroclastic deposits found at 1440–2250 m depths were
interpreted as being formed in the submarine environment (Gill et al. 1990; Wright
and Gamble 1999). When coring submarine sediment layers around the islands of
Sumatra and Java, it was found that deposits of pyroclastic flows more than 20 m
thick poured out during the eruption of Krakatau. This deposit was carried on the
sea’s surface and was deposited over a radius of about 70 km forming a semicircle to the northeast of the volcano, in the same direction as the tsunami wave of
1983 after the explosion of the Colo volcano on Una-Una Island.
Submarine Volcanic Explosion
We now know that underwater volcanism could include both explosive and quiet
types of events. Because seawater’s critical boiling point (315 bars and 374 °C)
corresponds to about 3000 m depth, it is impossible that explosive events could
take place at deeper levels, no matter how many volatiles are concentrated in the
magma. Even for the highly vesicular lava flows encountered on the ocean floor, it
would be impossible for the vesicles to nucleate, expand and grow at depths
greater than 3000 m.
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5 Earth’s Mantle Melting and Volcanism
explosive eruption in which fragmented magma and gas is released from a volcano
at high velocity is called a Plinian style of event. The etymology comes from Pliny
the Younger who first described this type of eruption taking place on Mount
Vesuvius in 79 A.D. Small-scale sub-aerial explosive volcanic eruptions inject
more than a million cubic meters of ash into Earth’s atmosphere every month
(Simkin and Siebert 2000).
Back in the sixties, scientists (such as McBirney 1963) thought that submarine
explosive volcanism could not take place at depths greater than a few hundred
meters. It was postulated that depths of no more than 500 m for depleted melt and
1000 m for alkali enriched melts, would be the limits for this type of volcanism.
Thus, it was believed that the fragmented debris (pyroclasts and hyaloclasts)
resulting from explosive eruptive events would be absent because the pressure of
the overlying water column would be sufficient to suppress juvenile gas exsolution.
In fact, during previous recovery of deep-seated ash and pyroclasts, the presence of
these products under the sea were often attributed to sub-aerial eruptions whose
explosive clasts and ash could have been subsequently transported either in the
atmosphere or by ocean currents.
Although fine ash is likely to be transported in the atmosphere as has been
observed during modern sub-aerial eruptions, it is evidenced that in situ deep-sea
explosion can also give rise to this type of deposit. Most of the records of oceanic
explosive events come from Archean ancient pyroclastic deposits found in continental areas. Others are from back-arc basins surrounded by island volcanism
such as the Okinawa Trough, the Lau Basin and the Tonga-Kermadec Ridge
submarine volcanoes. The pyroclastic deposits found at 1440–2250 m depths were
interpreted as being formed in the submarine environment (Gill et al. 1990; Wright
and Gamble 1999). When coring submarine sediment layers around the islands of
Sumatra and Java, it was found that deposits of pyroclastic flows more than 20 m
thick poured out during the eruption of Krakatau. This deposit was carried on the
sea’s surface and was deposited over a radius of about 70 km forming a semicircle to the northeast of the volcano, in the same direction as the tsunami wave of
1983 after the explosion of the Colo volcano on Una-Una Island.
Submarine Volcanic Explosion
We now know that underwater volcanism could include both explosive and quiet
types of events. Because seawater’s critical boiling point (315 bars and 374 °C)
corresponds to about 3000 m depth, it is impossible that explosive events could
take place at deeper levels, no matter how many volatiles are concentrated in the
magma. Even for the highly vesicular lava flows encountered on the ocean floor, it
would be impossible for the vesicles to nucleate, expand and grow at depths
greater than 3000 m.
130
5 Earth’s Mantle Melting and Volcanism