setting where the Pacific plate lithosphere is subducted underneath the volcanic
island arc chain, inside the Tonga-Kermadec Ridge. The subducted lithosphere’s
slab descends at a 50° angle to about 660–700 km depth in the Tonga Trench
(Barazanki and Isacks 1971). In the deepest central area, the trench narrows to
\10 km wide. It is likely that the slab thins out with depth, until it is only about
2–10 km thick.
It was in the middle of 1950s, during a mission of the RV. HORIZON from
Scripps Institution, when peridotites and dunites were first dredged from the Tonga
Trench at 9000–9400 m depths (Fisher and Engel 1969). This is another indication
that the crust in the subducting area of the trench is relatively thin, since deepseated rock formations were exposed on the trench’s floor surface.
Tonga Islands
The northern area of the Tonga-Kermadec Ridge, also simply called the Tonga
Ridge, forms a large volcanic plateau or platform. The Tonga Ridge consists
essentially of coralline-capped Eocene-Miocene volcanic islands and shallow
(\100–1000 m depth) seamounts. The basement of the Tonga platform exposed on
Eua Island is composed of depleted tholeiite, andesite and dacite with an age of
about 40–46 Ma (Ewart and Bryan 1972).
There are a total of 169 emerged islands but only 36 are inhabited. Thus, these
islands are mostly unpopulated and those with human beings living on them are
often used as mere vacation grounds or are cultivated for crops due to their rich,
volcanic soil. The population of Tongans is about 100,000 people, most of whom
live on the Island of Tonga itself. Tonga Island is part of Melanesia (the so called
‘‘dark islands’’ whose name was coined by the French explorer, Dumont
d’Urville).
The original Tongans came from the Fiji islands and settled in Tonga and
Samoa about 800 B.C. It is also near one of these islands, the volcano of Tofoa,
that Captain Bligh and Fletcher Christian had their altercation on board the HMS
Bounty, which later led to a rebellion (mutiny) on board the ship. Charles Wilkes,
commander of the 1838–1842 US Exploration Expedition around the World,
landed on Fiji and on the Island of Tonga to conduct geological observations.
Today most Tongans are Catholics, Protestants, Pentecostal and/or Seven Days
Adventists, as are the inhabitants of Pitcairn Island.
Volcanoes of the Tonga-Kermadec Ridge
As part of the Tonga-Kermadec Arc, it has been determined that the TongaKermadec Ridge hosts at least 94 volcanoes (de Ronde et al. 2005). A more
conservative estimate of 12 subaerial and 71 submarine edifices was given by
352
10 Subduction Zones
island arc chain, inside the Tonga-Kermadec Ridge. The subducted lithosphere’s
slab descends at a 50° angle to about 660–700 km depth in the Tonga Trench
(Barazanki and Isacks 1971). In the deepest central area, the trench narrows to
\10 km wide. It is likely that the slab thins out with depth, until it is only about
2–10 km thick.
It was in the middle of 1950s, during a mission of the RV. HORIZON from
Scripps Institution, when peridotites and dunites were first dredged from the Tonga
Trench at 9000–9400 m depths (Fisher and Engel 1969). This is another indication
that the crust in the subducting area of the trench is relatively thin, since deepseated rock formations were exposed on the trench’s floor surface.
Tonga Islands
The northern area of the Tonga-Kermadec Ridge, also simply called the Tonga
Ridge, forms a large volcanic plateau or platform. The Tonga Ridge consists
essentially of coralline-capped Eocene-Miocene volcanic islands and shallow
(\100–1000 m depth) seamounts. The basement of the Tonga platform exposed on
Eua Island is composed of depleted tholeiite, andesite and dacite with an age of
about 40–46 Ma (Ewart and Bryan 1972).
There are a total of 169 emerged islands but only 36 are inhabited. Thus, these
islands are mostly unpopulated and those with human beings living on them are
often used as mere vacation grounds or are cultivated for crops due to their rich,
volcanic soil. The population of Tongans is about 100,000 people, most of whom
live on the Island of Tonga itself. Tonga Island is part of Melanesia (the so called
‘‘dark islands’’ whose name was coined by the French explorer, Dumont
d’Urville).
The original Tongans came from the Fiji islands and settled in Tonga and
Samoa about 800 B.C. It is also near one of these islands, the volcano of Tofoa,
that Captain Bligh and Fletcher Christian had their altercation on board the HMS
Bounty, which later led to a rebellion (mutiny) on board the ship. Charles Wilkes,
commander of the 1838–1842 US Exploration Expedition around the World,
landed on Fiji and on the Island of Tonga to conduct geological observations.
Today most Tongans are Catholics, Protestants, Pentecostal and/or Seven Days
Adventists, as are the inhabitants of Pitcairn Island.
Volcanoes of the Tonga-Kermadec Ridge
As part of the Tonga-Kermadec Arc, it has been determined that the TongaKermadec Ridge hosts at least 94 volcanoes (de Ronde et al. 2005). A more
conservative estimate of 12 subaerial and 71 submarine edifices was given by
352
10 Subduction Zones