plateaus, plus the Shatsky and the Chatham Rises in the southwest Pacific, and in
the Caribbean igneous province (Ben-Avraham et al. 1981). In the eastern
Caribbean, silica-enriched volcanism has been observed on the Aves Ridge off the
northern coast of Venezuela.
These Large Oceanic Plateaus seem to be similar to the Large Igneous Provinces (LIP), thus they could be interpreted as being precursors of future continents.
Seismic studies done on these Large Oceanic Plateaus indicate that they are made
of a thicker crust (25–60 km in thickness), which is comparable to that found
underneath continental regions. Other Large Oceanic Plateaus with ‘‘granitic’’
crust but which are much older, such as the Seychelles Islands in the Indian Ocean,
which are Precambrian in age ([540 millions years), and the Aghulas Plateau
south of Cape Town in South Africa, are considered as being submerged continental fragments.
In summary, silica-rich igneous rocks are an integral part of all continental
Large Igneous Provinces (LIP), which erupted a great amount ([6500 km
3 ) of lava
from the oldest Precambrian ([3000 millions years ago) period to the youngest
Cenozoic times when the Large Oceanic Plateaus were created. Continental crust
has been formed during several cycles of accretion around a more ancient or
primitive «granitic nucleus» at different periods of Earth’s history up to today.
Eventually, the formation of continental crust and Large Igneous Provinces has
contributed to depleting the Earth’s mantle of its primordial incompatible (mostly
lighter) elements.
Volcanic Arcs: Possible Sites of Crustal Accretion
Once mantle convection began and early ‘‘silica-rich’’ crust was created forming
isolated nuclei (or patches of landmasses), the mechanism of continental crust
formation continued to evolve as the rigid plates began to drift. Evidence of the
existence of ancient intra-continental arcs is now lacking within the continents. It
is however possible that the earliest continental crust was first constructed during
the subduction of ancient lithosphere as early as the Precambrian period (more
than 500 million years ago) because during plate tectonics and ‘‘continental drift’’,
when the plates spread apart from the ridge axes, the crust along with its sediment,
basaltic rocks and a portion of the upper mantle’s intrusive rocks will be carried
away until it encounters less dense material where it could be subducted (pushed
underneath) the lighter material after a collision (see Chap. 10). If we look at the
fore-arcs of today’s subduction zones, we can see the accumulation of drifted
sediment, which could later be uplifted during tectonic events. Modern volcanic
formations (\150 million years old) have formed above subduction zones where
volcanic plates are recycled back into the Earth’s interior.
The total length of the arc margins where oceanic crust is being constructed is
about the same as the length of the volcanic arcs themselves, and this length is
estimated to be about 43,500 km (von Huenen and Sholl 1991).
40
2 Our Haven, Planet Earth
the Caribbean igneous province (Ben-Avraham et al. 1981). In the eastern
Caribbean, silica-enriched volcanism has been observed on the Aves Ridge off the
northern coast of Venezuela.
These Large Oceanic Plateaus seem to be similar to the Large Igneous Provinces (LIP), thus they could be interpreted as being precursors of future continents.
Seismic studies done on these Large Oceanic Plateaus indicate that they are made
of a thicker crust (25–60 km in thickness), which is comparable to that found
underneath continental regions. Other Large Oceanic Plateaus with ‘‘granitic’’
crust but which are much older, such as the Seychelles Islands in the Indian Ocean,
which are Precambrian in age ([540 millions years), and the Aghulas Plateau
south of Cape Town in South Africa, are considered as being submerged continental fragments.
In summary, silica-rich igneous rocks are an integral part of all continental
Large Igneous Provinces (LIP), which erupted a great amount ([6500 km
3 ) of lava
from the oldest Precambrian ([3000 millions years ago) period to the youngest
Cenozoic times when the Large Oceanic Plateaus were created. Continental crust
has been formed during several cycles of accretion around a more ancient or
primitive «granitic nucleus» at different periods of Earth’s history up to today.
Eventually, the formation of continental crust and Large Igneous Provinces has
contributed to depleting the Earth’s mantle of its primordial incompatible (mostly
lighter) elements.
Volcanic Arcs: Possible Sites of Crustal Accretion
Once mantle convection began and early ‘‘silica-rich’’ crust was created forming
isolated nuclei (or patches of landmasses), the mechanism of continental crust
formation continued to evolve as the rigid plates began to drift. Evidence of the
existence of ancient intra-continental arcs is now lacking within the continents. It
is however possible that the earliest continental crust was first constructed during
the subduction of ancient lithosphere as early as the Precambrian period (more
than 500 million years ago) because during plate tectonics and ‘‘continental drift’’,
when the plates spread apart from the ridge axes, the crust along with its sediment,
basaltic rocks and a portion of the upper mantle’s intrusive rocks will be carried
away until it encounters less dense material where it could be subducted (pushed
underneath) the lighter material after a collision (see Chap. 10). If we look at the
fore-arcs of today’s subduction zones, we can see the accumulation of drifted
sediment, which could later be uplifted during tectonic events. Modern volcanic
formations (\150 million years old) have formed above subduction zones where
volcanic plates are recycled back into the Earth’s interior.
The total length of the arc margins where oceanic crust is being constructed is
about the same as the length of the volcanic arcs themselves, and this length is
estimated to be about 43,500 km (von Huenen and Sholl 1991).
40
2 Our Haven, Planet Earth