the lithosphere as a plume or a ‘‘blob’’ having a lower density, due to its higher
heat and its different composition when compared with the surrounding material.
The plume rises from deep in the mantle, probably in the vicinity of (or exactly at)
the core-mantle boundary region, about 2900 km deep (Turcotte and Oxbourgh
1978). The simplest hotspot model implies that the magmatic source is from the
deep mantle and the hotspots are therefore relatively fixed with respect to each
other, to a precision of about 1–2 cm/yr. An alternative view is that the plumes
giving rise to hotspots are not ‘‘fixed’’, but rather they could be moved by convection currents in the mantle. The combined motion of the landmasses and the
movement of the oceanic lithospheric plates could affect the mantle’s convective
system, which would then alter the trajectory of a rising mantle plume.
One important characteristic of hotspots is that they are relatively long-lived
and their presence in a given place can be retraced up to 200 million years ago (Le
Pichon and Huchon 1984). Thus, hotspots have existed prior to the formation of
our present day ocean. The variation of ‘‘rock age versus distance’’ is an important
notion for the definition of a hotspot. If we are dealing with a hotspot, the age of
the volcanic rocks should increase at a distance from their point of eruption above
the hotspot, since the lithosphere moves above and beyond the hotspot. There are
notorious exceptions to this fact, but they were attributed to the existence of
remainders of magmatic reservoirs for a period of 3–4 Ma such in Hawaii (Jackson
1976).
Within continental regions, upwelling magma underneath the lithosphere will
contribute to the rupture of the plates and initiate oceanic opening. This happened
more than 120 million years ago in the Atlantic Ocean, during the separation of
America and Africa. Such an opening left a scar in the lithosphere thereby giving
rise to volcanic chains such as the Walvis Ridge, the Rio Grande Rise, the Sierra
Leone Rise in the Atlantic and to other similar structures in the Indian Ocean (i.e.
the Ninetyeast Ridge) and in the Pacific (Nazca and Cocos ridges) at the beginning of the break-up of PANGEA. This phenomenon of rupture of the oceanic
lithosphere is an on-going process.
French–German Cooperation: The ‘‘Volcanisme
Intra-Plaque’’ Program
Franco-German collaboration for studying intraplate volcanism related to hotspots
and their associated phenomena began in 1986. The project started after a telephone
conversation between Professor Peter Stoffers (Fig. 9.2a) from the University of
Kiel in Germany and Dr. Jean-Louis Cheminée (Fig. 9.2b) at Institut de Physique du
Globe in Paris (IPGP). Peter asked Jean-Louis if he was interested in collaborating
on the study of the Society hotspot, in the South Pacific. It was then that Jean-Louis
told Peter to call me at IFREMER to ask my opinion and to see if I were interested.
In fact, I was very interested in such an adventure because it could open a new
perspective, since it entailed the exploration of a new area of the ocean floor that
304
9 Hotspots
heat and its different composition when compared with the surrounding material.
The plume rises from deep in the mantle, probably in the vicinity of (or exactly at)
the core-mantle boundary region, about 2900 km deep (Turcotte and Oxbourgh
1978). The simplest hotspot model implies that the magmatic source is from the
deep mantle and the hotspots are therefore relatively fixed with respect to each
other, to a precision of about 1–2 cm/yr. An alternative view is that the plumes
giving rise to hotspots are not ‘‘fixed’’, but rather they could be moved by convection currents in the mantle. The combined motion of the landmasses and the
movement of the oceanic lithospheric plates could affect the mantle’s convective
system, which would then alter the trajectory of a rising mantle plume.
One important characteristic of hotspots is that they are relatively long-lived
and their presence in a given place can be retraced up to 200 million years ago (Le
Pichon and Huchon 1984). Thus, hotspots have existed prior to the formation of
our present day ocean. The variation of ‘‘rock age versus distance’’ is an important
notion for the definition of a hotspot. If we are dealing with a hotspot, the age of
the volcanic rocks should increase at a distance from their point of eruption above
the hotspot, since the lithosphere moves above and beyond the hotspot. There are
notorious exceptions to this fact, but they were attributed to the existence of
remainders of magmatic reservoirs for a period of 3–4 Ma such in Hawaii (Jackson
1976).
Within continental regions, upwelling magma underneath the lithosphere will
contribute to the rupture of the plates and initiate oceanic opening. This happened
more than 120 million years ago in the Atlantic Ocean, during the separation of
America and Africa. Such an opening left a scar in the lithosphere thereby giving
rise to volcanic chains such as the Walvis Ridge, the Rio Grande Rise, the Sierra
Leone Rise in the Atlantic and to other similar structures in the Indian Ocean (i.e.
the Ninetyeast Ridge) and in the Pacific (Nazca and Cocos ridges) at the beginning of the break-up of PANGEA. This phenomenon of rupture of the oceanic
lithosphere is an on-going process.
French–German Cooperation: The ‘‘Volcanisme
Intra-Plaque’’ Program
Franco-German collaboration for studying intraplate volcanism related to hotspots
and their associated phenomena began in 1986. The project started after a telephone
conversation between Professor Peter Stoffers (Fig. 9.2a) from the University of
Kiel in Germany and Dr. Jean-Louis Cheminée (Fig. 9.2b) at Institut de Physique du
Globe in Paris (IPGP). Peter asked Jean-Louis if he was interested in collaborating
on the study of the Society hotspot, in the South Pacific. It was then that Jean-Louis
told Peter to call me at IFREMER to ask my opinion and to see if I were interested.
In fact, I was very interested in such an adventure because it could open a new
perspective, since it entailed the exploration of a new area of the ocean floor that
304
9 Hotspots