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Tham khảo sách 'updates in volcanology – a comprehensive approach to volcanological problems_2', tài liệu phổ thông phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 6 Hotspot Concept The French Polynesia Complexity Claudia Adam CGE Univ. Evora Portugal 1. Introduction At the surface of the Earth volcanism is found in several tectonic contexts. It is largely concentrated at the plate margins at divergent plate boundaries mid-oceanic ridges where new tectonic plates are created and at convergent margins subduction zones where the lithospheric plates dive into the mantle. In the interior of the oceanic plates we find however linear volcanic chains composed of several volcanoes aligned along the direction of the plate motion. Their origin has been attributed to the drifting of the lithospheric plates over a fixed hot mantle upwelling deeply rooted in the mantle. Since several years this concept is debated and the existence of the plumes themselves is questioned. Here we focus on French Polynesia a region characterized by a great concentration of volcanism and situated on the South Pacific Superswell a wide area associated with numerous geophysical anomalies including anomalously shallow seafloor considering its age a dip in the geoid and a mantle characterized by slow seismic velocities. 14 of the active volcanism is concentrated in an area covering less than 5 of the globe. A wide range of volcanic features should be noted en echelon ridges isolated seamounts and chains of midplate volcanoes. The characteristics of these chains often depart from the classical definition of hotspots. In particular the broad depth anomalies surrounding the chains called swells display peculiar morphologies. These characteristics are however well recovered by a numerical model based on highly resolved seismic tomography model describing the first 240 km of the upper mantle. This demonstrates that a direct link exists between the surface observations and mantle flows. However even if the dynamics of the shallowest part of the mantle is sufficient to explain the surface observations the existence of the secondary plumes at the origin of the hotspot .
