Overblog
Suivre ce blog
Editer l'article Administration Créer mon blog

Earth of fire

Actualité volcanique, Article de fond sur étude de volcan, tectonique, récits et photos de voyage

Publié le par Bernard Duyck
Publié dans : #Actualités volcaniques

Le modèle proposé jusqu’à présent pour expliquer l’origine des volcans de points chauds était lié à celui de remontée de panaches de roches chaudes, enracinés en profondeur dans le manteau terrestre.

Une étude de Scott W. French et Barbara Romanowicz, de l’Université de Berkeley, basée sur technique d’imagerie sismique de l’entièreté du manteau, propose une nouvelle carte en haute résolution du manteau terrestre qui montre non seulement ces connections  de la plupart des points chauds de la planète, mais révèle aussi qu’à une profondeur d’environ 1.000 kilomètres, les panaches sont cinq fois plus larges que ce que les géologues pensaient, entre 600 et 1.000 km., et d’une température de 400°C supérieure à celle des roches environnantes.

La communication entre les panaches du manteau inférieur et les points chauds volcaniques n’est pas directe, parce que le sommet des panaches se déploie comme le delta d’un fleuve lorsqu’ils fusionnent avec le manteau rocheux supérieur moins visqueux.

Selon Romanowicz, « ces colonnes sont clairement séparées dans le manteau inférieur, et dans leur cheminement jusqu’à une profondeur voisine de 1.000 mètres ; ensuite, elles commencent à s’amincir dans la partie supérieure du manteau, elles serpentent et dévient … alors que les sommets de ces panaches sont associés aux volcans de points chauds, ils ne sont pas toujours sous-jacents à ceux-ci verticalement » 

Situation des panaches détectés dans le manteau inférieur / modèle SEMUCB –WM1, à une profondeur de 2.800 km. –  The background map represents the relative Vs variations at 2,800 km in this model, with respect to the global average at that depth. We identify three categories of plumes. ‘Primary’ plumes are those for which δVs/Vs is lower than –1.5% for most of the depth interval 1,000–2,800 km. These 11 plumes also correspond to regions of the lower mantle where the average velocity reduction over the depth range 1,000–1,800 km is significant at the 2σ level (see, for example, Supplementary Figs 3 and 4). Clearly resolved plumes correspond to vertically continuous conduits with δVs/Vs greater than −0.5% in the depth range 1,000–2,800 km. Somewhat resolved plumes have vertically trending conduits with δVs/Vs greater than −0.5% for most of the depth range 1,000–2,800 km, albeit not as clearly continuous. Plumes are numbered as listed in Extended Data Table 1. Green dots represent the global hotspot distribution according to ref. 27. Note that none of the plumes detected falls within a region of faster-than-average velocity at the base of the mantle, and that long-wavelength structure in this model agrees with that of previous tomographic models (see, for example, Supplementary Fig. 10). - From Broad plumes rooted at the base of the Earth's mantle beneath major hotspots / Scott W. French & Barbara Romanowicz -

Situation des panaches détectés dans le manteau inférieur / modèle SEMUCB –WM1, à une profondeur de 2.800 km. – The background map represents the relative Vs variations at 2,800 km in this model, with respect to the global average at that depth. We identify three categories of plumes. ‘Primary’ plumes are those for which δVs/Vs is lower than –1.5% for most of the depth interval 1,000–2,800 km. These 11 plumes also correspond to regions of the lower mantle where the average velocity reduction over the depth range 1,000–1,800 km is significant at the 2σ level (see, for example, Supplementary Figs 3 and 4). Clearly resolved plumes correspond to vertically continuous conduits with δVs/Vs greater than −0.5% in the depth range 1,000–2,800 km. Somewhat resolved plumes have vertically trending conduits with δVs/Vs greater than −0.5% for most of the depth range 1,000–2,800 km, albeit not as clearly continuous. Plumes are numbered as listed in Extended Data Table 1. Green dots represent the global hotspot distribution according to ref. 27. Note that none of the plumes detected falls within a region of faster-than-average velocity at the base of the mantle, and that long-wavelength structure in this model agrees with that of previous tomographic models (see, for example, Supplementary Fig. 10). - From Broad plumes rooted at the base of the Earth's mantle beneath major hotspots / Scott W. French & Barbara Romanowicz -

Une nouvelle image montre que la base des panaches est ancrée à la frontière du manteau et du noyau terrestre dans deux énormes taches / gouttes de roches chaudes, chacune de 5.000 kilomètres de diamètre, semblant plus denses que les roches environnantes. Ces deux ancrages, en directions opposées, l’un sous l’Afrique, l’autre sous l’océan Pacifique, ont formé un seul ensemble durant 250 millions d’années.

L’étude note de plus que leur taille cinq fois plus large que pensée jusqu’à présent suggère qu’une température plus élevée n’est pas seule responsable de la montée des panaches, mais qu’un facteur de différenciation chimique par rapport aux roches environnantes s'y ajoute.

Perturbations de vitesse relative par rapport à la moyenne mondiale à la profondeur de 410 km et position des îles volcaniques dans le Pacifique - Doc Berkeley University – From Broad plumes rooted at the base of the Earth's mantle beneath major hotspots / Scott W. French & Barbara Romanowicz

Perturbations de vitesse relative par rapport à la moyenne mondiale à la profondeur de 410 km et position des îles volcaniques dans le Pacifique - Doc Berkeley University – From Broad plumes rooted at the base of the Earth's mantle beneath major hotspots / Scott W. French & Barbara Romanowicz

Perturbations de vitesse relative par rapport à la moyenne mondiale pour chaque profondeur -  Relative velocity perturbations are shown with respect to the global average at each depth. For each panel, the location of the box is shown in the inset map ofFig. 1. The region is shown from above, with cuts at increasing depths: a, 410 km; b, 660 km; c, 1,000 km; d, 1,500 km; e, 2,000 km; f, 2,500 km. The following hotspot locations, projected down from the surface, are indicated by green cones in each box: Hawaii (top; north), Samoa (left; west), and the four Superswell hotspots: Tahiti, Pitcairn, Marquesas and MacDonald. Well defined vertically oriented conduits with central cores of velocity lower than −1.5% can be associated with each of the hotspots, particularly clearly in c and d. The low-velocity conduit beneath Hawaii stands out in b–d. In f, patches of much lower-than-average velocity start appearing within the Pacific LLSVP, continuing down to the CMB. On the other hand, at a depth of 410 km, the low-velocity conduits start spreading horizontally and merge into the depth range in which low-velocity fingers have previously been observed in the upper mantle21.  From Broad plumes rooted at the base of the Earth's mantle beneath major hotspots / Scott W. French & Barbara Romanowicz

Perturbations de vitesse relative par rapport à la moyenne mondiale pour chaque profondeur - Relative velocity perturbations are shown with respect to the global average at each depth. For each panel, the location of the box is shown in the inset map ofFig. 1. The region is shown from above, with cuts at increasing depths: a, 410 km; b, 660 km; c, 1,000 km; d, 1,500 km; e, 2,000 km; f, 2,500 km. The following hotspot locations, projected down from the surface, are indicated by green cones in each box: Hawaii (top; north), Samoa (left; west), and the four Superswell hotspots: Tahiti, Pitcairn, Marquesas and MacDonald. Well defined vertically oriented conduits with central cores of velocity lower than −1.5% can be associated with each of the hotspots, particularly clearly in c and d. The low-velocity conduit beneath Hawaii stands out in b–d. In f, patches of much lower-than-average velocity start appearing within the Pacific LLSVP, continuing down to the CMB. On the other hand, at a depth of 410 km, the low-velocity conduits start spreading horizontally and merge into the depth range in which low-velocity fingers have previously been observed in the upper mantle21. From Broad plumes rooted at the base of the Earth's mantle beneath major hotspots / Scott W. French & Barbara Romanowicz

University of California, Berkeley, seismologists have produced for the first time a sharp, three-dimensional scan of Earth’s interior that conclusively connects plumes of hot rock rising through the mantle with surface hotspots that generate volcanic island chains like Hawaii, Samoa and Iceland.

Plus de renseignements sur les sites "en Sources" ...

 

Sources :

- Nature - Broad plumes rooted at the base of the Earth's mantle beneath major hotspots - By Scott W. FrenchBarbara Romanowicz - link 

Nature 525,95–99 (03 September 2015) doi:10.1038/nature14876

Received 12 November 2014 -  Accepted 19 June 2015 - Published online 02 September 2015

Science Daily - CT scan of Earth links deep mantle plumes with volcanic hotspots - Scans prove that plumes of hot rock anchored at core-mantle boundary rise to form island chains - link

 

Commenter cet article

Articles récents

Hébergé par Overblog