Volcanism in Vanuatu, mechanisms of subduction, volcanic rocks.

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Introduction :

Le volcanisme du Vanuatu résulte de la subduction de la plaque australienne sous la plaque pacifique.

Au niveau de l’archipel du Vanuatu, ce sont deux plaques (lithosphère)  océaniques qui entrent en collision, produisant un volcanisme d’arc insulaire. Ces plaques ont une épaisseur de 8 à 30 Km.

La plaque australienne, plus dense, (plaque subduite) plonge sous la plaque pacifique (plaque chevauchante) et est à l’origine d’un volcanisme explosif par la formation d’un magma remontant à la surface.

arc-insulaire

Birth of magma in an oceanic subduction zone:

The dense, cold oceanic plate plunges under the overlapping plate into the warm mantle.

This subdued plate is full of water and covered with hydrated marine sediments.

Warming causes dehydration. The released aqueous stream percolates the overlying mantle. The conditions are then met for the partial melting of the materials of the mantle corner.

The overlapping plate consists of three layers. In the deepest layer, the upper mantle, it finds peridotites whose melting point is lowered by the water brought during the subduction of the plunging plate. These partial mergers occur at the corner of the mantle. These mechanisms occur at about 100 km depth.

At the level of the plunging plate, the basaltic crust and the sediments (such as quartz (SiO2), felspath and clays) follow this partial melting. The presence of isotope of Barium makes it possible to demonstrate this partial fusion of the sediments.

These magmas are a mixture of peridotite, lower lithosphere, basaltic-gabbroic crust, upper lithosphere, and low-temperature minerals of sediments driven by subduction.

The magmas generated, lighter than the casing, rise towards the surface.

As they continue to ascend, these magmas are stored in magmatic chambers where fractional crystallizations occur which modify their composition. There may also be a new rise of magma which further modifies the composition of the magma of the magmatic chamber, we speak of contamination.

All variants are possible and give different magmatic products.

Subduction remains a very complex phenomenon.

volcans_subduction_anime

Geochemical characteristics of magmatic rocks in subduction zones:

The magmatism of the subduction zones has its own characteristics.

The andesites, dacites, rhyolites, magmatic rocks most characteristic of the subduction zones, and associated rocks, are rich in silica, alkaline minerals and mineral hydrates, which is their primary characteristic. It would be the hydrated fusion that would explain the silica richness of these magmas.

There is also a decrease in the iron content throughout the calco-alkaline series, as the silica content increases.

The lavas emitted by the subduction volcanoes are therefore characterized by their richness of silica and this explains in part the extreme viscosity of the magmas and the explosive nature of the eruptions.

They are also rich in alkaline elements such as Potassium (K), Rubidium (Rb) and Barium (Ba), they are called calco-alkaline. Other differences in composition, particularly isotopic, constitute signatures of arc magmas.

Cristal_Fractio_1

The mechanisms of evolution of magmas:

Once it is stored in the magmatic chamber (s), the magma evolves in line with the volcanic edifice.

On cooling, fractional crystallization occurs. Some minerals are no longer soluble in magma and precipitate. The resulting magma has a composition that evolves over time.

The enclosure can also undergo a partial melting and contaminate the magma by changing its composition.

This explains the diversity of the rocks found in the subduction zones (basalt, andesite, dacite ...)

Vanuatu volcanic rocks

Ignimbrite of Ambae

This volcano shield is crowned with a large pyroclastic cone of large volume.

It is from this cone that the above-mentioned rock emerges.

ignimbrites:

These rocks result from cataclysmic eruptions involving considerable amounts of material and energy. The deposits often have a thickness of several tens of meters spread over large surfaces.

The placement of the ignimbrites is that of a foam or foam of magma

Guy ANGE

Docteur en pharmacie
Biologiste Médical

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