La Chaîne des Puys

Sur ces pentes maudites que les anciens peuplaient de génies et de fantômes maléfiques, nous croyons entendre le hurlement des damnés..

Guy de Saint-Cyr

Volcanos building

Stromboli (Eolian Islands)

Volcanicity and tectonics: an intimate marriage

We saw in the chapter on tectonics that there was a close link between the structure, the dynamics of the plates and the volcanos. It is noted that large the eruptive chains of the Earth are located on the borders of plates, in the zones of accretion and subduction. The plates can however " be bored " in their medium of a volcanic chain. The following eruptive families then will be distinguished:

- the accretion volcanicity, generally under sailor and very discrete, because the pressure of water limits the gas overflowing of the magmas. When the demonstrations are air, as in Iceland, the emitted magmas are effusive and low in gas.

- the volcanicity of subduction, always explosive and dangerous. This characteristic is due to the fact that the magmas with the complex composition (elements of plate under navy, water), are viscous and very charged out of gas.

 

Carte des volcans

Carte des Volcans (GIF 26 Ko)

 

- the hot spot volcanicity : it appears in medium of plate by forming chains of volcanos as in Hawaii, is characterized by gas magmas not very charged. The apparatuses, very effusive, generate very hot abundant lava.

 

Formation

Magmas mechanism increase

To a hundred kilometers to the interior of the Mantle, the temperature is about 1300°C. The fusion point of materials depends as well on the pressure. The reduction in this last (decompression) under particular conditions, can pull their change of state, which comes along with a variation of density. Ascents are made by difference of density. The magmatic tanks form under the earth's crust. The magma resulting from these tanks will migrate in the crust by faults. It can, either to spout out on the surface of the Earth, to give rise to a chain of volcanos, or to invade zones of the crust and to cool without never reaching surface. In this last case the magma crystallizes to form granitic bases, which will be put at the air by the erosion of the overlying rocks.

Various volcanic dynamisms

Characteristics of the magmas which arrive at the surface of the Earth depends the behavior on the volcanos. The classification of the eruptive dynamisms refers to volcanos having been the subject of studies on emitted materials. It takes account of the quantity

of gas contained in the magmas and their temperature. The nature of magma, dependent of their origin and their transformation in tanks, induce the dynamism. The viscous magmas very loaded in gas, cold (800°C), induce explosive volcanos (gray volcanos). In opposition, the fluid magmas, low in gas, very hot (1100°C), generate effusive volcanos with abundant emission of lava (red volcanos). In the following chapter we will describe the structure of the Strombolian and Pelean dynamisms, the other standards not being represented in the "Chaîne des Puys".


© Copyright Jan 2001 Photos et Tableaux: Bernard Dichamp