La Chaîne des Puys

Quant la loufoquerie d'une planète a remué des myriacubes de terre, on peut trouver cet astre brouillon, mais c'est le génie du désordre..

Alexandre Vialatte

Tectonic lesson


Stromboli crater 1


The Earth fire

The Earth structure

Our planet was formed there is 4 billion years by the accumulation of elements resulting from the system pre-solar system. They were arranged then to form three great structures, or zones. The seismic studies make it possible to highlight them, to evaluate the compositions and dimensions of them. Center of the ground to surface, follow one another the core, the Mantle and the earth's crust. One could add to it the atmosphere, resulting from the degasification of planet. The earth's crust, only visible part, supports the continents and the oceans.


Dimensions :


Diametre: 2900 km

Lower Mantle

Deep: 70 à 120 km

Upper Mantle

Deep: 25 à 70 km

Earth crust

Thickness: 12 à 70 km

coupe de la Terre

coupe du manteau

Cyclopean movements

The enormous gravitational pressures exerted on central materials apparently caused nuclear reactions which would be at the origin of the fusion of the core. The interior of the Earth is thus the seat of temperatures and raised pressures, where the components were organized. The Mantle is responsible for the effects on the earth's crust. Of an aspect which one could describe as " plastic ", the matérial of the Mantle causes to make " floating " the less dense, cold and solid earth's crust. The great variations in temperatures, the differences in density of materials, create movements of convection in the structure of the Mantle, with the image of the agitation of water when it is heated in a pan. The earth's crust, fragmented in great sectors, called plates, moves then with the likings of these gigantic movements involving mountains and basins oceanic. This phenomen, discovered during XXème century, is called " the continental drift " or the " tectonic plates".

The continents moving, tectonic plates




A changing surface: the mobility of the tectonic plates

Since its birth the Earth changed unceasingly. The many plates which form the earth's crust, involve the continents and the oceans in their expansion then in their disappearance. They are born in a movement from accretion, run up, are dislocated, worn, are lost under other plates. This dynamics, invisible on a human scale, combined has the erosion made of our planet a star in perpetual transformation and on the changing surface. The mountains, born from the confrontation of the plates, are the direct consequence. The other effects, perceptible to the man, are the earthquakes and volcanicity. The slow drift of the grounds nevertheless is highlighted by topographic measurements of high degrees of accuracy in the zones at strong displacement (several cm/an on the rift Is African). If internal energy disappeared, the Earth would cool. There would be no more mountainous folds, more oceanic basins. It would be flattened under the effect of erosion. Without relief, a planetary ocean would probably cover it..

Starting of the plates: the accretion (divergent boundary)

The plates are formed by the increase of the magmas of the higher Mantle. They build in a symmetrical way by the surge of the products resulting the depths of the Mantle. Then in a slow horizontal movement, the crust resulting derives on both sides from the zone from emission, while floating on the superior layers of the Mantle. This phenomenon is called the accretion and the median axis, the rift. Thus creates for itself the ocean floor, because in the majority of the cases, the accretion is under navy. There are however terrestrial rifts, as in Iceland, or in Afar. One of the characteristics of the rift is an intense volcanic activity. Thus, the Atlantic dorsal formed by the rift separating the American plates from the plates Europe and African, is a succession of underwater volcanos being prolonged as far as Iceland



Titanic shocks: collisions (Transform boundary)

Pulled by the movements of the Mantle, the plates " float ". When two plates slided, it rub one with the other. This confrontation is not carried out without consequences. One thus attends the most powerful earthquakes of planet, generating in the populated areas of the fatal catastrophes. It is the case on the Anatolienne fault in Turkey, where the Eurasienne plate runs up against the African plate. The efforts between plates are also at the origin of the great mountainous formations, like the Himalayas and Tibet, born from the meeting of India and the Asiatique plate. More close to on our premises the Alps resulting from are pushed from Italy on the Eurasienne plate. A plate does not consist of one only block. The slow drift, the collisions, make of it a very dislocated unit, composed of faults, elevated zones (mountains), basins of collapse (grabbens). These accidents are there also the seat of volcanicity and earthquakes.

The disappearance of the plates: subduction (convergent boundary)

The disappearance of a plate is a surprising phenomen. Unrelentingly pushed by the movements of the Mantle, it passes under its neighbor, re-entering in the depths of the Earth. The materials, swallowed by the Mantle, will join the internal magmas. The confrontation generates on the absorbing plate of the folds at the origin of the formation of mountainous chains or archipelagoes. Thus were formed the Andes or the Rocky Montains ones by the subduction of the Pacifique plate under the American plate. The Indonésiens arcs, Filipinos, or Japan testify to the disappearance of the Pacifique plate under the close plates. It is in these formations that one also finds the ocean deeps deepest of the sphere. The visible demonstrations of this dynamics are an intense seismic activity, due to frictions of the two plates, and fusion partial of deep materials creating a volcanicity with the gigantic eruptions, like Pinatubo in 1991, or the Holy Mount Helens in 1980.


Chaîne coté Sud

coulée Etna

© Copyright Jan 2001 Photos et Tableaux: Bernard Dichamp