Geysers : lycéenne en galère......

[jess]

bonjour a tous,

je ne savais pas trop ou poster ce message mais comme la plupart des geysers se trouvent dans des regions volcanique

je me demandais si vous pouviez me renseigner:actuellement je travaille sur les geysers dans le cadre des tpe ou l'on doit combiner phisyque chimie et svt

mes recherches s'orientent actuellement sur

1)formation d'une collone

2)apprivoisement en eau

3)sources de chaleur

étant donnés le peu d'infomation dont je dispose pour constituer un dossier solide (à savoir recherches sur des données plus technique), j'aimerais savoir si vous pourriez m'aider ou alors me donner des liens qui me guiderais dans mes recherches.

J'espere apprendre de vos lumière

[SVE Genève]

En réponse à votre recherche, ci-dessous une note à ce propos que nous avions déjà fait parvenir à un autre membre de geopolis. Bien cordialement.

Pour répondre un peu plus en détail à votre question concernant le fonctionnement d'un geyser, voici quelques explications physiques du phénomène :

Il faut savoir que le fonctionnement d'un geyser est avant tout un problème physique: C'est-à-dire comment un important volume d’eau bouillante peut être projeté vers le haut et en quelques minutes voire quelques secondes ? L’explication de ce phénomène se trouve dans les paramètres qui permettent à l'eau de passer de l'état liquide à l'état de vapeur.

Au fond de chaque geyser se trouve une cavité remplie d'eau très chaude et de gaz. Cette chambre est reliée à la surface par un conduit dont la paroi peut avoir été lissée et renforcée par les sels minéraux provenant de l'eau du geyser. Plus le conduit est haut et plus la pression régnant dans la chambre est élevée.

La durée et l'intensité des éruptions sont fonction de plusieurs facteurs : la température atteinte par l'eau, de son volume et de sa pression à la base du conduit ainsi que de la quantité de gaz dissous. Pour bien comprendre le fonctionnement d'un geyser, il faut rappeler que les différents états de l'eau dépendent du couple pression-température. A la pression atmosphérique normale, la vaporisation (passage du liquide au gaz) de l'eau se fait à 100°C; mais pour une pression supérieure la température d'ébullition sera plus élevée (principe de la cocotte minute). D'autre part, la pression de l'eau ne dépend pas de son volume mais est proportionnelle à sa hauteur.

Dans la première phase d'activité la chambre et le conduit sont envahis d'eau souterraine et par une partie de l'eau provenant de la précédente éruption. Ensuite cete eau est progressivement chauffée par le flux thermique d'origine magmatique. Comme la pression est grande dans la chambre, la température y dépasse les 100°C sans ébullition; alors que le sommet du conduit est plus froid. Le moment de l'éruption survient lorsque l'eau souterraine qui occupe la partie inférieure du conduit atteint sa température d'ébullition, se dilate en se vaporisant et chasse l'eau du conduit formant une "bulle" caractéristique du début de l'éruption. Puis la pression chute alors brusquement dans la chambre et l'eau qui s'y trouve rentre instantanément en ébullition et se vaporise sous l'effet de cette dépression; une partie est violemment expulsée par le conduit . L'eau peut alors de nouveau envahir la chambre et le processus recommence.

Durant la phase de remplissage, l'eau, soumise à une pression supérieure à la pression atmosphérique (101,3 kPa), monte en température et dépasse 100°C sans bouillir. La température de l'eau atteint alors la courbe de vaporisation à la base du conduit provoquant son expulsion. Puis, la brusque dépression engendrée par l'évacuation de l'eau du conduit vaporise l'eau de la chambre.

[jess]

:ye!: merci beaucoup vos explications vont être bien utile car je commencé à desespérer (je n'avais pas vu cette partie du forum) cepandant la formation de la colonne du geysers s'est faite par la rupture des plaques tectonique?ou un truc du genre?

[SVE Genève]

Les sources géothermales et les geysers se trouvent essentiellement au niveau des zones volcaniques. Il faut une source de chaleur profonde assez chaude (le magma) et une nappe d'eau souterraine surchauffée et portée à ébulition dans les conditions énoncées dans l'explication précédente pour qu'un geyser puisse fonctionner. Comme vous le savez sans doute, les zones volcaniques sur notre planète se situent dans trois cadres géotectoniques, soit en rebord des plaques tectoniques : les zones de subduction (enfoncement d'une plaque sous une autres; d'expansion (écartement de deux plaques tectoniques) et aussi dans les zones dit de point chaud, au milieu des plaques et à l'aplomb d'un panache provenant du manteau. Les geysers sont donc liés à l'activité volcanique.

[jess]

je sais que la divergence des plaques tectonique donnent l'apparition de volcns cepandant les plaques sont assez impresionantes et donc les volcans aussi

est-ce que les geysers se font grace aux fissure?

[pegasus]

Tiens, voici quelques trucs qui pourrait peut-être t'intéresser. Par contre, c'est centré sur l'Islande... Désolé, je n'ai pas d'exemplaire en français car moi je travail en anglais...

Iceland is a relatively young land which started to be formed less than 20 million years ago. It is nothing in comparison to the Earth which was born 4.5 billion years ago. At the origin, it is made of volcanoes which went up the ocean and which, after a series of eruptions, ended up by shaping this island whose landscapes are unique in the world!!! Indeed, Iceland abounds in sumptuous landscapes resulting from the battle the forces of nature fight against water, ice and fire…

The characteristics of Iceland are that it is an island situated halfway between two plates, on one side the American plate and on the other side the Eurasian plate. These two plates deviate. This phenomenon is completely normal because the Earth is covered by plates which coincide perfectly between them. These plates move because of the intense heat which reigns under ground. Due to the extreme temperatures, masses of rock go up towards surface. During this increase of temperature, these rocks cool down, consequently, to the approach of the earth's crust, the rocks, having become denser go back towards the core of the Earth. These movements, called movements of convection, move like a conveyor belt involving the plates which are above. Consequently, some draw aside (movement of divergence), others overlap (movement of convergence), others collide and creating mountain range such as the Andes cordillera. Iceland is precisely at the border of two plates which deviate, on the one hand, the American plate and on the other hand, the Eurasian plate.

The border between America and Europe is not clear; it corresponds to a series of parallel cracks such as in Thingvellir. These cracks, which are formed because of the thinness of the earth's crust at these places, have been forming for millions of year. At the bottom of these faults, one can't distinguish the “depths” from the Earth because they are filled with rock. Indeed, as soon as it opens, the earth's crust is full again. Because of the thinness of the crust on the level of the junction of the plates, they exert a lower pressure on the magma tic rocks. Consequently, these rocks are liquefied and engulfed in the faults to fill them. This phenomenon is called “accretion” that is why even if the plates can deviate, no “pit” is formed between the two plates. On the contrary, on the level of this junction, the earth's crust tearing regularly, the lava which spouts out in the form of volcanic eruptions accumulates and ends up forming a mountain range which is called a dorsal. We can thus observe at the Atlantic ocean bed, between the American and the Eurasian plates and in the prolongation between the South American plate and the African plate, an immense mountain chain : the mid - Atlantic dorsal which is 15.000 km - long and which emerges on the level of Iceland.

On the level of Thingvellir, we can observe an immense 30 km broad and it is in this fault called rift that the cracks resulting from the gap between the two plates are formed. Iceland is thus divided into two by this rift and grows of 2 cm per year. Consequently, it is the entire Atlantic Ocean which widens.

At the ocean bed at the level of the dorsal, the volcanic activity is intense. For example, we can observe seism and hot water springs like black smokers.

Iceland emerges from the ocean at this place because two forms of volcanism combine itself. A volcanism related to the formation of the dorsal, another related to the presence of a phenomenon called hotspot. Indeed, under ground the temperature is, in some areas, higher than elsewhere. The rocks which undergo these extreme temperatures, become less dense than the surrounding matter, consequently, they go up towards surface forming a plume from which the head comes and “crushes” itself against the earth's crust, the temperature is so high that its flux acts like a genuine blowtorch which ends up by piercing the crust and enables lava to spout out. In the North Atlantic, the hotspot emerges exactly on the level of Iceland between the two plates. The produced lava by the hotspot thus adds up to that resulting from accretion volcanism. For this reason the dorsal emerges on the level of Iceland by forming an island.

In 1783, in Laki, an enormous 25 km length crack opened and more than 100 volcanic mouths were formed at this place. This enormous eruption lasted 9 months and more than 10 km3 of lava spread in the plain on a surface of 550 km ² to the sea (~ 5 times the surface of Paris). It was the most important lava flow that Man had known. It was accompanied by an enormous emission of gas and ashes which contaminated most of the pastures of the island and decimated part of livestock. A catastrophic famine followed causing the death of almost a quarter of the population of this era. This eruption had consequences until Europe. Indeed, the produced gas cloud at the time of this eruption went up to 10 km of altitude and, because of displacements of masses of air on a large scale, covered all Western Europe. In France for example, the climatic consequences were catastrophic because they caused a consequent cooling of a few degrees, the weather remained cold until the end of June, which caused disastrous harvests this year and thus led to famines, 5 years before the French revolution…

In Iceland, it's the landscape which undergoes a revolution with each eruption because there exist an average of one eruption every four years such as the one of Krafla in 1984, one of the most active volcano of the island (which comprises 200 of them). The latter is very dangerous since it can awake at any moment… But Iceland comprises great uninhabited spaces, then even if the eruption is impressive, it rarely threatens its population.

However, it occurred once, on the night of January 23, 1973, on the only inhabited island of the Vestmann islands and larger harbour of Iceland, Heimaey. This city was almost completely destroyed by the lava flow and projections of ashes. The 5000 inhabitants had to be evacuated in catastrophe, all seemed lost. But it was without taking into account the ingeniousness of the Icelanders. Indeed, they had the idea to pump sea water and to project it on the lava in order to cool it down. Thanks to that, the city was partly saved. The eruption had considerably increased the island and the harbour was much improved and protected from the storms.

Another eruption is still famous. It is the one which happened on November 14, 1963 because it gave rise to a new island. As for any other Icelandic eruption, this one was due to the alliance of the dorsal and the hotspot. This piece of ground was baptized: Surtsey. Today, Surtsey is the youngest island of our planet. Surtsey is protected and only the scientists are authorized to go there. Over there, they can discover how the animal and vegetable lives appear on a virgin land.

In Iceland, the Vatnajökull glacier is the largest glacier of Europe. Indeed, larger than Corsica, it occupies 10% of the Icelandic territory with its 8000 km ². In some areas, its thickness can reach 1000 metres. Under this enormous glacier, there are many volcanoes, of which 3 are very active. On September 30, 1996, the powerful eruption of the Grimsvötn volcano began under the glacier. Released heat dissolved the ice and water, then gigantic steam plumes rose up to several thousands metres of altitude accompanied by an immense sheaf of ashes. The released heat made the ice melt. Then, thousands of cubic meters of water disappeared under the glacier. After fifteen days of eruptions, the volcano fell asleep again. All seemed over but 2 weeks later, at the foot of the glacier, mud torrents spouted out in the plain. A tidal wave flooded many square kilometres for 2 days. Its flow even reached 45.000 cubic metres per second.

Under the glacier, there is a magma tic room with an old chimney leading to a caldera. A caldera is a kind of basin formed by the depression of an old crater. Partly filled by ice, this caldera also contains water. Before the eruption, the displacement of the plates had involved the formation of new faults which allowed the lava of the magma tic room to go up by taking one of these new passages located upstream of the caldera and to spout out under the ice. Under the effect of heat, the ice melt and the water resulting from this cast iron filled the caldera located downstream. Thus while this caldera was being filled, a depression formed at the level of the eruption. After a few days of activity, the lava succeeded in piercing the ice. A gigantic amount of ash and steam, plumes of smoke rose in the Icelandic sky. Then the eruption ceased. However, the temperature being still very high at the top of the crater, the ice went on melting and running out in the caldera. The water of this caldera undergoing the enormous pressure of its dome of ice, ran out under the glacier by raising the ice thanks to this dome acting like a genuine piston. Then, all the water was driven outside causing a gigantic tidal wave called Joküllum (glacier which runs) by the Icelanders. Today, a new jökullum is feared under Myrdalsjökull because Hekla awoke 3 years ago. At the foot of this glacier the small village of Vik shelters 500 inhabitants. At the moment, it is the most controlled zone of Iceland.

When water and volcanism “mix” they give out curious phenomena such as fumaroles, hot and sulphurous gases which escape from the ground such as in Krysuvik. There is also water and mud pools which spout out and bubble at average temperatures of 80 or 100 °C. All these phenomena are due to the fact of the gap between the plates in Iceland. Indeed, because of this phenomenon, the earth's crust is not very thick and the magma is only 2 or 3 km deep.

We can also observe geysers such as on the site of Geysir which gave its name to all the geysers of the whole world. A geyser appears as a surmounted underground tank of a long conduit which emerges outside. It is fed by a whole network of fractures in which circulates rainwater which, in contact with the very hot rocks, reached very high temperatures, up to 120 °C. Usually, water, at this temperature, vaporizes at the atmospheric pressure but at the bottom of the tank, the pressure is much larger because of the water column which thus prevents water from vaporizing. The pressure not being uniform, the more it goes up in the conduit, the weaker the pressure is. Consequently, in the middle of the conduit, small bubbles are formed and go up onto the surface by evacuating little water. Consequently, the water column being less important and thus the lower the pressure is , the water contained at the bottom of its tank can vaporize. Bubbles go up on the surface by dragging with them the water contained in the conduit which spouts out in a powerful steam and water jet.

[Invité Rémi BORNET]

C'est de l'anglais technique, ce n'est pas très difficile à comprendre, je pense qu'elle devrait s'en sortir :ye!: C'est le type d'anglais que tout le monde devrait pouvoir utiliser, ce n'est cependant pas celui qui est enseigné... domage...

@+

[Mr Fouilletout]

Hello

La série "C'est pas sorcier" a présenté il y a quelques années un exellent documentaire "C'et pas sorcier en Islande"

qui présente entre autres les geysers.

Si tu peux trouver ce document, cela pourrait peut-être t'aider !

Mr Fouilletout

[pegasus]

Hello

La série "C'est pas sorcier" a présenté il y a quelques années un exellent documentaire "C'et pas sorcier en Islande"

qui présente entre autres les geysers.

Si tu peux trouver ce document, cela pourrait peut-être t'aider !

Mr Fouilletout

Je confirme, je l'ai est, euh...comment dire..."téléchargés" et ils sont très bien.

Il y en a deux : L'Islande, une île qui souffle le chaud et le foid

L'Islande, terre de glace et de feu

Ce dernier est spécialement consacré au volcanisme en Islande mais qui peut s'élargir sur certain point à tout le globe...

Si tu peut te les ""procurer"", ca t'aiderai surment !

[pegasus]

C'est le type d'anglais que tout le monde devrait pouvoir utiliser, ce n'est cependant pas celui qui est enseigné... domage... :ye!:

@+

Tout à fait d'accord !

[jess]

vous me sauvez la vie lol parce que j'avais du mal a cerner le probleme comment de grande plaque tectonique donne l'apparitio a de "petit" geyser je comprend mieux et aussi chque occasion est a prendre pour parfaire son anglais!

MERCI beaucoup

[pegasus]

Ce sont les titres exacts...

L'Islande, une île qui souffle le chaud et le foid

L'Islande, terre de glace et de feu

[jess]

et par quelle site de teléchargement?

[pegasus]

Ben par le P2P, eMule par exemple...

Enfin je ne sais pas si c'est vraiement le lieu pour évoquer celà ...

[jess]

donc en gros si j'ai bien compris grace aux fissures faites par des plaques divergentes et grace a la propriété des roches peu friable on forma un conduit pour le geyser?