les curieuses formations due à la lave


Les curieuses formations due à la lave


La Pointe de la Table


Cet endroit se situe à l'extrême Sud-Est de la Réunion, entre Saint Philippe et le Tremblet. Il a la particularité de s'être agrandi de 30 ha, il y a peu de temps, en mars 1986. Comme cela arrive dans 5% des cas, les éruptions peuvent être hors enclos, c'est ce qui s'est produit en 1986. Une sorte de plateau s'est mise en place au bord de la falaise. On se croirait à étretat, n'est ce pas ?

Une fissure s'est ouverte à quelques centaines de mètres de là. Cette fissure est longue de 200m et perpendiculaire à la mer. Par endroit, elle peut être profonde. La végétation a du mal à revégétaliser les abords , malgré les pluies abondantes de ce côté de l'île. Il apparait donc une cicatrice dans le sous bois.

Cette coulée a laissée des traces très interressantes. Tout d'abord , non loin de là, on rejoint un tunnel, long d'une cinquaine de mètres.  La lave sort des entrailles de la Terre à 1000°C environ, et s'écoule comme du caramel. Au contact de l'air elle se refroidit. Lorsque le débit est suffisamment important, seule une couche supérieure se refroidit et se solidifie. A l'intérieur la lave reste d'autant plus chaude, qu'elle est désormais protégée de l'air par la couche durcit. Elle continue à couler  dans le tunnel, tant qu'elle est alimentée. Dès que cette alimentation s'arrête, le tunnel se vide.

La photo montre clairement le chemin pris par la lave qui mène tout droit au tunnel. Comme je l'ai dit, la lave qui se refroidit comme par les parties en contact avec l'air. Lorsque la vitesse de la lave se ralentit, son refroidissement entraine l'arrêt de sa circulation. D'étranges formes se créent alors. Le refroidissement se fait sous forme de prismes, ce qui forment les orgues basaltiques. On en voit une belle formation en longeant la mer vers le puit arabe.

Ceci dit , les orgues basaltiques sont très nombreuses sur l'île. On en voit sur de nombreuses falaises, comme ici au bassin la Mer, près de St Benoit.


Les coulées dans la roche

L'érosion peut faire apparaitre les phénomènes qui se produisent à l'intérieur de la roche. Ainsi les chemins qu'empruntent le magma, avant de trouver une sortie par une fissure sont bien visibles, car de couleur et de texture différente du reste de la roche.

Ici, le magma a circulé verticalement, c'est un dyke (rivière St Denis). Lorsque le dyke aboutit à l'air libre, et donc conduit l'éruption du magma, on parle de cheminées.

Alors qu'ici, (rivière du mât, Salazie), les coulées sont horizontales, ce sont des Sills. Si elle sont si nombreuses, c'est dû à la présence toute proche d'une poche de magma, que l'on appelle chambre magmatique. C'est le cas ici (photo ci dessous), la rivière du mât traverse maintenant la chambre. Cet endroit se situe juste sous la passerelle lorsque l'on vient de l'ilet à Vidot.

Un massif volcanique comme le piton de la Fournaise, ou comme le Piton des Neiges, est constitué d'une multitude de chambres magmatiques plus ou moins grandes, souvent reliées entre elle. Ceci explique pourquoi il y a tant d'éruptions, car une éruption ne permet au mieux que de vidanger une chambre, qui se remplit par la suite. Si l'éruption a lieu par une faille en bas de la chambre, la vidange est quasi complète, le vide créé peut entrainer l'effondrement de la structure. C'était le cas en 2007 pour le cratère Dolomieu. Par contre certaines éruptions peuvent sortir par le sommet de la chambre. Ces éruptions sont souvent plus gazeuses et sous pression, elles entrainent alors des projections de cendres et de lapillis (cas du Formica léo).

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