About deep-sea volcanism |
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| Authors: | Haroun Tazieff |
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| Institution: | (1) Present address: Maître de Recherche au Centre National de la Recherche Scientifique, 15 Quai de Bourbon, Paris IV;(2) Institut International de Volcanologie, Catania |
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| Abstract: | Hyoclastites mainly result from underwater comminution of molten basalts initially explosively erupted out of the sea-floor and instantaneously pulverized by closely succeeding phreatic explosion (s).Many sea-mounts probably never were the alleged volcanic islands, later sea-level eroded into truncated cones and eventually drowned several km down, they are claimed to be. They are here considered as submarine polygenic volcanoes, the shape of which is congenital. Their building up probably started by accumulation of numberless flows of basalt, quietly poured out from a long-lived central vent; when this lava-volcano's crater, so progressively carried higher and higher, reached depths where explosive phenomena became possible because of lowered hydrostatic pressure, magmatic explosions occurred due to violent release of primitively dissolved (or combined) gases. Shattering of lava, 1 ) increases by several orders of magnitude lava's surface to volume ratios, so allowing huge quantities of super-heated steam to be engendered; 2) this super-heated steam trapped below the lava-lumps, as well as in their numberless holes, immediately explodes and comminutes the primary lavalumps; 3) so other super-heated steam is produced and further steam explosions are resumed in confined room until almost all the primitive heat content of the magma is transformed into kinetic energy and the lava is comminuted into glassy, ashy, hyaloclastites.This process also works above fissural eruptions. The difference is that fissural volcanoes, contrarily to large central ones, are usually monogenic (i. e. delivering one eruption only through the same vent instead of numberless ones for polygenic volcanoes). Linear effusive eruptions also produce quietly flowing basaltic flows but — because being monogenic — they cannot build up big, and eventually steep, reliefs as polygenic volcanoes do. When not poured over steep slopes where pillowlavas develop, submarine flows are characterized by 1) the lack of any scoriaceous, more or less thick, upper part (or jacket), and 2) a regular pavingstone-like surface, each polygon of which being the upper face of short prisms similar to ordinary columnar prismation, but one or two orders of magnitude shorter. As for central volcanoes, explosive activity along submarine fissures produces huge quantities of hyaloclastites, but these cannot be heaped up into steep ridges, as happens for subglacial eruptions, because sea-currents spread them far and wide.
Zusmmenfassung Die Hyaloclastiten entstehen hauptsächlich durch submarines Zerspratzen von Lava, die bei vulkanischen Explosionen im Meer ausgeworfen wurde.Zahlreiche  sea-monts waren wahrscheinlich niemals vulkanische Inseln, die später abgestumpft und überschwemmt wurden, wie es allgemein angenommen wird. Wir sind überzeugt, daß ein großer Teil der Vulkane sich unter Wasser gebildet hat aus Laven, die aus einem langlebigen Zufuhrkanal gefördert wurden und die allmählich nach oben wuchsen.Die Bildungsart der Hyaloclastiten, die hier beschrieben wird, erklärt die Tafelformen und die aus Palagonit bestehenden zackigen Berggrate, die Islands Unter-Eis-Vulkanismus kennzeichnen.
Résumé Les hyaloclastites (palagonites) sont formées essentiellement par la fragmentation en milieu aqueux des lambeaux de lave lancés par explosions volcaniques sous-marines (ou sous-lacustres ou sous-glaciaires). Cette fragmentation résulte de l'explosion de la vapeur prisonnière dans et sous les dits lambeaux.Beaucoup de guyots (sea-mounts) n'ont probablement jamais été, comme on le croit généralement, des îles volcaniques ultérieurement tronquées par érosion et englouties. Nous sommes convaincus qu'une forte proportion de guyots sont des volcans sous-marins, faits de coulées interstratifiées avec des hyaloclastites, et que leur forme tronconique est congénitale.Le processus de formation des hyaloclastites que nous décrivons rend compte également des montagnes tabulaires et des crêtes dentelées, constituées de palagonites, caractéristiques du volcanisme sous-glaciaire d'Islande.Les coulées subaquatiques subhorizontales offrent une surface polygonale de « basaltes en pravés ».
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