Sur le mécanisme des éruptions basaltiques sous-marines à faibles profundeurs et la genèse d'hyaloclastites associées

A basaltic eruption occurring in relatively shallow waters looks drastically different from the same occurring on land.

1. Explosive phenomena are exceptionally violent (height several times the height reached in subaerial basaltic eruptions); 2) frequency of explosions very much higher (20 to 90 explosions per minute vs 0.1 to 10); 3) proportion of pyroclasts much higher too, reaching up to 99% of the total outpour; 4) pyroclasts much finer than that of land basaltic eruptions, having a granulometry similar to that of acid pyroclasts.

According to the author, these differences are due to series of phreatic (steam) explosions rapidly following each violent magmatic (gas phase) event. By magmatic event, we mean actual explosion as well as the violent mashing of the upper part of the lava column; the necessary condition being an initial fragmentation of the lava enabling a comparatively large area of very high temperature material being put into contact with water to produce a first steam explosion; this phreatic explosion shatters the fragments from the magmatic explosion, letting new incandescent surfaces to be put into contact with water, thus producing a 2 d steam explosion; this brings former fragments to smaller shatters still, and so on. This process enables a very quick transformation of the heat content of a given volume of lava into kinetic energy, explaining the specific characteristics observed, as well as their absence when only quiet effusive flows (or lava lake) occurs under-water, where no initial magmatic explosion happens to expose large enough area of high temperature lava.