Cuisson à la vapeur et combustion des blocs de lave flottant à la surface-« Ballons de lave »De nombreux blocs de lave encore chaud ont été vus en arrivant à la surface au-dessus de la cheminée sous-marin à l'éruption en cours à El Hierro depuis octobre. Au cours de la journée, bon nombre de ces blocs sont à la vapeur, float pendant un certain temps et ensuite coulent. À la nuit, « glow » ou plus correctement flammes provenant de la combustion des gaz contenaient à l'intérieur que les blocs souvent creux sont perçus et étaient souvent confondues avec la lueur de la lave.
Un phénomène similaire a été observé lors de l'éruption sous-marine au large de l'île de Terceira La (Açores) en janvier 1999. C'est dans un stade similaire comme l'éruption de El Hierro actuelle. Il n'a pas manqué à la surface, ni atteindre surtseyen activité, interaction magma-eau c'est-à-dire explosive à faible profondeur.
Le diagramme suivant montre une interprétation de l'éruption de Azores sous-marin de 1999 près de Terceira, qui est probablement très similaire à ce qui se passe maintenant sur El Hierro.
Sources :
- geocrusoe.blogspot.com/2009/02/tipos-de-actividade-vulcanica-submarina.html
-Forjaz, V. h. ; Rocha, f. M. ; Medeiros, j. M. ; Meneses, l. f.
Extrait de "A small seismic crisis during November 1998 was monitored by the Azores Seismological Surveillance System (SIVISA). It was centered W of Terceira Island on the E of the so-called Graciosa Trough [38.783°N, 27.483°W; submarine vent at -500 m]. After four days of major activity seismicity decreased and remained at normal levels until 18 December; then, a few microseisms were registered by SIVISA and white vapor columns were reported by fishermen to be rising from the sea 8 km W of Terceira island. These phenomena were discontinuous but persisted until the night of 23 December when local residents observed orange lights several miles W of Terceira." -> Similar orange lights are being observed at El Hierro now."Although no unusual seismicity was noticed in the following interval, on 8 January fishermen again reported white vapor columns on the sea. Scientists from the Center of Volcanology of Azores University (CVUA) using a helicopter of the Portuguese Air Force took the first pictures of the phenomenon (figure 1). Lava blocks reaching a maximum of 3 m in length rose gently to the surface where they floated for a few minutes. White steam columns resulting from contact between hot lava blocks and seawater were observed, rising about 10 m above the surface of the sea. During the night the same activity was observed from the W coast of Terceira island and six different areas of incandescent, dispersed lava blocks were identified along a NE-SW axis. Activity continued at the same level until 13 January after which only intermittent short-period events were observed." Excerpt from Smithsonian's monthly bulletin:"A small seismic crisis during November 1998 was monitored by the Azores Seismological Surveillance System (SIVISA). It was centered W of Terceira Island on the E of the so-called Graciosa Trough [38.783°N, 27.483°W; submarine vent at -500 m]. After four days of major activity seismicity decreased and remained at normal levels until 18 December; then, a few microseisms were registered by SIVISA and white vapor columns were reported by fishermen to be rising from the sea 8 km W of Terceira island. These phenomena were discontinuous but persisted until the night of 23 December when local residents observed orange lights several miles W of Terceira." -> Similar orange lights are being observed at El Hierro now."Although no unusual seismicity was noticed in the following interval, on 8 January fishermen again reported white vapor columns on the sea. Scientists from the Center of Volcanology of Azores University (CVUA) using a helicopter of the Portuguese Air Force took the first pictures of the phenomenon (figure 1). Lava blocks reaching a maximum of 3 m in length rose gently to the surface where they floated for a few minutes. White steam columns resulting from contact between hot lava blocks and seawater were observed, rising about 10 m above the surface of the sea. During the night the same activity was observed from the W coast of Terceira island and six different areas of incandescent, dispersed lava blocks were identified along a NE-SW axis. Activity continued at the same level until 13 January after which only intermittent short-period events were observed."
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