Steaming and burning lava blocks floating to the surface - "Lava Balloons"
Many still hot lava blocks have been seen arriving at the surface above the submarine vent at the ongoing eruption at El Hierro since October. During day, many of these blocks are steaming, float for a while and then sink. At night, "glow" or more correctly flames from burning gasses contained inside the often hollow blocks are seen and were often confused with lava glow.
Steaming lava blocks on 27 Nov 2011 near La Restinga (El Hierro) photographed from the air by Guardia Civil / INVOLCAN
A similar phenomenon was observed during the submarine eruption off La Terceira Island (Azores) in January 1999. It was in a similar stage as the current El Hierro eruption. It did not breach the surface, nor reach Surtseyan activity, i.e. explosive magma-water interaction at shallow depths.
The following diagram shows an interpretation of the 1999 submarine Azores eruption near Terceira, which is probably very similar to what is going on now on El Hierro.
Schematic interpretation of the formation of lava balloons during the 1999 submarine Azores eruption near Terceira (from Forjaz et al., 2000)
- Forjaz, V. H.; Rocha, F. M.; Medeiros, J. M.; Meneses, L. F. & Sousa, C. (2000) "Notícias sobre o Vulcão Oceânico da Serreta, Ilha Terceira dos Açores" Ed. OGVA
"On the morning of 10 February 3 spots of white steam suddenly rose ~70 m into the air at a point 8 km from the coast. The steam was seen from land. ...
A sub-spherical floating block (a "lava balloon") from the Serreta eruption collected on an undisclosed day. Taken from Radiotelevisão Portuguesa (April 1999)
Lava produced by this submarine fissure eruption was basaltic, showing phenocrysts of olivine, pyroxene, and plagioclase in a highly vesicular groundmass. Analysis of the sample suggested that material rose from the sea bottom as hot lava balloons, lost gas from their interiors at the surface, and then sank. ..."
"Floating blocks were seen on the surface ... and some were collected. The scientific team attributed their seismic observations coupled with the floating blocks to the following mechanism:
The magma, being low in viscosity, moves very easily through the already opened fractures and was thought to escape without producing high seismic signals. Floating lava blocks could result from the detachment of pillow-lava edges followed by the ascent of blocks with sufficient gas content. It is also possible that hot, gas-rich lava fragments result from small submarine lava lakes or fountains.
A thin frozen skin of lava seals the gas cavity, and the block might then rise as a hot lava balloon. During ascent, the gas exsolves and nucleates inside the hot fragment while the blocks expand. Once at the surface the interaction between the hot blocks and the seawater produces white steam columns. At the same time, while cooling at the surface, the blocks crack slowly, lose their magmatic gas and sink. Sometimes when water enters inside the hot blocks, they blow up, violently throwing fragments several meters high."
from: Smithsonian / GVP monthly reports (Terceira)