Fire in the sea (extract)

Fire in the sea—Growth and destruction of submarine volcanoes
Hans-Ulrich Schmincke and Mari Sumita
GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstrasse 1-3, 24148 Kiel, Germany

El Hierro is the youngest (1.12 ± 0.02 Ma) and smallest of the seven major islands of the Canarian archipelago (Guillou et al., 1996). A shallow submarine eruption occurred between 12 October 2011 and 5 March 2012, ∼2 km off the southern coast of the island, just offshore the sites of prehistoric eruptions dated at ca. 4000 yr B.P. and 2500 yr B.P. The proximity to land and the shallow depth (the new volcanoes rose from a depth of 363 m and finally reached 89 m below mean sea level [bmsl]) provide a unique opportunity to study precursor phenomena on land prior to and during the eruption, and to repeat surveys across the eruptive sites. Eight swath bathymetry surveys were carried from 22 October 2011 to 24 February 2012 by Rivera et al. (2013, p. 355 in this issue of Geology). Most remarkable are the continuous changes in form and height of the new volcano, changing from a single cone into a linear array of four cones over 4 mo. Collapse of the edifice accompanied the rise of the volcanic structure from the start, with streams of debris moving down the valley in which the new volcano was growing. The total volume of the volcanic complex was calculated as 329 × 106 m3, with the actual cone making up one third of this bulk volume. More than 200 similar cones have been identified on the submarine flanks of El Hierro by Rivera et al., the largest with volumes of >1000 × 106 m3. The practically simultaneous episodes of collapse of the flanks were most interesting to monitor, but the large flank collapses known from all Canary Islands, as well as from other oceanic islands and volcanoes in general, have vastly larger volumes. For example, during the El Golfo debris avalanche, ∼50–180 km3 of material was moved (Masson et al., 2002), the equivalent of ∼500 of the volcanic structures of the type emplaced in 2011–2012 on El Hierro (Rivera et al., 2013).

All eruptions of submarine volcanoes are, as expected, accompanied by seismic swarms.
Most small (M <2.5) earthquakes on El Hierro started on 19 July 2011 along a north-south–oriented belt at 8–15 km depths (Ibáñez et al., 2012; Lopez et al., 2012). Seismicity migrated south during the second half of September, increasing in magnitude and peaking with an M 4.4 earthquake on 8 October at 14 km, followed on 12 October by continuous tremor, reflecting rapid rise of the magma to the surface, water discoloration, and steaming vesicular volcanic bombs floating on the water, ∼2 km south of the village of La Restinga in the south of El Hierro. Ibañez et al. (2012) infered that the initial earthquakes reflected magma migration from the upper mantle to crustal depths, where high fracturing would favor anomalously high b-values of the frequency-magnitude distribution of up to 2.25 (b-values in volcanic regions sometimes reaching 3). The larger-magnitude earthquakes with lower b-values (1.25) are interpreted by Ibañez et al. (2012) to reflect relaxation around the magma reservoir.

Monitoring of diffusive degassing is important for volcanic surveillance. At the beginning of the seismic activity, magma movement beneath El Hierro generated new fractures and microfractures, causing anomalously high, but diffuse, deep-seated degassing on land prior to the submarine eruption (Pérez et al., 2012). Helium ascended from depth through structures of high vertical permeability, thereby enhancing the helium content of soil gases, and with an increase in mantle-derived gases recorded by the 3He/4He ratio preceding seismic energy release (Pádrón et al., 2013).

Curious steaming bombs of highly vesicular scoriaceous basalt floated in the turbulent, turbid whirlpool above the submarine vent off Restinga, and resembled bombs from shallow-water submarine eruptions in the Azores (Gaspar et al., 2003) and Socorro Island (off Mexico) (Siebe et al., 1995). The balloons soon collapsed and fragmented, fragments sinking to the bottom within a few tens of minutes. Deposits composed of irregularly shaped, commonly curved, fragments of vesicular basalt are well-exposed in the upper part of the uplifted seamount complex of La Palma Island just north of El Hierro, and were interpreted as reflecting the growth of the seamount to near the surface (Staudigel and Schmincke, 1984; Schmincke and Sumita, 2010, their figures 4.20–4.23). © 2013 Geological Society of America

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