The 2011 submarine volcanic eruption in El Hierro (Canary Islands), Spain – Eruption overview

Last update: May 18, 2012 at 6:19 pm by By Armand Vervaeck

With special thanks to Dr. Carracedo (Geovol) allowing us to publish his report and Joke Volta for facilitating.

Dr. Juan Carlos Carracedo Gómez – ULPGC

Forty years after the Teneguía Volcano (La Palma, 1971), a submarine eruption took place off the town of La Restinga, south of El Hierro, the smallest and youngest island of the Canarian Archipelago. Precursors allowed an early detection of the event and its approximate location, suggesting it was submarine. Uncertainties derived from insufficient scientific information available to the authorities during the eruption, leading to disproportionate civil protection measures, which had an impact on the island’s economy based primarily on tourism, while residents experienced extra fear and distress.

El Hierro, 1.12 million years old, is the youngest of the Canary Islands. Located at the western end of the archipelago together with the neighboring island of La Palma, El Hierro rests on a ca. 3500 m-deep ocean bed.
The principal configuration of El Hierro is controlled by a three-armed rift zone system that gives rise to three ridges that extend from the center of the island in a characteristic ‘Mercedes star’ geometry(Carracedo, 1994), and host the larger part of El Hierro’s subaerial eruptions (Fig. 1A).
This triple-armed shape of El Hierro is further enhanced by the scars of several massive gravitational landslides that truncate all three flanks. The collapse of the north flank, that formed the spectacular El Golfo bay with an almost vertical 1400 m-high escarpment, is the youngest landslide of the entire Canary Archipelago with an age of less than 100 ka. Rift zones, however, also continue underneath the sea surface. The south rift stretches as a submarine ridge for more than 40 km (Fig. 1B), indicating that recent submarine eruptions have occurred there as well.

Fig. 1. A. Geological map of El Hierro (from Carracedo et al., 2001). B. colour shaded relief image of El Hierro viewed from above (from Masson et al., 2002). The subaerial and submarine parts of the South rift are indicated.

During the German research cruise Meteor 43/1 in 1998, lava samples were dredged from the submarine prolongations of the southern rift zones of La Palma and El Hierro. El Hierro samples taken close to the present eruptive site (<3 km distant) included fresh picrites and alkali-basalts and variably altered lapillistones and hyaloclastites. Further dredging along the submarine north-west and north-east rift zones during the Poseidon 270 cruise in 2001 recovered fresh alkali basalts from 21 young volcanic cones at depths of 800 to 2300 m together with ocean bottom sediments having a strong volcaniclastic component.
It appears overall that the density of seemingly young volcanoes on El Hierro’s submarine rifts is comparable to that on land, emphasizing the relevance of submarine eruptions during the growth of oceanic islands.

Precursors to the 2011 eruption

Numerous earthquakes were recorded by the Spanish Instituto Geográfico Nacional (IGN) from July 2011 onwards, the greater part of them insignificant from a hazard point of view, but were clearly precursors of a volcanic eruption. In particular, seismicity, initially of low magnitude (M < 3.0) and focused north of the island, increased while migrating southward. The greater part of the hypocentres were initially concentrated within the lower oceanic crust (Fig. 2), at depths of 8–14 km (ca. 200–400 MPa pressure), which is in agreement with pressure estimates from microscopic fluid inclusions in xenoliths from north-western El Hierro and phenocrysts from a recent eruption. The seismic and petrological data are thus in-line with a scenario of a magma batch becoming trapped as an intrusion horizon, near the base or within the subisland oceanic crust. Shifting seismic foci suggest that magma progressively accumulated and expanded laterally in a southward direction, causing a vertical surface deformation of about 40 mm at that time.
During this initial phase, the system remained active but showed no sign of having overcome the resistance of the oceanic crust. Hypocenters thereafter migrated south-east, approaching the submarine prolongation of the active South rift zone. From there, the magma progressed rapidly towards the surface, as indicated by the first time occurrence of shallow (< 3 km) earthquakes on 9 October 2011.
The scenario changed dramatically at about 4 am on 10 October, when the now frequent and strong seismicity (up to M 4.4) ceased and was rather abruptly replaced by a continuous harmonic tremor, indicating the opening of a vent and thus the onset of a submarine eruption.

Fig. 2. Seismic hypocentres beneath El Hierro between 19 July and 10 October 2011. Hypocentres migrated from North towards the South rift zone of the island, where they became shallower (< 3 km). The eruption commenced on 10 October. Most of the time, seismicity remained stable at the base of the oceanic crust (data from IGN, http://www.ign.es/ign/resources/ volcanologia/ html/eventosHierro. html)

The submarine eruption

On October 10, patches of pale-colored water that smelled of sulfur and were associated with dead fish, were found floating one mile south of the coast confirming the opening of a vent on the flank of the submarine part of the South rift zone. The surface expression of this eruption, including green and bright discoloration of seawater, was clearly observed in high-resolution satellite images featuring a large stain(locally known as ‘la mancha’) visible on the surface of the Las Calmas Sea (Fig. 3A). The eruption formed aNE–SW trending fissure outlined by strong bubbling and degassing (Fig. 3B), occasionally 10–15 m high, loaded with juvenile volcanic ash and pyroclasts (Fig. 3C).
However, information on the depth and precise location of the submarine vent was lacking in the first two weeks of the eruption because of the unavailability of adequate means for submarine surveying.
On October 24, the RV Ramón Margalef of the Instituto Español de Oceanografía (IEO) carried out the first survey of the area, previously mapped in 1998 by the Spanish RV Hespérides (Fig. 4A). Comparison of present and 1998 bathymetry outlined a 700 m-wide, 100 m-high new volcanic cone resting at about 350 m depth in a canyon on the flank of the South Rift submarine extension (Fig. 4B). On 4 December 2011, the eruption apparently waning, the RV Ramón Margalef carried out another campaign, detecting significant growth of the volcanic edifice. The initial single eruptive center (Fig. 4A,B) had now evolved to three cones of similar height, with their summit 180–160 m below the sea surface (Fig. 4D), still below the critical value to generate significant surtseyan explosions (about 100 m below sea level).
Lava flows and pyroclasts, confined by the canyon walls, caused the greater part of the erupted volume to flow downslope towards deeper parts of the ocean floor.

Fig. 3. A. Plume of dissolved magmatic gases and suspended matter producing green and bright discolouration of seawater (locally known as ‘la mancha’) commencing on 10 October 2011 and continuing for several kilometres to the south-west before drifting off into the Atlantic (Satellite image by RapidEye). Fig. 3. B. Plumes of gas on ocean surface showing a N–S trend, indicating a submarine eruptive fissure. Inset: Expansion of steam with decreasing water depth (modified from Schmincke, 2004). C, strong degassing with abundant rock fragments generated large ‘bubbles’, some of them 10–15 m-high, bursting to the surface off the nearby village of La Restinga (8 November 2011).

Fig. 4. A. DEM showing the pre-eruptive submarine canyon where the 2011 eruption nested (image taken from the RV Hespérides, 1998). B. DEM of the same area taken on 24 October by the RV Ramon Margalef after the onset of underwater activity. C. Geological map of the submarine eruption from the first DEM obtained on 24 October 2011 by the RV Ramon Margalef. D. Geological map of the same area on 4 December 2011.

Floating stones off El Hierro

Abundant rock fragments resembling lava bombs on a decimeter scale (Fig. 5) and characterized by glassy basaltic crusts and white to cream-colored interiors, were found floating on the ocean surface during the first days of the eruption. The interiors of these floating rocks are glassy and vesicular (similar to pumice), with frequent mingling between the pumicelike interior and the enveloping basaltic magma (Fig. 5B).These floating rocks have become known locally as ‘restingolites’ after the nearby village of La Restinga. Their nature and origin remained elusive at first, with suggestions from the scientific community including: (1) the floating bombs are juvenile and potentially explosive high-silica magma; (2) they are fragments of marine sediment from the submarine flank of El Hierro; and (3) that they are relatively old, hydrated volcanic material. However, none of these interpretations provides a satisfying fit to the available observation since for instance, high-silica volcanism is uncommon on El Hierro, and magmatic minerals (either grown in magma or as detritus from erosion) are entirely absent in the ‘restingolites’. Given that the involvement of highly evolved, high-silica magmatism would have implications for the explosive potential of the eruption, it was important to clarify the nature of the ‘restingolites’ swiftly in order to fully assess the hazards associated with the ongoing El Hierro eruption. Furthermore, should the ‘restingolites’ be shown not to originate from high-silica magma, then unraveling their genesis will most likely provide unique insights into the volcano–magma system beneath El Hierro.
All ‘restingolite’ samples are glassy and light in color and most are macroscopically crystal-free. However, occasional quartz crystals, jasper fragments, gypsum aggregates and carbonate relicts have been identified in hand specimens. X-Ray diffractograms mainly indicate the presence of quartz, mica and/or illite, and glass. There is a notable absence of primary igneous minerals from the XRD data. Microscopic quartz crystals have also been identified and analysed using a field emission electron probe micro-analyser (FE-EPMA), as well as the composition of the glass matrix, which ranges between ~65 and 90 per cent SiO2.
The high silica content coupled with overall low incompatible trace element concentrations, the occurrence of mm-sized relict quartz crystals and the lack of igneous minerals, plus the occurrence of carbonate, clay, jasper and gypsum relicts are all  ncompatible with a purely igneous origin for the cores of the floating stones. Igneous rocks on El Hierro do not contain any free (primary) quartz crystals (nor do igneous rocks on any of the other Canary Islands).
A potential source of the quartz crystals found in the floating rocks from El Hierro is likely to be the sediments of layer 1 of the pre-island ocean crust. These contain quartz crystals transported from Africa by both wind and turbidity currents and are characterized by a lack of igneous minerals due to their pre-island age.

Fig. 5. A. ‘floating rocks’ observed in October 2011 off El Hierro. B. ‘restingolite’ sample displaying typical features, such as a crust of basalt, primary sedimentary bedding, folding, high vesicularity, and mingling structures. C. hollow basaltic bomb of the late stages of the eruption. D. similar bomb from the Serreta Oceanic Volcano, Terceira, Azores (photograph by Ulrich Küppers).

The floating rocks found at El Hierro are thus most probably the products of magma–sediment interaction beneath the volcano (Fig. 6). Ascending magma mixes with the pre-volcanic sediments and the ‘restingolites’ were carried to the ocean floor during eruption while being melted and vesiculated during transport in magma. Once erupted onto the ocean floor, some of them were able to separate from the erupting lava and floated to the sea surface due to their low density (Fig. 6).

for more information and updates, go to:    http://earthquake-report.com/2012/05/18/the-2011-submarine-volcanic-eruption-in-el-hierro-canary-islands-spain-eruption-overview/