The island of El Hierro in the Canaries is the peak of a shield volcano (a volcanic mountain made up of layers of lava, with a wider, less cone-shaped, profile than the more notable stratovolcanoes) rising 1500 m above the sea. It is about 1.2 million years old, but has been relatively inactive during recorded history, with one observed eruption in 1793, and some lava flows dated to the 1600s. With no recorded history of large scale volcanic eruptions the island has developed as a popular tourist spot, and has a permanent population of over 10 000. Then in July this year (2011) El Hierro began to change.
The island of El Hierro.
From the start of July onwards seismologists began to record small earthquakes beneath El Hierro. These were minor, none larger than a 3 on the Richter Scale, but shallow, with an average depth of about 10 km, and as time went on they were becoming more frequent. This is something that volcanologists pay careful attention to, as it can be a sign of magma movements beneath a volcano, heralding a forthcoming eruption. This lead scientists from the Instituto Volcanologico de Canarias to place a network of GPS monitors around the island, to measure any movements. On 24 August they reported that the volcano had inflated by about 1 cm during the previous month.
By 3 September El Hierro was experiencing up to 250 minor tremors a day, with some of the quakes as shallow as 2 km. There was then a fall off in the number and intensity of quakes, lasting until the 27th, when the quakes abruptly resumed. The local government issued calls for calm, but closed schools on the island as well as the only significant road tunnel, Los Roquillos. On the 28th there were reports of rocks emerging from the Pico de Malpaso summit, though local authorities have denied this. People living close to the mountain were evacuated, but as a precaution against landslides. At about this point newspapers started to point out that there had been previous predictions that a major eruption in the Canaries could cause an Atlantic-wide tsunami. Authorities in the Canaries denied that there was any risk of this unless there was a submarine eruption on El Hierro, something thought highly unlikely.
On the 29th evacuees were allowed to return to their homes, as the area of seismic activity had moved offshore into the Las Calmas Sea, to the south of the island. This was thought to be caused by magma moving into a new chamber beneath the island, but was not seen as a cause for alarm, as only about 10% of magma movements result in an eruption at the surface. These quakes persisted for the next couple of weeks, waxing and waning in strength.
On 10 October 2011 a small submarine eruption in the Las Calmas Sea was reported, 5 km from La Restinga, the most southerly point on the island, at a depth of about 600 m. A significant number of dead fish were seen close to the sight. The next day the alert level for La Restinga was raised from yellow to red, and an emergency meeting was called at the village's football field, where the villagers were informed they were to be evacuated to the north of the island. A 4 nautical mile (7.4 km) shipping exclusion zone was also placed around the eruption.
On 12 October two new volcanic fissures were discovered, one at a depth of 750 m, 3.7 km from the shore and the other at a depth of 500 m, 2.8 km from the shore. These apparently turned the water around them green, and gave off a strong sulphurous smell. This combination of dead fish, water discolouration and odour was interpreted as indicating the vents were emitting gas rather than lava, and it was hoped that this might cause the pressure in the magma chamber to fall, decreasing earthquake activity and reducing the risk of a full-scale eruption.
Helicopter footage of one of the green patches.
A satellite image showing the extent of water discolouration to the south of El Hierro; to give a sense of scale El Hierro is about 25 km from east to west. Satellite Image courtesy of RapidEye.
On 15 October volcanic pumice (a volcanic rock with many gas filled pores) was observed floating on the surface of the sea, confirming that the eruptions were not confined to just gas. At this point some scientists began to talk about the possibility of a new island emerging to the south of El Hierro. A day later giant gas bubbles were observed reaching the surface of the sea close to the shore.
A giant gas bubble off the coast of El Hierro.
Throughout this period of eruptions there have been discussions of the possibility of a major tsunami originating from El Hierro. Scientists at the scene have repeatedly suggested that they feel this is a highly unlikely outcome, however this is largely based upon informed guesswork.
In 2001 Stephen Ward of the Institute of Geophysics and Planetary Physics at the University of California Santa Cruz and Simon Day of the Benfield Grieg Hazard Research Centre at the Department of Geological Sciences, University College London published a paper in the journal Geophysical Research Letters in which they speculated that a volcanic eruption on La Palma in the Canaries could provoke an Atlantic-wide tsunami with devastating consequences.
In October 2011 a team lead by Pablo Dávila Harris of the Department of Geology at the University of Leicester published a paper in the journal Geology discussing the likelyhood of a similar tsunami being caused by a volcanic eruption on Tenerife.
Ultimately we simply do not know the probability of an eruption on El Hierro, or many other Atlantic islands, causing a major tsunami. In the Pacific, where the dangers of a major tsunami are much more obvious, there is a well established tsunami warning centre, and a similar system is under construction for the Indian Ocean. The danger of a major tsunami in the Atlantic is clearly lower than in either of these other oceans, but should such an event occur it is likely to have devastating consequences, both economically and in human terms. While the costs of such a warning system are likely to be unattractive to governments in the current economic climate, a considerable amount of money has already been spent looking for extra-terrestrial objects with the potential to impact the Earth; a less likely scenario, and one which we would be less clear how to react to.
See also Tourists evacuated due to volcanic activity on El Hierro, Eruptions on Mount Nabro and Volcanoes on Sciency thoughts YouTube.