2011–12 El Hierro eruption
The 2011–2012 El Hierro eruption occurred just off the island of El Hierro, the second smallest and farthest south and west of the Canary Islands, in the Atlantic Ocean off the coast of Africa. The island is also the youngest in the volcanic chain. The October 2011 – March 2012 eruption was underwater, with a fissure of vents located approximately to the south of the fishing village of La Restinga on the southern coast of the island. Increased seismicity in June 2012 to the north-west of the vent did not result in another phase of eruptive activity. Until the volcanic eruption|2021 La Palma eruption], which started on 19 September 2021, this was the last volcanic eruption in Spain.
Earthquakes
Beginning on 17 July 2011, increased seismic activity was detected by the Instituto Vulcanológico de Canarias and National Geographic Institute's seismic monitoring station located in Valverde, [Santa Cruz de Tenerife|Valverde]. The seismic monitoring network was increased in density on 21 July to allow better detection and location of the seismic events. There was an earthquake swarm with in excess of 400 minor tremors between 20 July and 24 July; by27 July a further 320 earthquakes had been recorded. On 25 August there were reports that some horizontal deformation had been detected, but that there was no unusual vertical deformation. At that time, the total number of tremors had exceeded 4000. By the end of September, the tremors had increased in frequency and intensity, with experts fearing landslides affecting the town of La Frontera, and also a small possibility of a volcanic eruption through a new vent. In late September, emergency services evacuated several families in the areas at most risk, and made plans to evacuate the island if necessary.
Submarine eruption: October 2011 – March 2012
Between 4:15 and 4:20 am on 10 October 2011 the earthquake swarm changed behaviour and produced a harmonic tremor. Harmonic tremors are produced by magma movements and can indicate that an eruption has begun. That same day, patches of pale-coloured and sulphurous smelling water with dead fish were seen off the coast of La Restinga. A small submarine eruption had begun, 2 km south of La Restinga. Eruption 'jacuzzis', occasionally reaching high above the surface of the water, were seen during the most energetic eruptive episodes.In early November, the 600 residents of La Restinga were evacuated for the second time. A confirmed surtseyan type of eruption phase started at the fissure on 7 November 2011. On 25 November 2011 the eruption was ongoing with vigorous phreatic bubbles emerging.
Several separate plumes of material, aligned N-S, were visible from the air, showing that the eruption was of a fissure type.
The main volcano on the island, Tanganasoga, underwent rapid inflation and increased carbon dioxide gas emissions, which were a cause for concern. On 24 November there were various reports of a sulphurous smell in the El Golfo area of the island.
On 27 November the coast guard vessel Salvamar Adhara collected some pumice clasts, colloquially nicknamed 'floating lava bombs' or 'lava balloons', some of many that had been ejected by the underwater eruption and floated to the surface of the sea before sinking again.
In the following days, three scientific and seismic survey vessels studied the area: Sarmiento da Gamboa, Ramon Margalef and Cornide de Saavedra.
The depth of the volcano beneath the surface of the water was estimated at in early December; after detailed bathymetric surveys, the highest point was placed at between below the sea surface. By late February 2012 the depth of the highest point was below sea level.
On 21 December 2011 it was suggested that the eruption was subsiding, as the harmonic tremors and earthquakes are decreasing in frequency; however, in early January 2012 the earthquakes were increasing in frequency and the area of the eruption appeared to be increasing, with a wider area of sea producing the steaming pumice clasts, steam and general "jacuzzi" activity. By late February 2012 a decrease in seismicity, deformation and gas release was noted. In early March the authorities on El Hierro declared the eruption to be over; this was questioned by some vulcanologists. The webcams were taken down. In mid-April 2012 the top of the cone was measured at below the surface of the sea. Passive degassing of the underwater vent continued during 2012.
Climatic impacts of the October 2011 to March 2012 El Hierro submarine volcanic eruption were reported by Yim.