By MARK DUNPHY - Mon Sep 26, 12:42 pm
El Hierro (circled) in The Canary Islands. Google Earth
The Canary Islands Government has raised the alert level for the El Hierro volcano in the Canary Islands (Spain) to ‘Yellow’, the highest alert status since an unprecedented earthquake swarm commenced in mid-July.
Spanish seismologists, accompanied by the President of the Cabildo de El Hierro and the Minister of Security and Emergency Area, held a press conference on Sunday to reassure the 10,000 residents of the smallest of the Canary Islands that the raising of the alert level does not indicate that an eruption is imminent. They indicated, however, that the number of volcanic earthquakes detected beneath El Hierro continues to increase.
Hierro, a shield volcano, has had a single historic eruption from the Volcan de Lomo Negro vent in 1793. The eruption lasted approximately one month and produced lava flows.
The Canary Islands Government commenced an in-depth geological survey of El Hierro earlier this month in an effort to determine the source of an earthquake swarm.
The unprecedented seismic activity commenced on 19 July (the activity was first reported by iWeather Online on 26 July). In excess of 6,750 earthquakes have been recorded up to Monday, 26 September 2011. More than 50 earthquakes were recorded between midnight and 6:00 a.m. GMT on Monday alone. The earth tremors have ranged between 1 and 3 magnitude, the National Geographic Institute (IGN) reported.
The vast majority of the tremors have been recorded in the northwest of the 278.5-square-kilometre island at El Golfo, the location of a massive landslide that created a 100-metre high tsunami almost 50,000 years ago.
Speaking to the El Pais newspaper during the weekend, volcanologist Juan Carlos Carracedo suggested that an eruption on El Hierro would “not be a major surprise”.
He explained: “It is the youngest of the Canary Islands. There is a ball of magma which is rising to the surface and it is stationed at the limit of the earth’s crust. At the moment we do not know if that ball of magna will break the crust and cause an eruption.”
Earthquake Swarms
Earthquake swarms are events where a local area experiences sequences of many earthquakes striking in a relatively short period of time. The length of time used to define the swarm itself varies, but the United States Geological Survey (USGS) points out that an event may last for days, weeks, or months.
Latest seismic activity on El Hierro
4-D Plot Of El Hierro Earthquakes July-September 2011
El Hierro’s Volcanic/Seismic Past
El Hierro is situated in the most southwestern extreme of the Canaries. The island was formed after three successive eruptions, and consequent accumulations, the island emerged from the ocean as an imposing triangular pyramid crowned by a volcano more than 2,000 metres high.
The volcanic activity, principally at the convergence of the three ridges, resulted in the continual expansion of the island. A mere 50,000 years ago, as a result of seismic tremors which produced massive landslides, a giant piece of the island cracked off, crashed down into the ocean and scattered along the seabed. This landslide of more than 300km3 gave rise to the impressive amphitheatre of the El Golfo valley and at the same time caused a tsunami that most likely rose over 100 metres high and probably reached as far as the American coast.
According to the Global Volcanism Program, the massive Hierro shield volcano is truncated by a large NW-facing escarpment, seen here from the east, which formed as a result of gravitational collapse of the volcano. The steep-sided 1500-m-high scarp towers above a low lava platform bordering 12-km-wide El Golfo Bay, which is barely visible at the extreme left. Holocene cones and flows are found both on the outer flanks and in the El Golfo depression. The last eruption, during the 18th century, produced a lava flow from a cinder cone on the NW side of El Golfo.
El Golfo, El Hierro, The Canary Islands (Spain)
According to ElHierro.com: “Although over 200 years have elapsed since the last eruption, El Hierro has the largest number of volcanoes in the Canaries with over 500 open sky cones, another 300 covered by the most recent outflows, and some 70 caves and volcanic galleries, notably the Don Justo cave whose collection of channels surpasses 6km in length.”
El Hierro is located south of Isla de la Palma (population 86,000), currently the most volcanically active of the Canary Islands. About a half a million years ago, the volcano, Taburiente, collapsed with a giant landslide, forming the Caldera de Taburiente. Since the Spanish occupation, there have been seven eruptions.
Taburiente, La Palma, marked on Google Earth
Caldera de Taburiente. Image wiki
In a BBC Horizon programme broadcast on October 12, 2000, two geologists (Day and McGuire) hypothesised that during a future eruption, the western flank of the Cumbre Vieja, with a mass of approximately 1.5 x1015 kg, could slide into the ocean. This could then potentially generate a giant wave which they termed a “megatsunami” around 650–900 m high in the region of the islands. The wave would radiate out across the Atlantic and inundate the eastern seaboard of North America including the American, the Caribbean and northern coasts of South America some six to eight hours later. They estimate that the tsunami will have waves possibly 160 ft (49 m) or more high causing massive devastation along the coastlines. Modelling suggests that the tsunami could inundate up to 25 km (16 mi) inland – depending upon topography.
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