Thursday 5 July 2012

Fresh volcanic activity on El Hierro.

El Hierro is the southwestern-most of the Canary Islands, a Spanish territory off the coast of southern Morocco. It is a shield volcano (shallow-dome shaped volcano formed from layers of lava) rising 1500 m above sea-level. The current island forms part of the rim of the crater of a larger, more ancient volcano, El Golfo, which is thought to have collapsed 130 000 years ago. The center of this ancient crater lies to the southwest of the modern island. Until 2011 the volcano has been essentially inert throughout recorded history, with the last rumored (but unconfirmed) activity in the 1790s.

In July 2011 the island began to suffer frequent small tremors, which persisted throughout the summer, increasing in intensity sharply in September. In the second week of October these became more severe still, prompting a temporary evacuation of the southernmost tip of the island. This was followed by a series of submarine eruptions to the south of the island, marked by the production of turbulent gas rings and patches of floating lava (pumice). These persisted till March 2012, when they abruptly stopped. At the same time Earthquake activity on the island dropped abruptly, though it never actually stopped.

Annotated satellite photograph of El Hierro island on 26 October 2011 highlighting the emission area (discolored brown water, circled) and volcanic material transported by ocean currents, appearing light blue-green; settlement names are underlined. (Inset) Geographic location of El Hierro island, in the Canary Islands, off the West coast of Africa. Smithsonian Global Volcanism Program.

On 24 June 2011 the island suffered a magnitude 3.1 quake located of the north shore, according to the  Instituto Geogr√°fico Nacional. This marked the onset of a renewed increase in seismic activity, with a series of Earthqakes following that moved first south, then westward across the island, finally migrating off the southwest coast, where tremors continued into July. Such migrating quakes under or around volcanoes are often caused by the movement of magma bodies beneath the visible cone, and may signal the onset of new volcanic activity.

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