El Hierro is the southwesternmost of the Canary Islands, the tip of an ancient shield volcano that rises 1500 m above sea level. The Canary Islands are all volcanic in origin, sitting on a volcanic hotspot that has been moving east to west throwing up volcanoes for at least 60 million years (strictly speaking the hotspot has been staying still while the overlying Atlantic crust moves east). Technically El Hierro is part of the rim of the crater of an older volcano, El Golfo, which collapsed around 130 000 years ago. The island appeared above the sea about 1.2 million years ago, in the east the oldest of the islands, Feurteventura, is thought to be about 20.6 million years old.
There are seven major islands in the Canaries; El Hierro, La Palma, La Gomera, Tenerife, Gran Canaria, Fuerteventura and Lanzarote. All of these except La Gomera have their own volcano; La Gomera probably did but is highly eroded, to the extent that it is not possible to be certain where the volcanic eruptions that produced the island were centered. There are also a number of seamounts (submarine volcanoes) associated with the islands; notable South and Henry Seamounts, to the south and southeast of El Hierro, and Casablanca Seamount to the east of Fuerteventura.
Despite being the youngest of the islands, El Hierro has not been active for a long time. The last reported eruption on the island was in 1793, when lava erupted from a vent on the northwest of the island for about a month. Prior to this there may have been eruptions in 1692 and 1677. Significant lava flows on the island have been dated to 550 BC, 950 BC and 4790 BC.
In July this year (2011) the Instituto Geográfico Nacional began to detect small earthquakes occurring deep beneath the island. These quakes were happening at a depth of about 10 km, though all were small, with magnitudes bellow 3 on the Richter Scale. This sort of activity near a volcano is often indicative of magma movements beneath the surface which may lead to a future volcanic eruption. Since July there have been over 7200 such minor earthquakes on Hierro, but in September they suddenly became much more frequent, with over 900 occuring between the 8th and 19th of the month, three large enough to have been felt by people living at the surface.
The epicenters of earthquakes on El Hierro since July 2011.
On Wednesday this week (28 September) rocks were seen being thrown from the Pico de Malpaso summit, the highest peak on the island. At this point the local police made the decision to begin evacuating people from the immediate area. Initially 53 people have been evacuated from properties close to the mountain, but preparations are being made to evacuate up to 2000 of the islands' 11 000 inhabitants. The authorities do not expect a major eruption on the island, but are concerned about the hazards presented by volcanic bombs (hot rocks thrown from volcanoes).
The threat of a major volcanic eruption in the Canary Islands is taken seriously by geologists. In 1999 a team led by Simon Day of the Benefield Grieg Hazard Research Centre at University College London published a paper in the Journal of Volcanology and Geothermal Research in which they concluded that a major eruption in the Canary Islands could lead to a flank collapse, triggering a tsunami that could devastate the coasts of countries around the Atlantic Ocean, including the US, Brazil and Western Europe.
The evolution of a hypothetical tsunami centered on La Palma in the Canaries.
See also The dangers of a modern Laki-style eruption in Iceland, Scientists predict eruption of Axial Seamount and Volcanoes on Sciency Thoughts YouTube.