El Hierro, the youngest Canary.
Island of landslides

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Text: Annemieke van Roekel, geoscience journalist
www.vuurberg.nl.

This article was first published in Gea Magazine (September 2022).

El Hierro, the most southwestern island of the Canary Archipelago, is covered with many cinder cones and other young volcanic sediments. Its distinct three-armed island structure is determined by the structure of rift zones combined with the collapse of the flanks of the three ridges. Recent volcanism has made it a beautiful Geopark, with extensive areas of malpaís, rope lava and cinder cones covered by the Canary Island pine (Pinus canariensis).

El Hierro is the top of a volcanic structure that rises 5.5 km from the Atlantic Ocean floor and rises 1.5 km above water. What we see is just the tip of the iceberg. As a result of major landslides, extensive debris fans (aprons) lie all around the island (Fig. 1), with an estimated volume of three times the current section above sea level.

 

Fig. 1. Debris avalanche deposits on the sea-floor, as a result of lateral collapses off the flanks. Carracedo & Troll, 2016 (fig. 2.9C). With kind permission.

El Golfo
The last landslide in the Canary Islands was about 15,000 years ago, possibly below sea level, at El Golfo, in northwestern El Hierro (Masson 1996). But the first major landslide in El Golfo occurred much earlier, probably around 130,000 years ago, when the northern ridge collapsed. Both landslides occurred during a period with sea levels one hundred meters lower than today, during an ice age.

Since its subaerial existence, 1.12 million years ago, four major landslides have occurred on El Hierro, and these have largely determined the three-armed shape of the island. Intrusions of dikes have contributed to the instability of the flanks (Carracedo et al., 2016). The three-armed geometry is also the basic structure of La Palma and Tenerife, but on El Hierro the shape is more pronounced due to the young age of the island and the collapse of the flanks on all three sides.

It wasn't until 1972 that Hausen interpreted the 'crater' of El Golfo (Fig. 2) as the result of a mega-landslide, at that time without the knowledge of submarine debris fans. Before then, people thought of a real crater created by a volcanic eruption, succeeding the theory of fluvial erosion. Modern techniques with sonar data (Fig. 3) of offshore areas, and to a lesser extent seismicity, provided definitive evidence of large-scale prehistoric landslides on El Hierro and the other islands. Of all four major landslides on El Hierro, that of El Golfo is the most visible and accessible.

 

Fig. 2 (left). El Golfo 'crater' (Frontera), with Roques de Salmor at the horizon. The tiny rock are famous for the (extinct) Hierro Giant Lizard. Fig. 3 (right). Bathymetry mapped with sonar from three directions: north (top), west (center) and south (bottom). Seabed data are important to reconstruct and date landslides. Avalanche deposits from El Golfo have been observed up to 65 km from the coast. Image: CIMA / Instituto Español de Oceanografía.

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Copyright: Annemieke van Roekel
Last update: 28 December, 2022