Sunday 1 April 2012

The magma chamber beneath the Erta Ale Volcano.

Erta Ale (or Erte Ale) is an active shield volcano in the north of Ethiopia, close to the border with Eritrea. Geologically it is on the Ethiopian Rift, the boundary between the Nubian Plate and the Danakil Microplate. The African Plate is slowly splitting apart along the Ethiopian Rift and the East African Rift to the south (which is splitting the Nubian Plate to the West from the Somali Plate to the East). Arabia was a part of Africa till about thirty million years ago, when it was split away by the opening of the Red Sea Rift (part of the same rift system), and in time the Ethiopian and East African Rifts are likely to split Africa into a number of new landmasses.

Rifting in East Africa. The Danakil Microplate is the red triangle to the east of the Afar depression at the southern end of the Red Sea. Università degli Studi di Firenze.

In a paper in the April edition of the journal Nature Geoscience a team of scientists lead by Carolina Pagli of the School of Earth and Environment at the University of Leeds detail the mapping of a magma chamber beneath Erta Ale using an array of seismic monitoring stations across Ethiopia, combined with data from a number of satellites, and its implications for the Ethiopian Rift.

Map of Ethiopia showing the location of seismic monitoring stations used in the study (white triangles), epicenters of quakes monitored during the survey and location of Erta Ale (black box). The team were able to measure the shape of the magma chamber by monitoring the way pressure waves from Earthquakes were deflected. Pagli et al. (2012), supplementary information.

The study revealed that Erta Ale has a shallow elongate magma chamber extending from the volcano several kilometers to the northeast. The rock formations beneath the volcano were also crosscut by a dike (vertical sheet intrusion) and a sill (horizontal sheet intrusion).

Diagrammatic map of Erta Ale. Contour lines reflect surface elevation. The grey area is the magma chamber, the red line the dike, the blue squares on the grid of the sill. Pagli et al. (2012).

An elongate magma chamber has implications for the Ethiopian Rift. This is usually considered to be a slow moving rift as it is currently spreading at a rate of about 12 mm a year. Shallow elongate magma chambers are more usually associated with fast spreading rifts, over 40 mm per year, whereas volcanoes on slow moving rifts tend to have deeper and more-or-less circular magma chambers. Pagli et al. theorise that the extreme dry climate of northern Ethiopia may facilitate the preservation of a chamber structure that would be unstable in a wetter climate.

During the course of the study the team were able to monitor an eruption on Erta Ale in October-November 2008 (this is a very common event here), during which magma flowed through the magma chamber into the sill and dike system before erupting as a lava flow at the surface.

Fresh lava in the crater of Erta Ale in 2008. Stromboli online.