Mechanisms and implications of the 2011 eruption of Nabro volcano, Eritrea

Project summary

The 2011 eruption of Nabro volcano in Eritrea is of great scientific interest and has had substantial impacts in the remote part of Afar in which it is located. It is sited in the extensional zone of Afar, close to the Mesozoic crustal block of the Danakil Alps. It is a predominantly trachytic edifice, with an 8 km wide caldera and ignimbrites (Wiart & Oppenheimer, 2005). The current eruption began shortly before 20:42 UTC on 12 June following intense seismicity. It is the first eruption of Nabro on record, highlighting the potential of caldera systems to erupt without warning. It is also the first seismicity of note recorded in this part of the rift. Comparatively little is known about magma differentiation, storage and transport mechanisms and eruptive processes in such tectonic settings.

Several thousand people were displaced by the eruption and international aviation was disrupted by ash clouds. Satellite measurements rank the eruption among the largest sources of SO₂ to the atmosphere since the 1991 Pinatubo eruption (~2 Tg of SO₂ released according to retrievals of Ozone Monitoring Instrument data). The main aims of the project are to arrive at a detailed synthesis of the nature and causes of the eruption, to evaluate the events in the context of understanding restless calderas worldwide, and to compare and contrast activity of Nabro with the fissural basaltic systems that have been the focus of research by the NERC Afar Consortium. Our specific objectives are to:

• Investigate geometries and kinematics of intrusive activity related to the eruption;
• Constrain pre-eruptive storage conditions, volatile budget, masses of erupted lava and tephra, and mechanisms of magma differentiation and volatile concentration in the crustal reservoir;
• Establish the source of the emitted sulphur (and halogens);
• Investigate the magma plumbing geometry and regional stress field, and integrate seismic and infrasound records to understand a complex pattern of explosive events;
• Investigate the event in the context of regional extension and proximity to the Danakil Alps.

Team

• University of Cambridge: Clive Oppenheimer (PI), Amy Donovan, Talfan Barnie
• Imperial College: James Hammond
• Eritrea Institute of Technology: Berhe Goitom, Ghebrebrhan Ogubazghi
• Eritrea Ministry of Energy & Mines, Department of Mines: Goitom Kibrom, Ermias Yohannes
• University of Bristol: Jon Blundy, Mike Kendall
• University of Leeds: Jurgen Neuberg, Tim Wright, Derek Keir, Joanna Hamlyn
• Michigan Technological University: Simon Carn
• University of Alaska Fairbanks: David Fee
• L’Institut des Sciences de la Terre d’Orléans: Bruno Scaillet

Report on fieldwork carried out in October 2011

Berhe Goitom, James Hammond, Goitom Kibrom, Clive Oppenheimer, Ermias Yohannes

This brief report summarises the achievements of a successful field campaign in Denkalia. The key aims were to document the aftermath of the June/July 2011 eruption of Nabro, to assess the current state of activity, to service the local seismometer network, and to collect a suite of samples of 2011 products along with older volcanic rocks on Nabro.

The fieldwork was conducted between 8 and 15 October (we left Asmara on the 7^th and arrived back on the 16^th). We had two 4WD vehicles at our disposal and based ourselves first at Afambo and later at Wade. In between, part of the group spent a night on the caldera rim at the abandoned military camp. Berhe and James focused on servicing the seismic network, while Goitom, Ermias and Clive concentrated on sampling of lavas and tephra. Our drivers were Alem Teklemariam and the irrepressible Asresehey Ogbatsien. We were wonderfully supported back in Asmara by Dr. Ghebrebrhan Ogubazghi. We are grateful to many others who assisted and supported our mission, especially Dr. Alem Kibreab (Ministry of Mines and Energy, and Isaac Tuum (Administrator of Central Denkalia subzone), and the several military officers and soldiers and local guides who accompanied us in the field.

Lavas and tephra from 2011 eruption

Given the limited time spent in the field, the extent of the lava flow fields and tephra fall, and the proximity of the international border, a comprehensive sampling strategy was impractical. We did, however, sample lavas and tephra over quite a wide area within and beyond the caldera. The figure (below) shows our GPS tracks and sample locations.

Lava flows

The main lava flow extends for ~17.5 km and its width exceeds 1 km in some parts. Meaningful estimates of thickness were not possible in the field but flow margins were typically 5–10 m thick. Thicknesses in the interior could be substantially greater. The flow surfaces were well above ambient temperatures in places, and were still emitting fumes. The main (longest) flow was sampled towards its front. Within the caldera, three shorter flows were sampled. Compositions range from trachybasalt to basaltic trachyandesite.

Front of the main (longest) lava flow. Note Ermias for scale.

The main lava flow has overrun agricultural and settled areas within and beyond the caldera. It has also engulfed drainages.

View of the vent region within the caldera. Note shorter lava flows.

Pyroclastic rocks

Tephra from the 2011 eruption fanned out towards the SW sector of the volcano. In the NE sector of the caldera floor, there is no trace of the eruption and people were still in residence. We found a bomb field within 1 km of the vent, and near Sireru on the SW part of the caldera, damage to tree branches suggested bombs had fallen there, too (they were presumably buried by subsequent scoriae fallout).

Lava bombs within 1 km of the vent area.

The mantle of dark scoriae reaches up to 2.5–3 m near Sireru based on the depth to which acacia and other trees were buried (amazingly most of the trees had new shoots).

Before and after view from more or less the same spot near Sireru. Pre-eruption shot courtesy of Berhe Goitom.

Partially-buried acacia trees and buildings near Sireru.

Degassing

The 2011 vent region had a distributed fumarole field, with a number of incandescent vents. The plume associated with these was visible drifting across the international border on some days. The lava flows were still degassing in many places, some marked by deposits of bright yellow/orange condensates.

Night-time view of the vent region in the centre of the caldera, seen from the caldera rim.

Mofettes at the edge of the bomb field. At least one was present before the eruption, and was used therapeutically by the local population. But we were told that others were new. The established one also seemed too hot to be bearable suggesting temperatures have increased.

Older lavas and pyroclastic rocks

We opportunistically sampled pre-2011 lavas and tephra in several locations. In particular, we recorded stratigraphy and sampled ignimbrites exposed on N, E and S flanks of Nabro.

Seismic network

Both the Eritrea regional network and the local Nabro network were serviced during the fieldwork. The Eritrea regional network was working well with no problems reported at any of the stations. Five of the Nabro seismic stations were working well with no problems. Station 6 was completely flooded after recording one week of data. This station was removed and taken back to Asmara. Station 7 had problems with timing that could not be fixed and station 8 had minor data logger problems though no data were lost.

Prognosis

Given the intermittent local seismicity (e.g., mid-September, early November), high-temperature degassing in the 2011 vent region, sustained MODIS thermal anomalies, and reports from the local administration (30 October) of “smoke” and “noises” (corroborated by infrasound records for 27–30 October), it seems prudent to consider the possibility of further eruptive activity at Nabro during the next year or so.