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Lying 700m above the Danakiel Depression is the Erta Ale formation in the Afar region of northeastern Ethiopia. There are significant gaps in the literature due to the political instability of the region and hostility of the local population pre 1970’s, but some observations have been made. The summit caldera is elongate (1600m X 700m) and follows the NNW Eritrean trend (Oppenheimer 1997). It appears to be made up of three collapsed structures (Barberi et al 1973). Two of these structures, the northern and central, have had long lived lava lakes. Currently only the central pit has a lava lake, the northern pit’s lava lake was covered over in 1992 (Oppenheimer 1997).
Erta Ale is a volcano that is part of a basaltic volcanic chain, confusingly also called Erta Ale. Fissure volcanism, normal faulting and open tensional faulting dominate the region (Gibson 1970). This is due to the presence of a spreading axis due to a mantle plume below the region (White and Mckenzie 1989), generating the extensional tectonic environment. Erta Ale erupts subalkaline- to transitional basalt most of which was erupted during the quaternary from axial fissures (Barberi et al 1973). The compositions can be accounted for by varying degrees of fractional crystallisation (Bizouard 1980).
The central crater appears to have had a lava lake for at least a century, suggesting convective circulation between the lake and a deeper reservoir (Oppenheimer 1997). Such convection is possible if there is 8-14% crystillisation or 30-60° C cooling during magma upwelling according to modeling (Oppenheimer 2004). Both of the craters appear to be connected to the same reservoir as the magma levels rose 160m synchronously during 1968-1974. This rise, leading to an overspill in 1973 creating 1 Km long flows, is thought to be due to an influx of magma into the magma reservoir (Oppenheimer 1997). Interestingly the lava flow did not alter gas composition, thus the gases emitted are representative of the magma reservoir (Le Guern 1979). Gerlach 1989 noted that there is a high CO2 flux, suggesting magma is undegassed, and is not stored for long periods of time.
The volcanoes growth is thought to be mainly by intrusion. The total SO2 flux suggests a magma flux of around 500 kg/s, but observed lava flows only amount to 10kg/s, thus suggesting the other 490 kg/s is intruded (Francis 1993). The volcano constantly shows activity with sustained fountaining and fluctuations in magma levels central pits lava lake, and fumerolic activity in the northern pit crater. This activity is due to the continuous degassing of the volcano.
Barberi F, Chemin’ee J L, Varet J. Long-lived lava lakes of Erta Ale volcano. Revue de GeÂ ographie Physique et de GeÂ ologie Dynamique. 1973;15:347- 352.
Belvisio S, Nguyen B C, Allard P. Estimate of carbonyl sulfide (OCS) volcanic source strength deduced from OCS/CO2 ratios in volcanic gasses. Geophys Res Let. 1986;13:1:133-136.
Bizouard H, Barberi F, Varet J. Mineralogy and petrology of Erta Ale and Boina volcanic series, Afar rift, Ethiopia. J. Petrol. 1980; 21:401–436.
Francis P W, Oppenheimer C, Stevenson D. Endogenous growth of persistently active volcanoes. Nature. 1993;366:554–557
Gerlach T M. Degassing of carbon dioxide from basaltic magma at spreading centers: I. Afar transitional basalts, J. Volcanol. Geotherm. Res., 39 ,in: F. Le Guern, G. Sigvaldason, Editors , The Lake Nyos Event and Natural CO2 Degassing. 1989; 211–219.
Gibson I L, Tazieff H. The structure of Afar and the northern part of the Ethiopian Rift. Philos. Trans. R. Soc. Lond., 1970;A 267: 331–337
Le Guern F, Carbonelle J, Tazieff H. Erta'ale lava lake: heat and gas transfer to the atmosphere. J. Volcanol. Geotherm. Res., 1979;6:27–48.
Oppenheimer C, Francis P. Remote sensing of heat, lava and fumarole emissions from Erta 'Ale volcano, Ethiopia. Int. J. Remote Sens., 1997;18:1661–1692
Oppenheimer C, McGonigle A J S, Allard P, Wooster M J, Tsanev V I. Sulfur, heat and magma budget of Erta ‘Ale lava lake, Ethiopia. Geology, 2004;32:509–512.
Sawyer G M, Oppenheimer C, Tsanev V I, Yirgu G. Magma degassing at Erta 'Ale volcano, Ethiopia. J. Volcanol. Geotherm. Res., 2008;178:837-846.
White R S, McKenzie D. Magmatism at rift zones: the generation of volcanic continental margins and flood basalts. J. Geophys. Res. 1989;94:7685–7729.