The existence of elongated, shallow magma chambers beneath the axes of fast-spreading mid-ocean ridges is well established 1-8 . Yet, at slow-spreading ridges such shallow and elongated magma chambers are much less evident 9,10 . Simple thermal models 8 therefore predict that spreading velocity and magma supply may provide the main controls on magmachamber depth and morphology. Here we use interferometric synthetic aperture radar data to investigate the dynamics of the magma chamber beneath the slow-spreading Erta Ale segment of the Ethiopian Rift. We show that an eruption from Alu-Dalafilla in November 2008 was sourced from a shallow, 1 km deep, elongated magma chamber that is divided into two segments. The eruption was probably triggered by a small influx of magma into the northern segment. Both segments of the magma chamber fed the main eruption through a connecting dyke and both segments have been refilling rapidly since the eruption ended. Our results support the presence of independent sources of magma supply to segmented chambers located along the axes of spreading centres 11 . However, the existence of a shallow, elongated axial chamber at Erta Ale indicates that spreading rate and magma supply may not be the only controls on magma-chamber characteristics.Shallow (<2 km depth) elongated axial magma chambers have been detected at many places on fast-spreading ridges 1-7 , but magma chambers have been found at only a few slow-spreading ridges as deeper isolated magma pockets 9,10 . The depth and continuity of these chambers is thought to depend on spreading rate and magma supply 8 .Typically, an axial high and continuous shallow magma chamber (at ∼1-2 km depth) characterizes ridges spreading faster than ∼40 mm yr −1 (refs 1,2,8), whereas a median valley and isolated deeper chambers (≥3 km deep) are found at slower-spreading ridges 5,9,10 . Elongated axial magma chambers are typically 250-4,000 m wide 1,3,4 and tens of kilometres long. In contrast, the magma chambers at slow-spreading ridges are isolated, nearspherical bodies. Magma chambers at ∼3 km depth have been inferred both in Krafla 12 and Askja 13 , Iceland. At the Dabbahu segment in Afar, both the Gabho and Dabbahu volcanoes are also associated with axisymmetric chambers between 3 and 5 km depth 14 . The ∼120-km-long Erta Ale spreading segment separates the Nubian plate from the Danakil microplate in the Afar region of Ethiopia. It forms a continuous ridge made up of distinct volcanic centres 15 (Fig. 1a,b), including the Erta Ale volcano, which hosts an active lava lake 15,16 . The spreading rate at this latitude is estimated at ∼12 mm yr −1 (ref. 17). In November 2008, an eruption occurred in the Alu-Dalafilla volcanic centre that lies 30 km north of the
We present a synthesis of diverse observations of the first recorded eruption of Nabro volcano, Eritrea, which began on 12 June 2011. While no monitoring of the volcano was in effect at the time, it has been possible to reconstruct the nature and evolution of the eruption through analysis of regional seismological and infrasound data and satellite remote sensing data, supplemented by petrological analysis of erupted products and brief field surveys. The event is notable for the comparative rarity of recorded historical eruptions in the region and of caldera systems in general, for the prodigious quantity of SO2 emitted into the atmosphere and the significant human impacts that ensued notwithstanding the low population density of the Afar region. It is also relevant in understanding the broader magmatic and tectonic significance of the volcanic massif of which Nabro forms a part and which strikes obliquely to the principal rifting directions in the Red Sea and northern Afar. The whole-rock compositions of the erupted lavas and tephra range from trachybasaltic to trachybasaltic andesite, and crystal-hosted melt inclusions contain up to 3,000 ppm of sulphur by weight. The eruption was preceded by significant seismicity, detected by regional networks of sensors and accompanied by sustained tremor. Substantial infrasound was recorded at distances of hundreds to thousands of kilometres from the vent, beginning at the onset of the eruption and continuing for weeks. Analysis of ground deformation suggests the eruption was fed by a shallow, NW–SE-trending dike, which is consistent with field and satellite observations of vent distributions. Despite lack of prior planning and preparedness for volcanic events in the country, rapid coordination of the emergency response mitigated the human costs of the eruption.
Erta Ale volcano, Ethiopia, erupted in November 2010, emplacing new lava flows on the main crater floor, the first such eruption from the southern pit into the main crater since 1973, and the first eruption at this remote volcano in the modern satellite age. For many decades, Erta Ale has contained a persistently active lava lake which is ordinarily confined, several tens of metres below the level of the main crater, within the southern pit. We combine on-the-ground field observations with multispectral imaging from the SEV-IRI satellite to reconstruct the entire eruptive episode beginning on 11 November and ending prior to 14 December 2010. A period of quiescence occurred between 14 and 19 November. The main eruptive activity developed between 19 and 22 November, finally subsiding to pre-eruptive levels between 8 and 15 December. The estimated total volume of lava erupted is ∼0.006 km 3 . The mineralogy of the 2010 lava is plagioclase+clinopyroxene+olivine. Geochemically, the lava is slightly more mafic than previously erupted lava lining the caldera floor, but lies within the range of historical lavas from Erta Ale. SIMS analysis of olivine-hosted melt inclusions shows the Erta Ale lavas to be relatively volatile-poor, with H 2 O contents ≤1,300 ppm and CO 2 contents of ≤200 ppm. Incompatible trace and volatile element systematics of melt inclusions show, however, that the November 2010 lavas were volatile-saturated, and that degassing and crystallisation occurred concomitantly. Volatile saturation pressures are in the range 7-42 MPa, indicating shallow crystallisation. Calculated pre-eruption and melt inclusion entrapment temperatures from mineral/liquid thermometers are ∼1,150°C, consistent with previously published field measurements.
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