Shallow axial magma chamber at the slow-spreading Erta Ale Ridge

Abstract

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

The development of extension and magmatism in the Red Sea rift of Afar

“…9). Following the November 2008 basaltic eruption from a fissure on Alu-Dallafilla, Pagli et al (2012) demonstrated using analysis of InSAR data, that the eruption was accompanied by up to~1.9 m of subsidence across a~1 km-wide,~10 km-long zone beneath the volcanic system (Fig. 9).…”

“…Unlike elsewhere in Afar, the axial range does not form a heavily faulted graben, but instead forms an axial high composed of at least 4 volcanic centres (e.g. Tat Ale, Erta Ale, Dallafilla, Gada Ale) that reach up to~500 m above sea-level (e.g., Barberi and Varet, 1970;Pagli et al, 2012;Thurmond et al, 2006) (Fig. 3).…”

“…The Danakil depression is bisected by a linear,~NNW-striking, 10-20 km wide axial volcanic range (Erta Ale range) which is the locus of on-going volcanism and magmatism (Barberi and Varet, 1970;Pagli et al, 2012) (Figs. 3 and 8).…”

Off-rift volcanism in rift zones determined by crustal unloading

“…In more evolved rifts where protracted intrusion has significantly heated the plate, such as in Afar, <10-km-deep magma reservoirs beneath axial volcanoes are common 25,26 and feed episodic dykes along the axis 4,27 .…”

Reduction of atmospheric disturbances in PSInSAR measure technique based on ENVISAT ASAR data for Erta Ale Ridge

“…In the very crater of Erta Ale, there is an exposed lava lake [2]. Eruptions, closely related to the seismic shocks, can cause considerable dislocations, both in the vertical and in the horizontal plane [3].…”