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

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 magma-chamber 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.