A 60-kilometer-long rift segment ruptured in north central Afar, Ethiopia, in September 2005. This rupture followed a short but intensive period of volcanic and tectonic activity during which time more than 162 earthquakes (magnitudes 4.1—5.2) occurred and a 400-meter-long and 80-meter-wide volcanic vent opened (Figure l ). For its spatial extent and amount of opening, this event ranks as the largest observed rifting sequence to have occurred on land, comparable to the 10-yearlong Krafla (Iceland, 1975–1984) episode along a mid-ocean ridge.
The Afar depression is a triple rift junction where the Red Sea and Gulf of Aden spreading ridges emerge on land and meet the northern extension of the East African Rift (Figure 2a). The junction lies above or near a mantle plume [e.g., Courtillot et al., 1984; Montelli et al., 2004]. The floor of the depression is composed mostly of basalt lava emplaced during Plio-Quaternary times over the last three million years [e.g., Kidane et al., 2003]. Huge and elongate basaltic shield volcanoes conspicuously rise above basalt fields and sand plains. The Afar depression is one of the hottest and driest places on Earth, yet its many pastoralists support large herds by distilling water from geothermal vents (known locally as boinas).