Ol Doinyo Lengai

Ol Doinyo Lengai is the youngest of a series of Quaternary nepheline, phonolite and carbonatite volcanoes that lie on the eastern branch of the East African Rift, and is renowned as the only active carbonatite volcano on Earth. Even more noteable is that it\'s carbonatite products are alkali-rich natrocarbonite. Activity at Ol Doinyo Lengai is described as periodic: extended effusive eruptions of natrocarbonatite interspersed by explosive ash eruptions dominated by silica-undersaturated compositions. Most eruptions are small lava flows, pools and spatter cones (hornitos) confined to the summit crater, but five explosive eruptions have occurred in the last century, changing the volcano\'s morphology.
Following 25 years of dominantly natrocarbonatite activity, the most recent explosive eruption occurred from September 2007 to April 2008. The months leading up to the event (July to August), which conicided with a seismic crisis, were studied using InSAR by Biggs et al. (2009). The eruption began with subsidence that was attributed to ∼40 cm of normal motion on a NE striking fault to the south of the volcano. However, from 17th July deformation was dominated by the intrusion of ∼7-km-long dyke. Dyke opening increased gradually to a total of ∼2.4 m. From July 21, the collapse of a shallow graben above the fault dominated the near-field displacements.
A similar study by Baer et al. (2008) concluded the same sequence of events: attributing deformation to normal faulting in the first week of the event, followed by intermittent episodes of dike propagation, oblique dike opening and dike-induced faulting. They report a gradual decline in the intensity of deformation occurred over the final weeks.