Historic and Recent Eruptive Activity of Oldoinyo Lengai

Abstract

The eruptive history of Oldoinyo Lengai began <0.37Ma ago, with eruptions of nephelinitic and phonolitic tuffs and agglomerates from the southern, now extinct, crater. Following a period of erosion, black nephelinitic ashes were erupted starting about 1250 a from a new northern crater, which has been the site of subsequent activity; altered natrocarbonatite lava blocks occur in these tuffs. Natrocarbonatite lava was first recorded in the summit area in 1904, though earlier verbal reports of “snow” on the mountain may refer to carbonatite ash.

Due to the remoteness of the volcano, observation of the activity was sporadic before the last 30 years. From a combination of observations since the 1960s and earlier accounts, it appears that, during this century, violent eruptions of mixed carbonate-silicate ash have been preceded by periods of natrocarbonatite lava extrusion lasting about 8–10 years, during which the active northern crater has been filled. The ash eruptions have resulted in changes in crater morphology, and have been followed by periods of quiescence, prior to renewed lava effusion. Extrusion of peralkaline combeite nephelinite earlier this century (in 1917?), together with evidence for molten silicate material during the 1966 eruption, points to the coexistence of silicate and carbonate magmas at the volcano.