Lake Natron and the Oldoinyo Lengai Volcano

Abstract

Lake Natron is located in a desolate, relatively low-lying section of the Gregory Rift Valley, northern Tanzania. The lake is bordered to the west by the Western Escarpment, a major rampart constructed of layer upon layer of Pliocene-age volcanic lavas and tephra, and to the east by the giant Gelai Volcano. Natron is an extraordinarily toxic, soda lake (pH of 12) with extensive sodium carbonate salt flats. The symmetrical cone of Oldoinyo Lengai (3,188 m) rises some 2.5 km above the valley floor to the southwest of Lake Natron. Oldoinyo Lengai is one of the most famous Holocene volcanoes on Earth as it was here the coexistence of two immiscible magmas with radically different compositions was first observed in nature. The alkaline silicate magmas are the most abundant, but it is the highly unusual natrocarbonatite magmas for which Lengai is so well known. The natrocarbonatite is erupted at temperatures considerably lower than silicate lavas. The high sodium contant of the natrocarbonatitite lavas and ashes reacts rapidly with meteoric waters to create secondary, white coloured minerals within a few days. The altered, white or pale grey natrocarbonatite ashes that partially cloak the upper slopes resemble an ice cap when seen from distance. Flows of natrocarbonatite associated with eruptions from the early 1990s could formerly be observed in a shallow summit crater which had remained intact since 1960. This feature overflowed and collapsed during violent eruptions between 2007–2008 and the new crater is too deep and unstable to enter. Lengai erupts on average every 15–20 yr, although some of the recent eruptions have been of nephelinite rather than natrocarbonatite. Lengai presents a potential hazard to the local Maasai who live in the proximity of Lake Natron , and also to aircraft as ash columns may attain heights of tens of kilometres.

Photographs not otherwise referenced are by the author.