Geological Background of the Lethal Gas Bursts from Lake Nyos, Cameroon, August 1986

Abstract

Lake Nyos is a maar-type volcano formed by phreatomagmatic eruption along the fissure. The rock is weakly alkaline olivine basalt very rich in crustal xenoliths and mantle-derived spinel-lehrzolite nodules. Stratiied surge deposits made of scoria and ash drape the pre-Cambrian(?) granitic basement along the lake shore. Lake Nyos is a part of Cameroon Volcanic Line which contains many fields of monogenetic volcanoes. Among them is another maar-type crater lake Monoun, which erupted similar lethal gas(CO₂) in 1984 and killed 37 people. The rocks of these young monogenetic volcanoes range from weakly to strongly alkaline basalts and basanitoids. The older(Tertiary?), plateau-forming basalts which constitute immediate basement are also alkaline basalts and basanitoids.