Physical volcanological study of the Rungwe volcanic province, Tanzania

Abstract

Many potentially hazardous volcanoes in low-income countries have not been studied and are not regularly monitored. This thesis presents a first step towards detailed hazard and risk assessment for the Rungwe Volcanic Province (RVP) in Tanzania. The volcano-tectonic architecture of the RVP reveals a strong control of tectonic activity over the location of three central volcanoes that are considered active: Ngozi, Rungwe and Kyejo. From the preserved stratigraphic record, it is clear that Rungwe volcano was dominated at least in its Late Holocene history by explosive eruptions generating pyroclastic fallout deposits. Tephra fallout is therefore the most important volcanic hazard at Rungwe. Ngozi had two major eruptions on a regional scale both of which possibly contributed to the formation of its present-day caldera. The < 1 ka Ngozi Tuff deposit suggests that Ngozi has been active until recently, and that it is capable of producing significant pyroclastic density currents. The ca. 4 ka Plinian-style Rungwe Pumice eruption is closely documented and physically characterized, enabling to infer key eruptive parameters. The deposit volume is estimated at 3.2 – 5.8 km³. The Rungwe Pumice classifies as a magnitude 5 Plinian eruption with a maximum eruption column height of 30 – 35 km. The Rungwe Pumice is a unique scientific case study. The eruption happened in nearly wind-free conditions, as evidenced by empirical and semi-analytical modelling and dispersal maps. Second, it lacks pyroclastic density current deposits which are usually found in other similar Plinian eruption deposits as a result of fountain collapse. Field, petrological and geochemical data are consistent with a 5 – 6 km deep magma chamber. Compositional variations between Rungwe magmas are interpreted to result from fractional crystallization in a main trachytic magma body that is regularly replenished by a less evolved magma from a deeper source, resulting in a certain proportion of magma mixing and mingling. The findings presented in this thesis provide a major step forward in the understanding of processes at work in the RVP and Rungwe volcano in particular.