par Switten, Cyril 
Président du jury Mattielli, Nadine
Promoteur Fontijn, Karen
Publication Non publié, 2021-08-27
Président du jury Mattielli, Nadine
Promoteur Fontijn, Karen
Publication Non publié, 2021-08-27
Mémoire
| Résumé : | Mt. Meru is a historically active stratovolcano located in Northen Tanzanian DivergenceZone within the Eastern Branch of the East African Rift. This polygenetic volcano has erupteda variety of products during its development. Amongst them, tephra deposits and pyrcolasticflows show its Plinian character. During the Quaternary phase three major to moderate ex-plosive eruptions occurred and produced phonolite deposits, MXP-1, MXP-2 and MXP-3. Thelatter categorised as a Plinian eruption, is better preserved and allows analysis to constrainits eruption dynamics. MXP-3 is divided into 3 distinct tephra deposits U7, U8, U9 and apyroclastic flow U10. Granulometry and componentry analysis are performed on the tephradeposit and highlights an unimodal grain size distribution with a Mdφandσφof -2.15 and1.93 respectively. The componentry analysis show a similar content of the major components(pumice, lithic and free-crystals), but variations are observed in crystal content. U7 is a coarseand poorly sorted sequence with a rich in feldspars of large sizes. U8 is the finest and mostwell-sorted sequence with the higher content in pyroxenes. And U9 is the coarsest and lesssorted sequence with an intermediate trend in crystal content compared to the other two se-quences. A bulk density of 550kg/m3is obtained with a volume 1.05km3according to thedispersal and thickness data from tephra deposits. However the volume seems to be underesti-mated and yields an isomass map which is inconsistent considering the bulk density obtainedand the thickness deposit observed. On the basis of the analysis performed a scheme on ofthe eruption dynamics is proposed. The eruption started with the U7 phase symbolised by animportant mass discharge rate and evolved phonolitic magma rich in feldspars. Subsequentlya lower intensity coupled with an input of a less evolved magma took place, producing thefiner, more pyroxene-rich U8 sequence.This was followed by the U9 sequence characterized byan intermediate composition between the two previous phases, with the most important inten-sity, forming a thicker sequence, composed of coarser clasts. The phase probably ended with adecrease in intensity that led to the production of a PDC (U10), ending the MXP-3 event |
