Browsing by Author "Jacobs, Patric"
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Item The ~4-ka Rungwe Pumice (South-Western Tanzania): A wind-Still Plinian Eruption(2011-05) Fontijn, Karen; Ernst, Gerald G. J.; Bonadonna, Costanza; Elburg, Marlina A.; Mbede, Evelyne I.; Jacobs, PatricThe ~4-ka trachytic Rungwe Pumice (RP) deposit from Rungwe Volcano in South-Western Tanzania is the first Plinian-style deposit from an African volcano to be closely documented focusing on its physical characterization. The RP is a mostly massive fall deposit with an inversely graded base. Empirical models suggest a maximum eruption column height HT of 30.5–35 km with an associated peak mass discharge rate of 2.8–4.8 × 108 kg/s. Analytical calculations result in HT values of 33 ± 4 km (inversion of TEPHRA2 model on grain size data) corresponding to mass discharge ranging from 2.3 to 6.0 × 108 kg/s. Lake-core data allow extrapolation of the deposit thinning trend far beyond onland exposures. Empirical fitting of thickness data yields volume estimates between 3.2 and 5.8 km3 (corresponding to an erupted mass of 1.1–2.0 × 1012 kg), whereas analytical derivation yields an erupted mass of 1.1 × 1012 kg (inversion of TEPHRA2 model). Modelling and dispersal maps are consistent with nearly no-wind conditions during the eruption. The plume corner is estimated to have been ca. 11–12 km from the vent. After an opening phase with gradually increasing intensity, a high discharge rate was maintained throughout the eruption, without fountain collapse as is evidenced by a lack of pyroclastic density current deposits.Item Fundamental Changes in the Activity of the Natrocarbonatite Volcano Oldoinyo Lengai, Tanzania(2010-05) Kervyn, Matthieu; Ernst, Gerald G. J.; Keller, Jörg; Vaughan, R. Greg; Klaudius, Jurgis; Pradal, Evelyne; Belton, F.; Mattsson, Hannes B.; Mbede, Evelyne I.; Jacobs, PatricOn September 4, 2007, after 25 years of effusive natrocarbonatite eruptions, the eruptive activity of Oldoinyo Lengai (OL), N Tanzania, changed abruptly to episodic explosive eruptions. This transition was preceded by a voluminous lava eruption in March 2006, a year of quiescence, resumption of natrocarbonatite eruptions in June 2007, and a volcano-tectonic earthquake swarm in July 2007. Despite the lack of ground-based monitoring, the evolution in OL eruption dynamics is documented based on the available field observations, ASTER and MODIS satellite images, and almost-daily photos provided by local pilots. Satellite data enabled identification of a phase of voluminous lava effusion in the 2 weeks prior to the onset of explosive eruptions. After the onset, the activity varied from 100 m high ash jets to 2–15 km high violent, steady or unsteady, eruption columns dispersing ash to 100 km distance. The explosive eruptions built up a ∼400 m wide, ∼75 m high intra-crater pyroclastic cone. Time series data for eruption column height show distinct peaks at the end of September 2007 and February 2008, the latter being associated with the first pyroclastic flows to be documented at OL. Chemical analyses of the erupted products, presented in a companion paper (Keller et al. 2010), show that the 2007–2008 explosive eruptions are associated with an undersaturated carbonated silicate melt. This new phase of explosive eruptions provides constraints on the factors causing the transition from natrocarbonatite effusive eruptions to explosive eruptions of carbonated nephelinite magma, observed repetitively in the last 100 years at OL.Item Holocene Explosive Eruptions in the Rungwe Volcanic Province, Tanzania(2010-07) Fontijn, Karen; Ernst, Gerald G. J.; Elburg, Marlina A.; Williamson, David; Abdallah, Edista; Kwelwa, Shimba; Mbede, Evelyne I.; Jacobs, PatricThe Holocene explosive eruptive record of Rungwe and Ngozi volcanoes of the Rungwe Volcanic Province in Tanzania was reconstructed based on detailed stratigraphic field evidence combined with whole-rock major and trace element analyses of tephra samples. This reconstruction is supported by 25 new radiocarbon dates on palaeosols that provide additional constraints on the Holocene tephro-chronostratigraphy. We show evidence of two catastrophic Ngozi eruptions and five Rungwe pumice fallout deposits, and also identify several more intercalated poorly preserved pumice and ash deposits. The Ngozi eruptions probably played a role in shaping the present-day caldera. The Rungwe record includes a ca. 2.2 km3 deposit of a Plinian-style eruption dated at ca. 4 ka, a sub-Plinian one at ca. 2 ka and at least three additional smaller-scale fallout deposits. The Rungwe explosive eruptive record shows that the volcano has been frequently active in its late Holocene past. This study highlights the need for volcanic monitoring in the region and presents herewith the first basis of future volcanic hazard assessment.