Research Repository :: Browsing by Author "Ernst, Gerald G. J."

Browsing by Author "Ernst, Gerald G. J."

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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, Patric
The ~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.
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, Patric
On 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.
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, Patric
The 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.
Remote Sensing Study of Sector Collapses and Debris Avalanche Deposits at Oldoinyo Lengai and Kerimasi Volcanoes, Tanzania
(2008-10) Kervyn, M.; Ernst, Gerald G. J.; Harris, A. J. L.; Belton, F.; Mbede, E.; Jacobs, P.
Evidence for volcano collapse and debris avalanche deposits (DADs) at Oldoinyo Lengai (OL), Tanzania, has been obtained from mapping and fieldwork. Three major DADs have been identified, named Zebra, Cheetah and Oryx DADs. Field evidence indicates geologically young ages. On this basis a remote sensing (RS) study of the active carbonatite volcano OL and the surrounding rift plain was carried out, using Shuttle Radar Topography Mission (SRTM) digital elevation data, Landsat and ASTER imagery, geological maps and aerial photographs. The SRTM digital elevation model (DEM) allowed morphological characterization of OL and reassessment of the volcano volume to 41±5 km3. This enabled the identification of collapse scars, fields of large hummocks (>300 m across), sharp deposit edges typical of DADs, and estimation of the minimum thickness of the DADs. Multispectral and topographic RS data interpretation allowed mapping of the extent and estimation of the volume of two sector‐collapse scars and three DADs. The DADs extend up to 24 km from OL and have volumes ranging from 0.1 to ∼5 km3. Striking radial ridges and grooves were identified in some parts of the DADs. The morphological variability for ridges and grooves in different DADs is attributed to contrasting flow dynamics and avalanching material. A volcano collapse and the corresponding DAD, ∼1 km3 in volume, were also characterized by RS at the nearby Kerimasi volcano. The presence of young DADs highlights the need for routine monitoring of ground deformation and seismicity at OL to anticipate hazardous events.
The Rungwe Volcanic Province, Tanzania – A Volcanological Review
(2011-11) Fontijn, Karen; Williamson, David; Mbede, Evelyne I.; Ernst, Gerald G. J.
The Rungwe Volcanic Province in SW Tanzania is a densely populated area that is considered volcanically active. As part of the East African Rift System, a significant control of tectonic activity seems to exist on the location and also potential destabilization of volcanic edifices. Three large volcanoes, Ngozi, Rungwe, and Kyejo, dominate the landscape and all show contrasting eruptive behaviour in the recent geological past. Kyejo volcano is a flow-dominated volcano that had a historic lava flow eruption. Lake sediment cores, drilled in Lakes Malawi, Masoko, Rukwa, and Tanganyika, provide a record of frequent explosive eruptions in the last few tens of thousands of years. In combination with on-land stratigraphic observations, they constrain the minimum eruptive frequency of especially Rungwe and Ngozi volcanoes. Both volcanoes had Plinian-style eruptions in the Holocene. The most striking documented Rungwe eruption, the ca. 4 ka Rungwe Pumice, is a rare case of a Plinian eruption in near-wind-free conditions. Furthermore, the Rungwe Pumice, just like any other Rungwe tephra deposit, does not show any evidence of pyroclastic density current deposits. Apart from explosive eruptions at a range of scales happening every few hundred years at Rungwe, the volcano also experienced at least two sector collapse events generating debris avalanches. All existing evidence shows that the Rungwe Volcanic Province is prone to future significant explosive eruptions. To further assess, quantify and mitigate volcanic hazard risks, extensive and systematic multidisciplinary geological research, and both volcanic and tectonic monitoring are needed.
A Semi-automated Algorithm for Monitoring Small Scale Thermal Activity at Oldoinyo Lengai Volcano, Tanzania
(2006-09) Kervyn, M.; Belton, F.; Mbede, Evelyne I.; Jacobs, P.; Ernst, Gerald G. J.; Harris, A.J.L.
Active and potentially hazardous volcanoes in East Africa (Tanzania, Kenya, Ethiopia) are not currently monitored on a regular basis. Among the most active ones, Oldoinyo Lengai has been almost continuously active since 1983. Activity has been confined to small-scale effusive and explosive eruptions of low temperature (~550°C) natrocarbonatite within the summit crater. Historical eruptions’ accounts illustrate that Oldoinyo Lengai also repeatedly displays much more explosive (i.e. violent strombolian or sub-plinian style) eruptions of silicate magma. Daily MODIS images offer a low-cost route to monitor volcanic thermal activity. Existing methods applied on a worldwide scale, such as the MODVOLC algorithm developed by the University of Hawaii, are insufficiently sensitive to monitor small-scale thermal anomalies. MODLEN is an adapted semi-automated algorithm, using MatLab, which allows daily record of the thermal emission at Lengai and the identification of high intensity activity. MODLEN is calibrated and validated using the activity reports from field visits. Despite the small-scale and low temperature of the eruptive activity at Lengai, MODLEN is able to detect all periods of increased activity observed in the field. A 5-year time-series dataset, resulting from analysis of more than 2500 night-time MODIS scenes, has been acquired as a baseline for future near-real time monitoring. It provides insights on the factors controlling the temporal distribution of high intensity activity events. Improvement of the method, to avoid false detection due to partial cloud coverage and detection of the full extent of thermal alerts are also discussed. Although recent activity has been moderate at Lengai, a more intense explosive eruption is overdue. Monitoring is needed to try and anticipate the transition to more hazardous eruption styles and to gain insights into the controls on eruption intensity at Lengai.
Thermal Remote Sensing of the Low‐Intensity Carbonatite Volcanism of Oldoinyo Lengai, Tanzania
(2008-10) Kervyn, M.; Ernst, Gerald G. J.; Harris, A. J. L.; Belton, F.; Mbede, Evelyne I.; Jacobs, P.
Although Tanzania, Kenya and Ethiopia contain a number of active and potentially hazardous volcanoes, none of them are routinely or continuously monitored. Of these, Tanzania's Oldoinyo Lengai (OL) has been active almost continuously over the past two decades (since 1983). Recent activity has been confined to small‐scale effusive and explosive eruptions of natrocarbonatite within the summit crater, with lava flows occasionally overflowing the crater rim and extending onto the volcano flanks. The automated MODVOLC algorithm falls short of detecting all thermal anomalies within OL's crater. The sensitivity of the algorithm is insufficient to detect anomalies of the size and magnitude presented by those at OL. We explore how Moderate Resolution Imaging Spectroradiometer (MODIS) infrared (IR) bands can still be used to monitor activity. We cross‐verify our observations against field reports and higher resolution satellite images (ASTER, Landsat ETM+). Despite the limited extent and low temperature (∼585°C) of natrocarbonatite lavas, relative variations in eruption intensity and periods of increased activity alternating with periods of reduced or no detectable activity can be observed using the MODLEN algorithm. Although activity in the past two decades has been moderate at OL, a more intense explosive eruption is overdue and there is a need for routine monitoring in the future. Our work makes a case for low‐cost thermal IR monitoring as an essential component of such a monitoring programme at several Tanzanian, Kenyan and Ethiopian volcanoes. The approach presented here is already available for routine use.
Voluminous Lava Flows at Oldoinyo Lengai in 2006: Chronology of Events and Insights into the Shallow Magmatic System
(2008-02) Kervyn, Matthieu; Ernst, Gerald G. J.; Klaudius, Jurgis; Keller, Jörg; Kervyn, François; Mattsson, Hannes B.
The largest natrocarbonatite lava flow eruption ever documented at Oldoinyo Lengai, NW Tanzania, occurred from March 25 to April 5, 2006, in two main phases. It was associated with hornito collapse, rapid extrusion of lava covering a third of the crater and emplacement of a 3-km long compound rubbly pahoehoe to blocky aa-like flow on the W flank. The eruption was followed by rapid enlargement of a pit crater. The erupted natrocarbonatite lava has high silica content (3% SiO2). The eruption chronology is reconstructed from eyewitness and news media reports and Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data, which provide the most reliable evidence to constrain the eruption’s onset and variations in activity. The eruption products were mapped in the field and the total erupted lava volume estimated at 9.2 ± 3.0 × 105 m3. The event chronology and field evidence are consistent with vent construct instability causing magma mixing and rapid extrusion from shallow reservoirs. It provides new insights into and highlights the evolution of the shallow magmatic system at this unique natrocarbonatite volcano.

Browsing by Author "Ernst, Gerald G. J."

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