Browsing by Author "Belton, F."

Now showing 1 - 4 of 4
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    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.
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    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.
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    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.
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    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.