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High spatial resolution topographic data are crucial as a prerequisite for evaluating and modelling volcanic hazards. Remote sensing now provides some of the best methods of retrieving digital elevation models (DEMs) over extensive volcanic regions. Here we compare the advantages and limitations of Shuttle Radar Topography Mission (SRTM) DEMs, derived from radar interferometry, and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) DEMs, derived from digital photogrammetry. Applications are presented for Mauna Kea (MK), Hawaii and Oldoinyo Lengai (OL), Tanzania. We quantitatively assess the accuracy of the respective DEMs to document the size of moderate-sized volcanic features. ASTER DEM accuracy depends on spectral contrast within the image and on the availability of high-quality ground control points. Unlike the ASTER DEMs, which are user-derived, processed SRTM DEMs are provided without estimations of the vertical accuracy for each scene, and the end-user has no control over the processing method. From comparison with a 10-m spatial resolution DEM derived from 1 : 24 000 scale topographic maps of MK, we estimate root mean square errors at 8, 10 and 13 m for SRTM 30-m, SRTM 90-m and ASTER 30-m DEMs, respectively. For scoria cones (<200 m high, <2 km basal diameter), SRTM 30-m, SRTM 90-m and ASTER 30-m DEMs underestimate cone height by 9.5, 27 and 14%, respectively, mostly because of the averaging effect of decreasing spatial resolution. For height estimations of volcanic features higher than ∼100 m in the OL region, all of the DEMs tested were found to be consistent. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]