Emplacement and inflation of natrocarbonatitic lava flows during the March–April 2006 eruption of Oldoinyo Lengai, Tanzania
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The most voluminous eruption of natrocarbonatite lava hitherto recorded on Earth occurred at Oldoinyo Lengai in March–April 2006. The lava flows produced in this eruption range from blocky 'a'a type to smooth-surfaced inflated pahoehoe. We measured lava inflation features (i.e. one tumulus and three pressure ridges) that formed in the various pahoehoe flows emplaced in this event. The inflation features within the main crater of Oldoinyo Lengai are relatively small-scale, measuring 1-5 m in width, 2.5–24.4 m in length and with inflation clefts less than 0.4 m deep. Their small sizes are in contrast to a tumulus that formed on the northwestern slope of the volcano (situated ~1140 m below the crater floor). The tumulus is roughly circular, measures 17.5 × 16.0 m, and is cut by a 4.4 m deep axial inflation cleft exposing two separate flow units. We measured the elastic properties (i.e. shear- and bulk moduli) of natrocarbonatitic crust and find that these are similar to those reported for basaltic crust, and that there is no direct correlation between magmastatic head and pressure required to form tumuli. All inflated flows in the 2006 event were confined by lateral barriers (main crater, erosional channel or erosional gully) suggesting that the two most important factors for endogenous growth in natrocarbonatitic lava flows are (1) lateral barriers that prevent widening of the flow, and (2) influx of new material beneath the viscoelastic and brittle crust.
KeywordsNatrocarbonatite Oldoinyo Lengai Inflation Pahoehoe Tumuli Elastic moduli
The fieldwork to conduct the present study was funded by research/travel grants from the Swedish Research Council (VR), Nordic Volcanological Center (NORDVULK), and the Royal Swedish Academy of Sciences (KVA). Fabio Ferri and Luigi Burlini assisted with the analyses of mechanical properties of natrocarbonatitic lavas at ETH Zürich. Matthieu Kervyn, Jurgis Klaudius, Luca Caricchi, Alex Teague and Peter Ulmer are gratefully acknowledged for many interesting discussions on various aspects of Lengai volcanism. HBM would also like to thank Melanie Finn and Matthew Aeberhard for their generous hospitality during the 2007 visit to the Lake Natron area. Finally, we would also like to thank Andreas Jallas (for excellent dry-preparation of thin sections). Insightful reviews by Laszlo Keszthelyi and an anonymous reviewer are gratefully appreciated.
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