Jurgis Klaudius

Remote sensing study of sector collapses and debris avalanche deposits at Oldoinyo Lengai and Kerimasi volcanoes, Tanzania

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.

Gem-Quality Yellow-Green Haüyne from Oldoinyo Lengai Volcano, Northern Tanzania

GEMS & GEMOLOGY FALL 2009 ldoinyo Lengai volcano, located in the Gregory Rift, northern Tanzania, is the only active carbonatite volcano in the world (Dawson, 1962; Keller and Krafft, 1990; Mitchell and Dawson, 2007). It is a typical stratovolcano and 2,952 m high (figure 1). It consists of unusual silica-undersaturated volcanic rocks including nephelinite and phonolite, as well as natrocarbonatites. The rocks occur as ash, lapilli, tuff, and agglomerates interlayered with lava flows. The agglomerates contain blocks of unusual feldspathoid-rich plutonic rocks such as urtite and ijolite. While visiting Oldoinyo Lengai volcano in 2003, researchers from Freiburg University in Germany found a block of plutonic rock ~0.35 m in longest dimension, which proved to have a composition intermediate between ijolite and urtite. The block was coarse-grained and pegmatitic, consisting primarily of nepheline and diopside with subordinate amounts of magnetite, apatite, and perovskite. It also contained large (up to 7 cm) anhedral gemquality pale greenish yellow crystals that were subsequently identified as haüyne (figure 2), ideal formula Na6Ca2(Al6Si6O24)(SO4)2. However, published data on haüyne show that it is characterized by variable contents of Ca and Na, typically contains some K, and in addition to (SO4) 2− also contains Cl and S2− (e.g., Deer et al., 2004). Thus, a more general formula of haüyne can be written as (Na,Ca,K)4–8(Al6Si6O24)(SO4,S,Cl)1−2.