Oldoinyo Lengai
Oldoinyo Lengai Geology
Photo courtesy of Laura Carmody
Oldoinyo Lengai, located in the East African Rift in Tanzania, is the only volcano on Earth which produces carbon - rich magmas in the form of sodium carbonatite, known as Natrocarbonatite, as well as silicate material in the form of nephelinite / phonolite. The low viscosity natrocarbonatite lava is effusively erupted almost continuously from the active northern crater at the summit of the volcano. The lava is hygroscopic and so is rapidly hydrated after eruption, changing from dark grey to white over a matter of months / years. Standing 2960m high, Oldoinyo Lengai is a classic stratovolcano shape composed of alternating layers of silicate and carbonatite lavas and pyroclastics with two summit craters, the active northern crater and inactive southern crater which are separated by an ash saddle. Oldoinyo Lengai forms part of the Younger Volcanic group within the rift, thought to have initiated 0.4 million years ago. The Older Volcanic group of the region are composed of large basaltic - trachyte shield volcanoes such as those of the Crater Highlands and date back to around 2.0 million year (Dawson, 2010). It's remarkably steep slopes (~55 degrees at the summit) have resulted in collapse of the volcano during periods of dormancy and erosion, resulting in a large landslide scar on the eastern slope (Eastern Chasm). This collapse is thought to have occurred during the early generation of the cone resulting in a shift in activity site from the southern to the northern crater. Other landslides originating from the northern flank have left deposits up to 20km away from the volcano on the shores of Lake Natron (Kervyn et al, 2006).
Oldoinyo Lengai's eruptive behaviour can be crudely classified into two types;
- The effusive eruption of low viscosity, relatively low temperature natrocarbonatite from vents, lava lakes or hornitos within the northern crater.
- Explosive eruptions of mixed silicate / carbonatite ash.
The explosive episodes of activity occur intermittently with the most recent explosions occurring in 1966 and then again in 2007. In between these episodes the volcano resumes effusive activity with natrocarbonatite flows contained primarily within the newly excavated crater which over time fills with material. Small flows can often overflow the crater rim and flow down the flanks of the volcano.
Degassing at Oldoinyo Lengai
Photo courtesy of Laura Carmody
As a carbon - rich volcano it is no surprise that Oldoinyo Lengai is one of the top CO2 producing volcanoes on the globe. In 1994 aerial measurements of carbon dioxide, using spectrophotometers, within the plume of Lengai was estimated at 0.05 - 0.06 X 1012 mol/yr (roughly 6000 - 7200 tonnes per day), with 98% of this value emitted from vents within the active crater and <2% from diffuse emissions from the flanks of the volcano (Brantley et al, 1995 & Koepenick et al, 1996). This value was later confirmed by a study of the fumarole gases generated at the hornitos present in the crater during 2005 (Fischer et al, 2009). This study by Fischer et al, 2009, also concluded that the isotopic signatures, determined by gas chromatography performed within the laboratory of gas samples from the vents within the crater, had a mean value of
δ13C
-2.79 parts per mil.
Prior to the explosive eruption in 2007 the majority of the degassing was through hornito structures on the crater floor. These fumaroles were around 120°C and sampled via teflon tubes and evacuated flasks. Post eruption degassing from the crater originates from open vents on the inaccessible crater floor and fractures around the crater rim.
