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Ol Doinyo Lengai, the world’s only active carbonatite volcano is situated within the Lake Natron Basin (figure 1.0), which is part of the East African Rift (figure 1.1). It was first known to be active in 1883 (Fischer, 1885). Violent eruptions of ash occurred in 1917, 1926, 1940, 1966 and 1967 (Dawson et al., 1995). Extrusions of natrocarbonatite lavas in 1983 then followed these ash eruptions and minor natrocarbonatite extrusion continues to the present day. Ol Doinyo Lengai is a stratovolcano, with alternating layers of lava, lithified ash and debris avalanches. Debris avalanche deposits are found in the area surrounding Ol Doinyo Lengai, and are thought to have formed by the partial collapse of the volcano in the past. The area around the summit consists of natrocarbonatite lava deposits.
Figure 1: Location maps a) East African Rift Zone (Sherrod et al., 2013) b) Zoomed in map from 1a, showing MAP AREA, Regional location map of Ol Doinyo Lengai in relation to Lake Natron and neighbouring Gelai volcano (Sherrod et al., 2013)
Ol Doniyo Lengai sits upon a sequence of Neogene alkaline volcanic rocks which consist of lavas and tuffs, deposited in the centre of the Gregory Rift Valley, south of Lake Natron, on Pre-Cambrian basement rocks (Dawson, 1962). Ol Doinyo Lengai is situated at a triple junction between north-south trending faults, forming the western escarpment of the Gregory Rift Valley (Dawson, 1962) and NE-SW trending faults forming the eastern side (USGS Open-File Report 2013-1306; Sherrod et al., 2013) (Figure 1). The tectonic plate system involves complex plate rotation.
Ol Doinyo Lengai is of particular geological interest due to the carbonatite composition of the lavas. These carbonatite lavas are unusual, erupting at ~500-600°C, which contrasts with more common basaltic lavas eruptive temperature of ~1000°C. Natrocarbonatites are alkali-rich and contain high amounts of sodium and potassium. They are dark black during their eruption and turn white upon cooling due to reaction with atmospheric water.
Aims and Objectives
Previous work on the volcano was undertaken by Dawson in 1962. He divided the main-cone rocks into various units of tuffs and agglomerates, interbedded with lavas. The lower flanks of the volcano consist of debris-avalanche deposits as well as tuff rings, and tephra cones. However, since the discovery of extrusion of natrocarbonatite lavas (Dawson, 1962), subsequent work has focused almost exclusively on the unusual mineralogy and chemistry of the carbonatite and mixed silicate-carbonatite magmas and their eruptive products. Although recent lavas and the 2007-08 ash eruption were observed, very little is known of the volcanology of the earlier 20th century deposits, the material that builds the lower part of the stratocone, and the volcanic pile on which the volcano sits (and through which active faulting occurs). Consequently, Ol Doinyo Lengai presents a unique opportunity to understand two important aspects of volcanology and volcano-tectonics.
Physical volcanology of carbonatite and mixed silicate-carbonatite deposits
The older pyroclastic deposits and lavas will be studied to determine the nature of carbonatite and associated eruptions. Mapping and detailed logging of the pyroclastic rocks will be carried out in order to reconstruct a detailed and accurate eruption history.
Questions we want to answer:
ï‚· What is the nature and distribution of the pyroclastic ash-fall and density currents?
ï‚· What is the frequency and magnitude of the eruptions?
ï‚· How do the eruptions evolve chemically?
Methodology:
I. Detailed field geological mapping of pyroclastic deposits (and lavas) in the lower slopes of Ol Doinyo Lengai and in more distal regions near Gregory Rift Valley faults.
II. Detailed logging of pyroclastic deposits (and lavas) in the lower slopes of Ol Doinyo Lengai and in more distal regions near Gregory Rift Valley faults.
III. These data will be used, where possible, to construct isopach and isopleths maps of the pyroclastic deposits. Such maps will inform future risk assessments at the volcano.
IV. Samples will be collected for detailed petrographical, geochemical and micro-structural analysis as appropriate.
Volcano-tectonics in an active rift
The older pyroclastic deposits of Ol Doinyo Lengai and more distal earlier eruptive products display a complex relationship with faults of the Gregory Rift Valley. The USGS map displays Holocene deposits covering faults and Pleistocene deposits being cut by faults (USGS Open-File Report 2013-1306; Sherrod et al., 2013). Two different fault trends in the area are observed corresponding to the triple junction: north-south trending faults in the north west of Ol Doinyo Lengai and north-east- south-west trending faults located in the east. Mapping of these faults will be undertaken in order to estimate tectonic strain and orientation of the stress field. By using previous dating of volcanic layers, this will allow reconstruction of strain rates.
Questions we want to answer:
ï‚· How tectonically active is this part of the rift system?
ï‚· What are the kinematics in this triple junction?
ï‚· How is the volcanic activity influenced by the tectonics?
Methodology:
I. Identify rift faults, determine nature of faults, thickness of fault cores and movement of faults, and fault offsets. Measure fault and fracture sets, striations and offsets.
II. Structural analysis will be used to reconstruct kinematics of this part of the rift, to estimate strain rates, and to potentially develop a model of volcano-tectonic interaction at this triple junction.
Areas of geological field work
As outlined above our focus will be on the older 20th century (and earlier) volcanic deposits and faulting within the area. Three main areas have been identified where we will conduct our research.
Area 1
This area is located to the north west of Ol Doinyo Lengai, where north-west trending faults, (solid black lines on the map) interact with lavas (orange, QTel) and tuff rings (brown, Qsk) (Figure 2).
Figure 2: Geological Map of Area 1 (Sherrod et al., 2013).
Area 2
This area is located to the north-east of the volcano and also displays interaction between faults (oriented north east-south west) and lavas (orange Qvg), tuff rings (brown, Qntr), and tuff deposits (pink, Qnet) (Figure 3). The faults located here are orientated north east-south west ,which is different to the direction of faults displayed in area 1. We want to understand
why these faults have a different direction and their relationshipwith the volcanic deposits.
Figure 3: Geological Map of Area 2 (Sherrod et al., 2013).
Area 3
This area will focus on the pyroclastic deposits and the unusual natrocarbonatite lavas located within the lower part of the Eastern Chasm on the north-east flank of Ol Doinyo Lengai (Figure 4). The area consists of undifferentiated lavas and pyroclastic deposits (lilac, Qom), and tuff rings (brown, Qntr) (Figure 5).
Figure 4: Geological map of Area 3 (Sherrod et al., 2013).
Figure 5: a) Eastern flank of Ol Doinyo Lengai showing Oltatwa tuff ring and a phonolite lava flow (Sherrod et al., 2013).
Figure 5: b) Tuff and lapilli-tuff deposits of the lower part of the Eastern Chasm (Sherrod et al., 2013).
We will spend approximately one week in Area 1 and approximately 10 days working in Areas 2 and 3, as there is more to cover in these areas. As we will be teaming up with students from the University of Dodoma we are able to split into smaller groups in order to cover as much area as possible in the short time we have. There will be approximately 6 groups, depending on confirmed numbers, of approximately 4 people in each group.
What we hope to gain from our research
By reconstructing the early eruption history of the volcano we will gain an understanding into the workings of carbonatite eruptions, which will inform risk assessments/hazard maps of Ol Doinyo Lengai volcano. Such data will be of benefit to the local community, UDOM and government agencies, as well as the wider academic community. Our investigation of volcano-tectonics will also provide insight on how volcanism is controlled and effected by active faulting. Together, these data will ideally be published in academic journals.
Other Research Projects
Hazard Mitigation and Human Impact
Volcanoes pose a considerable risk to people and certain infrastructure, primarily because there is no model to follow regarding their eruptions, which have the potential to cause catastrophic loss to surrounding civilizations (Evershed, 2010). It is therefore not only of great interest to further an understanding of the volcanic activity itself, but also to document the human perception to such a risk and how it shapes local society. Studies into the relationship between human civilization and volcano risk have been conducted previously in Iceland, where Saltykivskii (2012) discussed how the Eyjafjallajokull eruption of 2010 caused severe disruption on a local level to farmers who lost livestock and crops, and on an international level where flight disruptions due to the spreading ash cloud adversely effected the economy. Bird et al. (2010) considered how this information on general volcanic hazards needs to be communicated and also how to create early warning systems to enable safe working on the volcano, specifically for tour operators. Haulle (2012) discussed how earthquake risk in Rungwe, Tanzania, 700 km south-south-west of Ol Doinyo Lengai is interpreted by locals as a natural occurrence and that there are few impact reduction measures in place.
On the University of Glasgow Tanzania Expedition in 2015, we aim to work with locals, using interviews, focus groups and surveys to study how the risk of volcanic activity from the active Ol Doinyo Lengai volcano impacts them and suggest ways in which assistance can be given to reduce risk in future. We will also study the impact of tourism to the volcano on the local area, how the people are adapting to accommodate tourists, and how closer integration with the Western world has influenced their culture.
Re-take Photography
Alongside the geological and human research we will be running a re-take photography project. The main aim of this project is to illustrate the changing landscape due to both volcanic activity and shifts in land use. We will source numerous photographs from previous expeditions to the volcano and return to the location from which the pictures were taken to take the same picture 10+ years on. We have already been given access to over 50 photographs taken between 1998-2006. By re-taking these pictures we will see the impact of the 2007-08 eruption along with any development of the surrounding pasture lands.