The lava at Ol Doinyo Lengai is cool enough that it only glows at night.

The viscosity of the lava is so low that it froths as it erupts

Fresh lava from the volcano is still black, but lava just a few weeks old has already weathered to a gray/white color.

Erupting lava forms small cones called hornitos.

above photos coutesy of National Geographic

image courtesy of Jeffery Brown

image courtesy of Jeffery Brown

Related Links

National Geographic Article on Ol Doinyo Lengai

Ol Doinyo Lengai: The Mountain of God - pictures and links

The Oldoinyo Lengai web site Celia Nyamweru's website

Ol Doinyo Lengai - good pictures

photo courtesy of Brian Rippon - aerial views of the crater

photo courtesy of  http://perso.club-internet.fr/acatte/ Lengai_general-views.htm  view from inside the crater

Ol Doinyo Lengai

Introduction

Ol Doinyo Lengai is a stratovolcano located in Northern Tanzania. It looms 9,524 feet above the East African Rift Valley.  The name Ol Doinyo Lengai means "The Mountain of God" in the Maasai language.  It is the only active volcano in Tanzania and one of a select few that are active in the East African Rift Valley.  The volcano has erupted many times since it first began to be observed by geologists.  Major eruptions have occurred in 1880, 1914-15, 1926, 1940-41, 1958, 1960-66, 1983-93, 1994-1998, and the mountain continues to erupt mildly to this day.

Geologic Processes

The lava produced at Ol Doinyo Lengai is unlike any other lava on earth. Unlike most lavas, the lava at Ol Doinyo Lengai has a very low silica content. The lava from Ol Doinyo Lengai is a carbonatite, meaning it has more than a fifty percent carbonate content. Carbonatites are quite uncommon in the geologic record, and even less common at the surface as a liquid. Carbonatites are usually found as intrusive dikes, volcanic plugs, or cone sheets (Nyamweru, 2001). Furthermore, most carbonatites are calcite carbonatites, meaning that they are composed primarily of the mineral calcite, which is calcium carbonate (CaCO3). In contrast, the lava at Ol Doinyo Lengai is composed largely of sodium and potassium carbonate (Na,K(CO3)2). Called natrocarbonatite, sodium and potassium carbonates are even more rare than calcite carbonatites.

The minerals that dominate the lava at Ol Doinyo Lengai are nyerereite and gregoryite, carbonates that contain a high percentage of sodium and potassium. Both of these minerals are anhydrous and react with moisture in the atmosphere very rapidly (Mitchel, 2000). As a result of this, the runny black lava that spits from the mountain quickly turns gray or white as the minerals absorb water.
The lack of bulky silica molecules in the lava at Ol Doinyo Lengai allows the melt to flow extremely easily, making it the least viscous lava on earth. The natrocarbonatites of Ol Doinyo Lengai stay liquid at extremely low temperatures (550 C) (Nyamweru, 2001) relative to normal basaltic lavas (1100 C) as a result, they can be studied closely without protective gear. This is one of the most intriguing aspects of Ol Doinyo Lengai to geologists.

Products of the Geologic Process

Most lavas on earth have a silica-rich basaltic or rhyolitic composition. Basaltic and rhyolitic lavas form when material from the crust and upper mantle melt and flow to the surface, essentially maintaining their chemical composition.

Carbonatite lavas do not represent typical mantle or crust composition. There are two dominant theories for explaining this. One theory proposes that they are formed in a ‘primary’ fashion. This means that they are the melted product of carbonate-rich rocks found in the crust. Essentially, this theory states that carbonatite volcanoes form where there is carbonate-rich rock that is melted to form lava.  It is unlikely that this is the case, because carbonatites have been associated with alkali-rich parent rocks (Dawson 1998).

The second theory proposes that carbonatites are the result of a separation of magma into chemically separate units- a process called differentiation (Harmer and Gittins 1998). One type of differentiation is liquid immiscibility. Silicate minerals are crystallized as a melt cools. Because carbonate is not included in silicate mineral formation, the relative abundance of carbonate builds up until the melt is supersaturated with respect to carbonate. At this point, the carbonate-saturated magma is able to physically separate itself from the rest of the magma. If the carbonate-saturated melt is then brought to the surface, it is possible to have a carbonate-rich lava (Morton 2003).

Another type of differentiation that may occur at Ol Doinyo Lengai is fractional crystallization. In fractional crystallization, newly formed crystals float, sink, or are otherwise prevented from chemically interacting with the rest of the melt. Usually, iron and magnesium rich crystals form, leaving behind more silica and aluminum rich melt. In the case of Ol Doinyo Lengai, carbonate-enriched melt is left behind as silica bearing crystals are separated.

Both of these differentiation processes could result in the carbonatite lava that we see at Ol Doinyo Lengai. In reality, it is most likely that a combination of all three of these processes take place at Ol Doinyo Lengai (Bell et al. 1998). The differentiation of the magma beneath Ol Doinyo Lengai had probably been occurring long before Ol Doinyo Lengai was created. It was not until rifting began pulling the African continent apart that the carbonatite magma was able to reach the surface (Alden 2003).

Impacts

Why is the study of Ol Doinyo Lengai important?  First of all, Ol Doinyo Lengai is the world's only active carbonatite volcano.  This makes it a very important site for the study of carbonatites.  The world's largest deposits of rare earth elements (REEs) are found in carbonatite complexes.  It is believed that REEs are rare because they cannot incorporate themselves into minerals when they form deep in the earth.  REEs that cannot fit into silicate minerals are concentrated in the immiscible carbonate portion of the melt (Harmer 2002).  This results in REEs present at the surface with carbonatites.  The largest REE deposits in the world are in Bayan Obo, China and Mountain Pass, CA.  Both of these locations have carbonatite complexes.

Another interesting impact of the study of natrocarbonatite lava at Ol Doinyo Lengai is its application to planetary geology.  There are volcanic features on Venus that have characteristiscs of river systems on earth.  These include meander bends, braded flow systems and deltas.  It is suspected that the lava that created these features on Venus is similar to the lava produced at Ol Doinyo Lengai (Nyamweru 2001).

Literature Cited

Alden, A. 2003. The big rock candy mountain. URL: http://geology.about.com/library/weekly/aa031499.htm(4/3/03)

Bell, K. Kjarsgaard, A. and Simonetti, A. 1998. Carbonatites- into the twenty-first century. Journal of Petrology 39: 1839-1845.

Dawson, J.B. 1998. Peralkaline nephelinite- natrocarbonatite relationships at oldoinyo lengai, tanzania. Journal of Petrology 39: 2077-2094.

East african rift and ol doinyo lengai URL: http://web.umr.edu/~rhagni/ear-ol.html(4/3/03)

Harmer, R.E. and Gittens, J. 1998. The case for primary, mantle-derived carbonatite magma. Journal of Petrology 39: 1895-1903.

Harmer, R.E. 2002. Mineralisation of the phalaborwa complex and the carbonatite connection in iron oxide-Cu-Au-U-REE deposits. In Porter, T.M. (Ed), 2002 - Hydrothermal iron oxide copper-gold & related deposits: a global perspective, volume 1; PGC Publishing, Adelaide, pp 331-340.

Mitchel, Roger, H. 2000. Expedition to the natrocarbonatite volcano oldoinyo lengai.
(online version) newsletter of the mineralogical association of Canada 63. 9-10

Morton, P. 2003. Ways to form magmas of different compositions. URL: http://www.d.umn.edu/~pmorton/geol2300/classnotes/Igneousprocesseslecturepart1.doc
(3/30/03)

Nyamweru, Celia. 5 May 2001 Oldoinyo lengai URL: http://it.stlawu.edu/~cnya/index.html (3/19/03)

Ore deposits associated with carbonatites (REEs). URL: https://courseware.vt.edu/users/bekken/GS4604/Lecnotes/carbonatites.html (4/6/03)

Copyright © 2003 Earlham College. Revised 8 April 2003. Send corrections or comments to graveti@earlham.edu