EAST AFRICAN RIFT AND OLDOINYO LENGAI
EAST AFRICAN RIFT
The great outdoors is the laboratory for the geologist and geophysicist. In the field, geologists study the minerals, rocks, rock structures, volcanoes, hot springs, and other features of nature. A recent field trip that I took to Kenya and Tanzania illustrates some of the types of geological features that geologists study in the field. The most prominent structural feature in those two countries is the East African Rift that trends roughly north-south through Kenya and Tanzania. The Rift is a feature of plate tectonics in which large portions of the earth's upper mantle and crust have moved slowly (about 5 cm per year) but persistently through geological time to form the shapes of the continents, oceans, and mountains as we know them today. Places where the earth's crust has formed deep fissures and the plates have begun to move apart develop rift structures in which elongate blocks have subsided relative to the blocks on either side. The East African Rift is a world-famous example of such rifting. It is characterized by 1) topographic deep valleys in the rift zone, 2) sheer escarpments along the faulted walls of the rift zone, 3) a chain of lakes within the rift, most of the lakes highly saline due to evaporation in the hot temperatures characteristic of climates near the equator, and some of the lakes are the largest in Africa, 4) voluminous amounts of volcanic rocks that have flowed from faults along the sides of the rift, and 5) volcanic cones where magma flow was most intense. Due to the East African Rift, the total volume of phonolite (alkaline extrusive rocks) lava in Kenya exceeds that present elsewhere in the world by several orders of magnitude.
* Kenya Geology Photographs
OLDOINYO LENGAI CARBONATITE-PHONOLITE VOLCANOE
The volcanic mountains associated with the East African rift in Kenya and Tanzania are famous for their heights above the rift floor and for the character of their volcanic rocks. Perhaps the best known is Mt. Kilimanjaro in northeast Tanzania. Kilimanjaro is the highest mountain in Africa, one of the highest mountains in the world, and rises to an elevation of 19,344 feet above sea level. Mt. Kenya in south-central Kenya is the highest mountain in Kenya, the second highest mountain in Africa, and it height of about 17,000 feet promotes a rainfall that has developed the beautiful forests along it flanks.
Geologically, Oldoinyo Lengai, 9,442 feet high, is one of the most famous and interesting of the volcanoes associated with the East African Rift. It is the world only active carbonatite volcanoe. Carbonatite is a rock consisting mainly of calcite (CaCO3) or other carbonate minerals that was emplaced as a molten magma (hot liquid rock materials). Carbonatite lava has extruded recently (1993) forming the white top of Oldoinyo Lengai. That white peak, in the heart of Maasailand, is thought to represent the beard of the Maasai god by the Maasai people who called the mountain, Oldoinyo Lengai, "the Mountain of God". Small amounts of carbonatite magma is currently being extruded from small cones within the floor of the crater at the summit of the mountain. Violent eruptions of volcanic ash have taken place in 1917, 1926, 1940, 1958-67, 1983, and 1993. It was not until after the eruptions of 1960 that the carbonatitic nature of the volcanics was first recognized. Since that time, the soda (Na)-rich carbonatites of Oldoinyo Lengai have played a major role in the understanding by geologists of how carbonatite have formed. Understanding the origin of carbonatites is important because of their very large role in the development of concentrations of certain ore deposits. Examples of ore deposits associated with carbonatites include: 1) Mountain Pass, California (until recently, the world's largest rare earth deposit), 2) Bayan Obo, Inner Mongolia, China (the world's largest rare earth deposit), 3) Palabora, South Africa (one of the world's largest copper deposits; and with significant amounts of by-product apatite, titaniferous magnetite, Zr, U, Th, and vermiculite), 4) Okorusu, Nambia fluorspar deposit, 5) Amba Dongar, India fluorspar deposit, 6) Mato Preto, Brazil fluorspar deposit.
The carbonatite lavas at Oldoinyo Lengai uolcanoe are unusual in many respects:
1) They are extremely alkali-rich, with >30 wt% Na2O.
2) They have very low ferromagnesian element contents.
3) They have extemely high incompatible element concentrations that indicate a highly fractionated magmatic melt. -
4) They are enriched in volatiles, yet they were anhydrous.
5) They have a mineralogy dominated by phases unknown elsewhere (or found rarely as high-temperature magmatic minerals): nyerereite and gregoryite,
K-Na-Ca carbonates.
6) Their eruptive temperatures of the carbonatite at Oldoinyo Lengai of 500-590ƒC are among the lowest yet measured for terrestrial lauas.
7) Their viscosities measured for erupting Oldoinyo Lengai carbonatite lavas are among the lowest yet measured for terrestrial lauas.
These chemical and mineralogical features have been interpreted by geologists to indicate that the magmas that formed the carbonatite lava at Oldoinyo Lengai were generated in the earth¼s upper mantle by processes that changed the chemical composition of the magma (differentiation with crystal fractionation) and separated one liquid from another liquid (liquid immiscibility). The discovery under the microscope of spheres of carbonatite rock (formerly liquid spheres) within phonolite and vice versa have proven the former liquid character of the carbonatite and support the concept of liquid immiscibility.
* Oldoinyo Lengai Photographs