Geologists have discovered volcanoes that erupt carbonate magma - known as carbonatites - all over the world. But a volcano in the East African Rift is the only known volcano to be currently erupting carbonatites. Oldoinyo Lengai Volcano is a mystery for volcanologists, who wonder what the source is for this volcano's carbon rich magma.
The East African Rift Valley
Oldoinyo Lengai Volcano is a composite volcano located in Tanzania along the East African Rift Valley. The East African Rift Valley extends 3,700 km (2,300 miles) from Mozambique to Ethiopia and is home to many of Africa's volcanoes, such as the inactive Mount Kilimanjaro and the active Mount Nyiragongo. The rift valley is a product of mid-continent rifting, where a rift in the middle of the continent is pulling apart the crust in the Eastern region of Africa. The African plate is considered one continental plate, but where the rift is located two new plates are forming. These plates – or subplates – are called the Somali Plate and the Nubian Plate.
Where the crust is being pulled apart, magma is able to rise to the surface. This has led to an area that is heavy with volcanic activity that varies from volcano to volcano. Some volcanoes are related to hotspot activity, some are related to rifting, and some are related to a combination of the two. Moreover, volcanoes along this rift valley are the highest and lowest in the continent - from the towering Mount Kilimanjaro (5, 895 meters above sea level) to the vents of Ethiopia's Danakil Depression, which lie below sea-level. However, geologists will agree that the most unique volcanic acitivity along this rift valley is caused by the carbonate volcano Oldoinyo Lengai.
Oldoinyo Lengai Volcano
This carbonate volcano has puzzled volcanologists since its discovery in 1960, when a geologist witnessed the volcano erupting carbonate lava. Before this, it was generally accepted that rocks composed mainly of carbonate material were sedimentary, and any rocks suspected to be igneous that had high carbonate content, where actually sedimentary. When Oldoinyo Lengai was discovered, however, geologists soon began to realize that in fact volanoes could erupt carbonate lava, forming carbonate rich igneous rock.
Some of the ways that carbonate magma is different from silica magma are:
- Magma composition can vary, but most magma contains more silica than any other element. Mafic magma, like basalt, contains 52 percent silica and felsic magma, like andesite, contains 64 percent silica. Carbonate magma at Oldoinyo Lengai contains only 3 percent silica.
- Carbonate lava is the coolest on Earth. It erupts at 500 to 600 degrees Celsius (930 to 1,100 degrees F), whereas silica based magma erupts at temperatures between 700 to 1,200 degrees Celsius (1,300 to 2,200 degrees F).
- Lava flows, such as those erupting from Kilauea's active vent, glow red at night and during the day. Carbonate magma glows red only at night and appears much like a brown or black mud flow during the day.
Origins of Carbonate Magma
For many years, the origin and source of Oldoinyo Lengai's carbonate magma remained a mystery to scientists. But a recent paper titled, "Upper-mantle volatile chemistry at Oldoinyo Lengai volcano and the origin of carbonatites," published in the May 7, 2009 issue of Nature provides an explanation for the source of this unusual magma. The authors, a team of scientists from the University of New Mexico, Scripps Institution of Oceanography at UC San Diego and Centre de Recherches Petrographiques et Geochimiques in Nancy, France, used geochemistry analysis of the volcano's gas emissions. From this analysis, the scientists discovered that the geochemical signature of the mantle gases coming out of Oldoinyo were almost identical to mantle gases associated with mid-ocean ridges – areas tectonically similar to mid-continent rifts, but under the ocean. From these findings they were able to conclude that carbonate magma can form from the partial melting of minerals in the Earth's mantle and there does not need to be an extreme carbon content in the magma source.
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