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. Furthermore, most carbonatites are calcite carbonatites, meaning that they are composed primarily of the mineral calcite, which is calcium carbonate. In contrast, the lava at Ol Doinyo Lengai is composed largely of sodium and potassium carbonate. 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. 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) 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.

The second theory proposes that carbonatites are the result of a separation of magma into chemically separate units- a process called differentiation. 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.

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. 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.

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. 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.

credited to earlham and wikipedia