A Holy Volcano
Ol Doinyo Lengai is a stratovolcano. Lengai is a holy site for those who study alkaline rocks. And, it is a sacred mountain for Maasai tribesmen.
PICTURE LEFT: Black lava erupting from a chimney. It is the coldest, the fluidest and the strangest known lava of any volcano, a form of molten carbonates, unstable and in fusion.
ERUPTIVE HISTORY
Lengai is unique in that it is the only volcano in the world that is erupting black carbonatite lava, glowing red at night. When the black lava flow solidifies it turns white within 24 hours. It actually turns concrete-grey or cocoa-brown, "sweating out" the white soda salt on its surface. After about six months, with the help of the elements, the lava decomposes to yellow-brown sand. This means that the crater is actually a large dust bowl. During heavy rains, it turns to a muddy mixture, making it difficult to walk across.
This strange lava was for the first time identified only in the 1960s because Lengai is so steep and the hike so arduous, that despite its uniqueness, scientific expeditions have been extremely rare. It was only in 1960, when Barry Dawson of the then Tanganyika Geological Survey made the first successful attempt to reach the active crater.
The eruptive history of Lengai began less than 370,000 years ago, from the southern, now extinct crater. The cone-building stage of the volcano ended about 15,000 years ago. The first verbal accounts of the volcano were gleaned from Arab traders that there existed, in the remote heart of Maasailand, sacred mountain to Maasai. Before the onset of European exploration, missionaries reported quoting native sources: Above the mountain there is a black cloud. Thunder and sounds like cannon firing are constantly heard.
The first major explosive eruption documented took place from January to about June 1917. Ash was deposited as much as 40 - 50 km away, and killed the formerly luxuriant vegetation on the lower slopes of the mountain; it has not recovered fully to the present day. A deep summit crater replaced the flat lava platform. Another explosive eruption may have occurred for several months in 1926.
The first eruption of which there is a fairly complete record took place between July and December 1940. This eruption was identified by three major phases; (1) a preliminary stage of small explosions that discharged old material from the vent, (2) a Plinian-phase during which great quantities of gas were discharged, preceded by violent explosions that ejected blocks and bombs, (3) a phase during which mainly ash was ejected. At the close of this eruption there was a deep funnel-like hole in the summit crater and the slopes of the volcano were blanketed with white ash. Ejecta of carbonatitic composition were distributed over 100 km from the volcano, destroying grazing land and contaminating water holes.
From 1958 to 1966, the activity was characterised by the effusion of small-volume lava flows and spatter onto the floor of the crater from a succession of small transient cones, vents and lava pools.
In mid-August 1966 a violent explosive eruption resumed. A thick column of black ash was rising for approximately 4 km above the volcano. A series of violent eruptions continued over the following months. In December 1966 the eruption ended.
A major explosive eruption was reported on 8 - 9th July 1967. Ash fell at Arusha (110 km southeast) and in Nairobi, 190 km to the northeast. After this the volcano seemed to have remained inactive for several years. A new, deep pit occupied the northern part of the main crater
From January to March 1983 a number of small ash eruptions occurred; resulting in a slight fall of fine grey ash at Olduvai Gorge, 65 km to the west. Since then the activity in form of minor eruptions of highly fluid carbonatite lava from numerous small cones and lava pools has resumed. The crater has been progressively filled with lava flows and spatter cones. By early 1984 the pit crater was almost full and the level of the crater floor reached the base of the upper ash slopes between July and October 1984. The crater floor has continued to raise as more lava flows formed, and so the crater walls were becoming progressively lower. In late 1998 lava began to overflow the low points on the crater rim in both the north-westerly and the easterly direction.
Radiating cracks cross north and west crater floor, some of them are extending up through the crater wall. The cracks range from a few centimetres up to 1 metre in width and some of them are several metres deep, emitting sulphurous gases. These radiating cracks may indicate that the crater floor is bulging upwards as a large amount of magma rises slowly towards the surface. When it reaches the surface, as in 1966, an episode of powerful explosive activity will occur that will restore the crater to the steep-sided form that it had prior to 1983. It can happen in the relatively near future.
Carbonatites are unusual igneous rocks that contain more than 50% carbonate minerals. Their origin has been the subject of considerable controversy. Until 1985, some observers described the substance erupted as bubbling black mud! Despite its unique carbonatite composition, the lavas behave like fluid basalt, discharging both aa and pahoehoe flows onto the crater floor. Newly solidified lava is black and contains crystals that sparkle brightly in the sun, but any contact with moisture rapidly turns the lava grey and white. The rapid change in colour takes place because new minerals are formed as water from the atmosphere reacts with the sodium and potassium carbonates.
The magma that is currently being erupted at Oldoinyo Lengai is very unusual for two main reasons. Firstly, it is an extrusive carbonatite. Secondly, the rock at Oldoinyo Lengai contains a high proportion of alkalis, dominated by sodium carbonate, which is why it is called natrocarbonatite. Analyses of the carbonatite lava show that it contains less than 1.2% silica, about 32% carbon dioxide, and about 55% calcium, sodium and potassium oxides. Natrocarbonatite is made up largely of two minerals, nyerereite and gregoryite. These minerals are both carbonates in which sodium and potassium are present in significant quantities. Both of these minerals are anhydrous (i.e they contain no water) and when they come into contact with the moisture of the atmosphere, they begin to react extremely quickly.
