The volcano lies next to one of the big fault scarps that define the western wall of the Rift Valley. These faults were probably formed about 1.2 million years ago (Dawson 1992) and volcanic activity in the area occurred both before and after this faulting. Among the older volcanoes in this area are Ngorongoro, Ketumbeine, Gelai, Shombole, Mosonik and Kerimasi. Oldoinyo Lengai is less than 0.37 million (370,000) years old, and is the youngest big volcano in this part of the Rift Valley. These volcanoes are composed of alkaline rocks, relatively low in silica and rich in compounds of sodium and potassium.

Oldoinyo Lengai was formed by a complex sequence of events, including explosive eruptions of tuffs and agglomerates and effusive eruptions of lava. Over most of the cone the rocks are not well exposed, but on its eastern slope a large valley provides a partial cross-section; see the photographs below. The left hand photograph was taken by Celia Nyamweru in December 1988. It shows the eastern chasm partly in shadow; the route up the eastern slope runs to the right of the three triangular peaks in the centre of the image. The right hand photograph was taken by Celia Nyamweru from low on the eastern slope of the cone; the eastern chasm is in full sunlight and the white patch on the upper right hand side of the cone is weathered ash from the 1966-67 eruption.

J.B. Dawson mapped the volcano in 1960 (Dawson 1962) and established the following sequence, from oldest to youngest:

Yellow tuffs and agglomerates with interbedded lavas. These make up the main bulk of the volcano, the result of many episodes of explosive activity. The tuffs are made of crystals of nepheline and pyroxene, set in a fine-grained yellow matrix of zeolite, limonite and carbonate. The lava flows within the pyroclasts are composed of nephelinite and phonolite. These rocks have been correlated with rocks exposed in the Olduvai Gorge succession which range in age from about 0.15 to 0.4 Ma (Dawson et. al 1995).

Grey tuffs and agglomerates make up parasitic cones and craters on the outer slopes of Oldoinyo Lengai. They contain blocks of the older yellow agglomerates. Some are lithic tuffs, composed of small lapilli of nephelinite lava; others are crystal tuffs composed mainly of mica, pyroxene, nepheline and olivine crystals.

Black tuffs and agglomerates were laid down on a deeply eroded surface of the yellow pyroclasts. They occur on the lower slopes of the mountain and high on its western and north-western slopes. They include crystal tuffs (nepheline, pyroxene and mica), lithic tuffs (lapilli of nephelinite, ijolite and fenite) and agglomerates, with blocks of nephelinite, phonolite, urtite, ijolite and other alkaline rocks. These tuffs correlate with an ash in the Olduvai succession which is dated at about 1250 years. The earliest evidence for natrocarbonatite lava is found within these tuffs.

Melanephelinite extrusives (lava flows and parasitic cones) are found at three localities on the outer slopes of the volcano; see photograph to the right, taken by D.A. Kreulen on 14 May 1975.

Grey semi-indurated tuffs, up to about one metre thick, consisting of nephelinite lapilli and mica plates cemented by carbonate. These tuffs overlie the stunted remains of trees and are tentatively assigned to the eruption in 1917 that killed off most of the forest on the outer slopes of Oldoinyo Lengai.

Poorly consolidated black ash overlies the grey tuffs on the north, west and south slopes, and has also been observed at Olduvai Gorge, 45 miles to the west-southwest. The ash has formed spectacular barchan dunes; see the photograph on the left, taken by Celia Nyamweru in 1972. This was probably formed during the eruption in 1940/41.

Variegated (light green, pale yellow, pink and white) deposits of soda ash, up to 20 feet thick on the southern wall of the summit crater. These may have formed in 1954/55.

Recent natrocarbonatite lavas of the northern crater floor.

To learn about the eruptions of 1917, 1940/41, 1954/55 and others earlier this century, click here.

To learn about the most recent eruptions on the northern crater floor, click here.

For references to J.B. Dawson's work on Oldoinyo Lengai, click here.