Lava bubbled from a new vent (T20) [but see 17:6] in the center of the crater floor (figure 24) . . . on 12 March. A very recent lava flow (F33) extended from the new vent across the S depression, and was still cracking, suggesting that it had formed the previous day. The new vent was in roughly the same position as former vent T18 (observed in June-October 1991, but not in December 1991, perhaps after being covered by flow F32; 16:8 and 17:02), and may represent a renewal of activity from that vent. The largest of the remaining cones (T5/T9), estimated at 20 m high, had heat waves emanating from its vents. None of the other cones showed visible signs of activity

Figure 24. Sketch map of the summit craters of Ol Doinyo Lengai, 12 March 1992. Prepared by M. Peterson, with modifications from C. Nyamweru. Sketch from an oblique airphoto taken 24 July 1992, looking N across Ol Doinyo Lengai's crater. Fresh lava is shown emerging from hornito T20. The former feature T11 is no longer visible. Courtesy of F. LeGuern.

Geological Summary. The symmetrical Ol Doinyo Lengai is the only volcano known to have erupted carbonatite tephras and lavas in historical time. The prominent stratovolcano, known to the Maasai as "The Mountain of God," rises abruptly above the broad plain south of Lake Natron in the Gregory Rift Valley. The cone-building stage ended about 15,000 years ago and was followed by periodic ejection of natrocarbonatitic and nephelinite tephra during the Holocene. Historical eruptions have consisted of smaller tephra ejections and emission of numerous natrocarbonatitic lava flows on the floor of the summit crater and occasionally down the upper flanks. The depth and morphology of the northern crater have changed dramatically during the course of historical eruptions, ranging from steep crater walls about 200 m deep in the mid-20th century to shallow platforms mostly filling the crater. Long-term lava effusion in the summit crater beginning in 1983 had by the turn of the century mostly filled the northern crater; by late 1998 lava had begun overflowing the crater rim.

Information Contacts: C. Nyamweru, St. Lawrence Univ; M. Peterson, Arusha.