During a previously unreported 26 February climb by David Peterson, Howard Brown, and students from St. Lawrence Univ, activity was continuing from one cone (T20) . . . . Periodic gurgling and rumbling noises from the cone were audible from the crater rim. As Peterson and several students approached the active cone, lava fragments were ejected, one of which struck a student on the leg, causing a small burn. Crater photographs show a small dark vent at the summit of T20, but no dark (fresh) lava was evident on its flanks. However, by . . . 12 March, T20 had extruded a lava flow that covered much of the W part of the crater floor (17:03).

Brown's 26 February photographs show . . . T5/T9 as tall but pale gray, with no fresh, dark patches of lava. T15 was composed of jagged dark-gray pinnacles with medium-brown lower slopes and no sign of fresh lava. T8 and T8A seemed little changed from recent photographs, with slight yellow coloring at T8's summit. T14 appeared to have been surrounded by younger lava, which had turned pale gray to white. Some dark patches were visible around its summit vent. No dark fresh flows were evident on the crater floor.

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; D. Peterson, Arusha; H. Brown, Nairobi, Kenya.