Although no lava emission was observed during crater visits, the presence of new lava flows indicated continued activity through December. Photographs taken on 9 October by members of the St. Lawrence Univ Kenya Semester Program, guided by D., M., and T. Peterson, showed no significant changes from 13 August. The crater floor was pale brown and light gray, with no sign of fresh dark lava during the visit. Dark stains were visible on the upper part of cone T5/T9, suggestive of recent spatter, and a considerable amount of young lava (pale gray and pale brown) was apparent around the base of cone T8. A large flow (mid-gray, but with large white areas), possibly from a low dome W of the cones (T18), covered much of the W part of the crater floor, reaching the W wall.

On 7 December, John Gardner reported a large "black jagged" lava flow (F32) extending N-S across the crater floor. The lava was still warm to the touch, with steam being emitted from cracks in its surface, suggesting that the flow had formed within a few hours of Gardner's visit. Steam was reportedly emitted from the estimated 15-m-high cone T5/T9, from cracks in the lava on the crater floor, and from the E rim and E crater wall. Gardner also reported a cone . . . that might be a new feature.

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, M. Peterson, and T. Peterson, Arusha; J. Gardner, Nairobi, Kenya.