The following report on August and September activity . . . is from sources (in parentheses) who have climbed the volcano several times in the past few years.

Between 2 and 4 August a group visited the floor of the N crater and noted major morphologic changes since they last saw it in December 1985 and May 1986. They estimated that new lava flows had raised the crater floor ~40 m. The saddle that separates the larger, active part of the N crater from its small S corner or shelf (figure 3) was only ~7-8 m high . . . . Highly liquid, very hot, jet-black lava splashed out from the single active vent in the S central crater floor (1/3 of the way across the crater from the S edge), and formed small thin flows. The group estimated that ~25% of the crater floor had been covered by liquid lava during the 24 hours preceding their visit. Two inactive vents were observed closer to the N side of the crater floor in addition to many fumaroles. However, the party did not see any large holes in the crater floor or hear any of the rumbling noises that they noted in December 1985. (Bill Waldron)

During a visit 9-10 September 1987, lava bubbled up and flowed from one vent on the N side of the crater. A number of small inactive vents were noted on other parts of the crater floor. An observer who paced the crater floor estimated the dimensions to be 275 x 260 m and confirmed that the slope from the S saddle down into the N crater floor was on the order of 7 m high. (Thad Peterson)

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, Kenyatta Univ.