According to Frederick Belton's website, visitors whom climbed Ol Doinyo Lengai on 18 June reported that the new active cone covered the former crater floor entirely except for an area N of the summit. The new cone's W, N, and E sides stood about 30 m above the rim of the former crater and enclosed a deep crater. The visitors saw a few small vents on the crater's floor. Frequent emissions of ash-poor plumes originated from the SW part of the crater's floor, producing light ashfall. They heard continuous loud rumbling noises, occasional gas-jetting sounds, and rockfalls.

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.

Source: Ol Doinyo Lengai (Fred Belton)