Only few effusive eruption periods on Lengai have been reported since end of March 2006 with an overflow to the west far down the slopes and the followed collaps of the central area of the crater. Evidence of lava flows have been given by remote sensing data (MODIS) analyzed by Matthieu Kervyn 20 June 2006. The volcanologist Steven Beresford has noted only some days old lava flows from hornito T37B on 13 June 2006. This activity has been confirmed by locals. Activity with surface lava flows have then returned on 20 June 2007 due to information of the local Lengai observer and guide Burra Ami Gadiye. Through July 2007 many tectonic earthquakes have been reported from the Rift Valley area around Lengai, some with epicenters as close as 20 km and only at 10 km depth. Christoph Weber has visited O. Lengai with an excursion group and noted as well many earthquakes between 18 and 24 August 2007. On midnight between 3 and 4 September 2007 activity has culminated with a very strong and gasrich eruption ("paroxysm"(1)) causing a some kilometer high eruption column and damage to the vegetation on the volcano slopes, documented on a ASTER satellite image on 4 September. Ash fall out of about 1 cm has been reported from the tourist facility Moivaro camp at the shore of Lake Natron. The access to the volcano has been closed for visitors by local authorities.
Visit of O. Lengai by Chris Weber 21 and 23August 2007.
With an excursion group Chris Weber started to climb O.Lengai with some local porters on 21 August early morning. With the knowledge of strong effusive eruptions inside the crater and because of clowdy and foggy conditions that day, the porters (some Masai and some of other tribes) were told not to enter the crater without a volcano guide. But some ignored the warning and crossed the crater by themselves, which caused a terrible accident at around 0800 that day. One local Masai fell into a running quite deep lava flow, more than 500 degrees celsius hot. Somehow he managed to get out of the lava, but leaving both legs and one arm of him completely burnt. Half an hour later Chirs Weber arrived with his group at the crater of O. Lengai. The well equipped group started first aid treatment and organized an effective and immediate rescue down the steep volcano slopes with the help of some tough men such as Burra Ami Gadiye and Othman Swalehe. After treatment in Arusha hospital financed by volcano Expedition Int., the Masai was on the way to recover mid of September, which is some kind of a wonder because of his severe burning injuries.
On 23 August Chris Weber and his group reached the crater of O. Lengai at 0815 again. Rough degassing natroncarbonatitic activity from several central vents took place this day. Lapilli and ash was even carried by the wind outside the crater onto the western climbing track. With very good weather conditions Chris Weber took an overview picure of the crater (see figure 1.) and a view from the the SE to the central area of hornitos (see figure 2.). This eruption period, which started on 20 June, had filled the collapsed central pit area completely with new lava. Some hornitos were grown up again at locations T40C, T52, T58, T57B and new located at T57C and all of those were active with lava degassing and being spitted out. Active lava lakes occurred futher more at the east side of the remaining fragments of the collapsed T49B hornito and at T49 location (see figure 3.). The most violent activity occurred on 23 August at T56 located lava lake with lava fountaining up to 40 m high and lapille thrown out even much higher. Lavaflows mainly to the western crater area accompanyed this activity. From 1200 until 1400 massive lava flows drained ouside the crater via the west overflow as far down the volcano slopes as 1500 m altitude.
from left: view of old T49B and the erupting lava lake T56; T56; west overflow
from left: lava lake T56 at day and night
Lava temperatur was measured with 516°C at a Pahoehoe lava flows in slow motion. Fumarol (F1) temperatures were measured with 82°C. Lava samples were taken and will be analysed by Prof. Jörg Keller, IMPG, University of Freiburg, Germany. Most noteable expansions of crater rim overflows are only to the north.
During the night from 3 to 4 September activity on O. Lengai culminated into some kind of "paroxysm"(1) with lapilli and ash eruptions about 3 km high. Due to observation pictures taken from a plane on 5 September, the morphology of the crater and hornitos had not changed dramatically. Satelite pictures showed vast areas of burned vegetation on the south, west and northwest slopes of O. Lengai. The burning at the south was caused by a bush fire which started already before 20 August (eyewitnessed by Chris Weber), while the burnt areas to the west and northwest have been caused by lava flows on the slopes of O. Lengai. This is confirmed by MODIS images for the date of 31 August and 1-2 Septermber 2007, analysed by Matthieu Kervyn.
Today it is uncertain, if this "paroxysm"(1) of acitivity and/or the unusual strong seismic activity at the African Rift Valley part may lead to a strong explosive eruption like 1966/1967.
Some of the information are provided by Fred Belton (see listed reports at his website), Celia Nyamweru, Matthieu Kervyn, Jurgis Klaudius and Prof. Joerg Keller.
(1) "paroxysm" definition as unusual strong culmination of an eruption period or notable strong single eruption, not to be missunderstood as an vulcania, plinian or ultra plinian eruption, though it could be one of those as well.