Over the past three weeks, Lengai has quietened down significantly. Observers report a complete pause in activity lasting about two weeks, and then resuming of relatively small ash eruptions. A thermal anomaly is still visibile on satellite date in the crater of the ash cone.

Lengai's activity has been continuing to increase. Over the past days, several ash eruptions have produced plumes up to more than 10km height, the largest one possibly being on 4 March rising to about 14 km, as evident from satellite data, observations by passing aircraft and spectacular photos taken from the ground posted on Fred Belton's website.
Possibly, some of the eruptions have also produced pyroclastic flows travelling part way down the steel flanks of the mountain. It should be noted that visiting Lengai, even its lower flanks must be considered EXTREMELY dangerous. There are no safe observation spots nearby, particularly as the topography of the mountain puts the surrounding flat areas into a high risk zone of pyroclastic flows.

The VAAC reported an ash plume from Ol Doinyo Lengai was rising to an altitude of 11.6 km at 15 February.
 

The activity at Lengai seems to be increasing. In the past two weeks, explosions have ejected ash plumes rising several kilometers. On 15 Feb., Dutch pilots observed and photographed an eruption plume rising to estimated 12 km (36,000 ft).

Typical ash eruption from the new ash cone in the N crater

A small group from VolcanoDiscovery, local mountain guides and partners stayed near and on Lengai volcano during 17-21 January. During this period, Lengai continued to erupt ash to several 100 meters above the new ash cone during phases lasting several hours alternating with periods of quiet when only a weak plume of very fine gray ash and gas was issuing out of the new ash cone.
A detailed report of our observations can be found here.

Based on information from the Tanzania MVO, the Toulouse VAAC reported that Ol Doinyo Lengai erupted on 7 November and remained active. According to Frederick Belton's website, an observer saw a "smoke" plume rise to an altitude of 3.2-3.5 km (10,500-11,500 ft) a.s.l. and drift WSW towards the Gol Mountains on 10 November.

New photos from Ol Doinyo Lengai by Tim Leach about three weeks old.

Photo by Thomas Holden

Lengai volcano as seen from the north-east on 27 Oct 07: thick plumes of ash rise from the active crater and recent ash and lava deposits can be seen on the flank.

Recent photos of Ol Doiny Lengai by pilots of Martinair

Recent photos of Ol Doinyo Lengai eruptions in September 2007 by MartinAir-Pilot Jelle Schouten and his Co-Pilot Stan Brouwer you can find now at the "Volcano Photo Gallery" at this link: Air Photos of Ol Doinyo Lengai

As more evidence about the large eruption at Lengai volcano on 4-5 Sep is pieced together, it appears that the eruption was probably a mainly explosive event while evidence of significant lava flows on the flanks becomes less. The observed "lava flows" could have been bush fires ignited by hot ejecta. In any case, there is little doubt that the observed ash cloud was the result of a significant explosive eruption and not just landslides on the steep flank.

In the meanwhile, the mountain has calmed down and only weak thermal anomalies are being detected on the summit. It would be imprudent to say that the eruption is over. If this is indeed the start of a major explosive cycle, more eruptions could follow in the next weeks and months. Field investigations are under way.

  

Satellite image of the eruption at Lengai volcano (image: NASA/GSFC/METI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team)

A large eruption seems to be taking place at Lengai volcano, this time for real: On 4 Sept., reports started coming in that a large (natrocarbonatite?) lava flow is descending the West flank. A considerable ash plume was visible on satellite data.

Over 30 thermal anomalies have been detected by the MODIS team since August 23 - more than during the large eruption in March 2006. On 4 and 5 Sep, the thermal anomaly at the summit was extremely strong. From this and satellite imaginery, it seems that there was a short overflow to the East and a major overflow to the West starting on September 1st (it could be a bush fire on the volcano flank ignited by lava). New overflows on 5 Sep seem to be taking place on the W and NW flanks.

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Source: Matthieu Kervyn De Meerendre, Iniversity of Gent (Belgiim)

As more news has become available, the news about an effusive flank eruption at Lengai around 15 July turns out to be probably false. The observed "ashcloud" was dust from a landslide. This demonstrates how difficult it is in this area to have robust information. The ongoing seismic activity in the Gregory Rift, however, is quite peculiar.

After strong earthquakes (6 Richter) a new eruption at the volcano Ol Doinyo Lengai started and lava is flowing at its flanks. Many villagers have been spending nights in the cold and are terrified after many houses were distroyed by the earthquake.
 
About 3,000 villagers were displaced last year after another volcanic eruption from the mountain.
 

Thomas, our correspondant in Arusha climbed Lengai last week and reports indirect signs of activity near the western rim of the crater, at the place where lava overflowed in the March 2006 eruption.
This is his report:
"Hi Tom, Just back from Lengai late last night. It was very cloudy up there…couldn’t see anything. Looks to me like a lot of activity has happened ently. The central crater is growing, and there looks to be lots of recent flow over the western rim where the big eruption spilled in March ’06. The largest hornito on the eastern edge of the central crater is collapsing into the crater. It was too cloudy to get too close, and crevasses in the surface around the central crater looked too daunting to get near. On the very western rim near the place where the lava is spilling over the edge is a new cone. I could hear the lava swishing around inside but couldn’t get close enough to look inside. We sat down on the place where the lava had obviously just flowed a short time ...

Glow from nearby (but hidden) lava pooling somewhere beneath the collapse pit in Lengai's crater (taken on evening of 31 Jan 07).

During our latest expedition to the active crater of Lengai volcano from early 31 Jan - 2 February 2007, no eruption of natrocarbonatite lava occured from the summit vents.
According to local Maassai guides, the central area of the crater with the large collapse pit near the tall hornito T49b had not seen significant changes since last summer.
From an open vent in the NE corner at the bottom of the pit at the base of the hornito, loud washing-machine like sounds of boiling lava was audible all the time. A large pool of lava must be found somewhere at very shallow depths in some caverns beneath that area. This assumption is confirmed by the glow of lava that was clearly visible one night from a second, smaller vent located about 30 m south of the large vent in the base of the collapse pit. One guide confirmed he had seen spattering of lava from this vent some two weeks earlier.
In addition to the loud sound of revolving lava underground, a constant, deep rumbling could be heard from...

More detailed information about the recent activity at Lengai has emerged: there was probably no explosive eruption as reported earlier, but one or two very large effusive paroxysms: the first eruption occurred on 29 March, the second probably on 3 April. The eruptions produced large-volume natrocarbonatite lava flows that overflowed the crater to the West near the climbing route, travelled for more than 1 km length through a small canyon before stopping less than two km from a Maassai village at the base of the mountain.
Buring vegetation ignited by the lava might have produced smoke that had been mistaken for ash. No evacuations have taken place and no one has left the area, different from early news reports.

The activity at Lengai volcano, that started around 30 March is still enigmatic. According to information compiled by our colleague Fred Belton, a lava flow and lava fountains were emitted on the 30th, but there were no ash emissions. A steam plume was visible that may have been mistakenly described as ash in news reports. According to an observer in Tanzania, on 4 April a very large lava flow was visible on the volcano's W flank. The lava flow was over 1 km long and had traveled down the flank of the volcano and into a gorge. There were no signs that the flow was still hot. Photographs revealed that another lava flow may have traveled W of the volcano on 3 or 4 April. On 7or 8 April, active lava was contained within a new lava lake (?) at the summit. There are contradictory reports about evacuations evacuations from villages near the volcano.

There has been a possibly major summit eruption on Lengai volcano on the 31st of March 2006, involving ash fall and lava flows from the summit, according to the following news report, posted on IPPMedi