Jan 2005: Bernhard Donth reported, that during his visit on 10 January 2005 hornito T49B erupted in many effusive lava flows escaping first time over the northern edge of the crater. 

Feb 2005: On 3 February 2005 Chris Weber and others spend 5 days on O. Lengai. The hornito T49B was as well active at that time with lava flows travelling to the north. Pahoehoe lava flows in motion (little Levées) at flat terrain were measured from 520 degrees celsius up to a maximum of 561 degrees celsius. The fumaroles at F1 had a temperature of 84 degrees celsius and at T46 hornito a maximum of 91 degrees celsius. No change in distance was measured at CR1-3 cracks cutting the upper crater walls. The hornito T58C had grown to approximately 2870 m altitude. A Spitting Cobra was seen close to the summit of O. Lengai.

May 2005: Martin Haigh climbed Lengai on 15 May.  “Our guide told us that the last lava flows were in March, so we didn't see any of the glowing, orange stuff.  On the crater rim path up to the summit, my guide pointed out fresh leopard paw prints.” The photo below, courtesy of Martin Haigh, shows the central crater on 15 May.

July 2005: (The following report is courtesy of Bernard Marty)  A Tanzanian-French-American scientific team composed of Pete Burnard (CRPG-Nancy, France), Tobias Fisher (Univ. New Mexico, USA), Chamba Makene  (Geological Survey of Tanzania), Fredrick Mangazini (Univ. Dar el Salaam), Bernard Marty and Fabien Palhol (both at CRPG-Nancy, France) visited the crater to conduct gas sampling from 3 - 5  July,  2005.

At the time of arrival (08h00 local time, 03/07/2005, sporadic lava flow of the aa type was emitted from a vent situated at the base of T56B, which was itself disrupted by an explosion, leaving an open cavity of about 15 m in diameter. The lava was flowing eastward towards the eastern overflow, but never reached the crater rim. At approx. 11h30, lava was observed spilling out violently of the eastern fractured lip edge of T58B and flowing towards the eastern overflow. Sampling had to be interrupted at T46 due to the risk of a sudden lava flood in the sampling area, a real possibility if the thin western wall of T58B fractured following thermal erosion of the wall by molten lava in the lava lake. One hundred meters south of T46, close to the base of T47, a deep hole approx. 1.5 m dia, boarded by lava splashes and piercing a pahoehoe lava flow was emitting high temperature gases with no visible steam.  A lava pond, not directly observed but for which bubble explosions were clearly visible, was discharging surges of lavas towards the eastern rim while the adjacent T58C cone, now higher than T58B, was discharging high velocity gases  which from time to time splashed lava. The eruption lasted the whole day and the following night, with variations from steady state outpouring with lower degassing to burstles of large bubbles with enhanced lava emission. By the afternoon the lava, which was now overflowing the eastern slope of the volcano, ignited a bush fire. The lava outflow was estimated to be approx. 0.3 m³/s, with a speed of approx. 2 m/s in the flat area towards the volcano flank.

The camp originally proposed by the Masai porters was to be close to T58B, S-SE of the active vents at the foot of the southern wall.  This part of the crater was protected from the wind by some rockfalls and the cook wanted to avoid sand blowing into the meals. However, the emplacement was evaluated to be risky by the team and the tents were instead placed on the S-SW side of the crater; here, the camp was much further away from the active vents, was in the opposite direction to the lavas, was shielded from any potential lava flows by some rockfalls and also permitted direct access to the crater rim.  However, the cook established the "kitchen" in the original S-SE camp. On the evening of July 4th, a small lava flow went close to the natural entrance of the "kitchen". The Scientific Team moved the scientific equipment out of the "Kitchen" and urged the 5 Maasai remaining in the crater to move to the tent area, without success. At 5 AM in the morning of July 5th, lava flows suddenly invaded the "kitchen" area to a depth of over 1m; fortunately the Maasai sleeping there escaped safely, and even removed the food to safety. The breakfast was finally prepared directly on the 1 m thick lava that had just invaded the area that was proposed first as a camp. This episode which could have had dramatic consequences illustrates well the fact that there is no safe area within the northern crater. Camping should be limited to the southern crater or, as a bivouac, in elevated areas in the northern crater as far as possible from vents and directions of lava flows.

July – August  2005:  Fred Belton's expedition group, which included Josh Gordon, Adrienne Dickson, James Bell, Elizabeth Zbinden, local cook Isack Saractayi, local guide Paul Mongi, and 31 Maasai porters, arrived at the active North Crater of Ol Doinyo Lengai on the morning of 19 July, 2005. A camp was established in the inactive South Crater.  Many nights were spent by one or two observers at a bivouac tent on the SE crater rim.  On 26 July four members of the group descended, leaving Belton, Gordon, and  Dickson to remain for an additional two weeks, until 9 Aug, 2005.  For photos see 2005 Expedition.

Appearance of the crater: Activity during the past year has been confined to the central part of the crater. Since its initial violent eruption from a hole in the crater floor on 15 July 2004, T58C has grown to a height of ~12m and is now the second highest cone in the crater. It is a tall narrow cone and there are large blocks scattered about the crater floor below it which indicate that it has experienced some flank collapses in recent months.  T49B has grown significantly taller since July 2004 and is now the tallest feature in the crater, rising at least 15m above its base.  T56B, which is open to the SE as it was in July 2004, has grown a small cone on its SE flank.  The new cone, which has not been named because it is so high on the flank of T56B, is becoming undermined by a large tunnel. The tunnel begins at the SE base of the small unnamed cone and extends under it to the open vent of T56B.  Clearly the tunnel was a recent active feature and the tunnel entrance appears to be the result of collapse during or after an eruption. T58B contained a large and deep (10-12m) open pit which clearly indicated the presence of past lava lake activity at two levels.  The solid crust of a former lava lake about 4m below the rim contained a hole near its center that opened into a much deeper chamber.  Sometime during the night of  4 Aug  this deeper pit was filled by lava so that only the upper level remained.  T57B appeared the same as in July 2004 until its southern half was covered by a thick layer of fused spherical lapilli from a short but powerful eruption of T58C on 20-21 July.

Spatter cones T40, T46, T47, T51, T45, T37, T37B, and the ash cones have continued to be gradually covered by lava flows from the active cone group in the central crater.  In particular, T40, which was the primary active cone during much of 1999, is now well on its way to disappearing under the lava.  Lava flows from the central cone area have continued to build up the height of the mound in the central crater so that several cones are easily visible from Engare Sero village. T53 no longer exists.  A deep hole in the crater floor, just north of T47 and surrounded by recent pahoehoe flows from T58B, is possibly all that remains of the summit vent of T39.  The hole was degassing at a high temperature but did not show any evidence of recent effusive activity.  An area in the eastern part of the North Crater which is sheltered by outcroppings and sometimes used by campers had been covered by a thick flow of slabby pahoehoe lava from T58B. A large amount of vegitation on the E flank of Lengai had also been burned by lava flowing across the E Crater Rim Overflow. That activity was seen during 3-5 July 2005 by members of a Tanzanian-French-American expedition. (See July 3 – 5, 2005, above)  There are now several locations where small lava flows have crossed the north crater rim.  These new overflows were seen in the making by Bernhard Donth on 10 Jan 2005.

Measurements of crater rim overflows:

Activity: There was no activity on 19 July.  An unobserved eruption may have begun around 2300 on 20 July and probably ended by 0130 on 21 July, according to a camping group in the W part of the crater floor. The activity reached its peak between 0100 and 0130 and involved strong lava fountains from a vent about halfway up the E side of T58C. The fountains deposited a deep (at least 0.5 m) bed of lapilli around the base and on the south flank and top of cone T57B, and sent pahoehoe and aa flows toward the east. One of the flows traveled tens of meters down Lengai's eastern slopes. The lapilli consisted mostly of hollow spheroids with diameters up to 1.2 cm. Much of the lapilli was fused and sections of the field broke apart under its own weight, leaving a blocky, fissured surface. On 21 July at 1511 lava flowed from T58B's active vent during a 12 minute eruption. At 2100 on the same day a third eruption sent strong surges of lava from the same vent and continued throughout the night. Atmospheric cloud prevented detailed observations, but clear conditions at 0500 on 22 July revealed that a large lava channel had formed just east of T58B and had been thermally eroded to a depth of more than one meter.  Maximum flow rate during the eruption  was estimated to be 0.5 m³/sec. The eruption continued until about 1800 and deposited pahoehoe flows over a large part of the SE crater floor. With the exception of a very minor lava flow from T58C at 1930 on 26 July and the brief (unobserved) appearance of a lava lake in T58B on the night of 4 Aug, no further activity occurred through 9 August.  There were frequent sounds of lava at depth near the base of T58C and inside the open vent of T58B.  The lava lake that briefly occupied T58B filled up its deep inner pit and then solidified, leaving a flat surface of new lava about 4m below the lowest place on the cone's rim.  From 4 – 9 Aug lava could be heard moving near the surface somewhere inside T58B's vent, but it never became visible.  Observations ended at 0800 on 9 Aug.

Sept 2005:  Kees DeJong reports: "We climbed Oldonyo Lengai September 13, 2005, arriving at the crater rim 7.10 am. Tourists that had camped in the crater said that eruptions began  at midnight (and that there were no eruptions the previous days). Lava kept flowing until about 8.45 am; we left the crater at 1 pm that day." Photos that Kees made indicated that lava was flowing out from about halfway up the side of T48B and across the crater floor toward the west, but was not a particularly large eruption.  Earlier, probably on Sep 11, a small amount of lava flowed down the north flank of T49B and a short distance across the crater floor.  Other photos indicate (by comparing specific lava flows on the crater floor with the same lava flows in photos from Aug 7) a high probability that there were no lava flows at all on Lengai from Aug 9 to Sep 11.  On 19 Sept Burra Gadiye reported to Roger Mitchell that he had seen no activity that day.  Anatoly Zaytsev climbed on 30 Sept and reported: "Unfortunately, we did not see eruption during our visit (between 11 am and 3 pm, 30 September) - we have seen some natrocarbonatite lavas with age of two-three weeks." These are probably the flows from 13 Sep described above.

 Oct 2005: Jaco de Borst reports: "We have visited the volcano on 1, 2, and 3 Oct. On the first of October there was also no activity, only smoke from several cones and cracks. On the second of October there were several "small" eruptions. In daylight and night (very beautiful by the way), The cone which was erupting was the biggest and close to the overflow where the trail reaches the crater. The lava only reached the foot of the cone, (He is describing T49B.) We left in the morning of the third October, I think about 10.00 hour. When we left there was no activity, else than some noise and smoke. The night eruptions we saw was also on the 3 October, I think about 5 o'clock in the morning."

Dec 2005:  David Bygott climbed on 22 Dec and found the crater to be inactive.  A narrow flow of pahoehoe lava had recently emerged from the NW flank of T49B and flowed across the NW crater rim overflow.  David's photo of the flow, which was still warm and making some cracking noises, showed white deposits around cracks which indicate that it probably was active on Dec 21, possibly in the afternoon or evening.  Another photo indicates a wide pahoehoe-textured lava flow emerging from T56B toward the SE.  The lava in that flow has mostly turned white and appears to be several days to a week old.