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Oldoinyo Lengai field mission December 2007
Jörg Keller and Jurgis Klaudius
Mineralogisch-geochemisches Institut
Albertstr. 23B
D-79104 Freiburg
joerg.keller@xxxxxxxxxxxxxxxxxxxxxx
jurgis.klaudius@xxxxxxxxxxxxxxxxxxxxxx
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Jörg Keller and Jurgis Klaudius stayed in the Natron – Oldoinyo Lengai
area from Dec. 5th (arrival) to Dec. 11th (departure).
Purpose was to observe the type of eruptive behavior of the ongoing
activity, to characterize the composition of the new material involved
and the stratified sampling of the ashes of the Sept. 4th paroxysm and
later ash eruptions.
With the explosive eruption of September 4th Oldoinyo Lengai
terminated an about 25 years long activity dominated by the effusion
of highly fluid natrocarbonatite lavas within the summit crater. The
deep pit crater from the 1966/67 explosive eruptive period had become
gradually filled by about 1999/2000. The last days of August 2007 were
characterized by seemingly increased lava output (Chris Weber
reporting on Belton 2007). A natrocarbonatite lava of August 23rd,
collected by C. Weber, was analyzed by J. Keller at Freiburg
University and was close to the average or standard composition for
natrocarbonatite from the last 20 years. Roger Mitchell and Barry
Dawson collected fresh ash, which fell on Sept. 24th.
Although the eruptive activity and changes of the crater morphology
were documented by photographs from the air (Fig.1) and from the base
of the volcano, very few visits to the summit are recorded in the
whole period since Sept. 4th. It appears that, to our knowledge, only
Tim Leach from Ngare Sero Mountain Lodge/ Ngare Sero Lake Natron Camp
could visit the summit with his Maasai guides and helpers in exploring
a new route from the south or east.
Record of the activity early December:
The intermittent explosive activity during the period of our stay,
with ash plumes increasing in height to up to several thousand meters
above the volcano alternating with periods with minor puffing or
degassing or with seemingly dormant phases up to several days long,
was obviously rather representative for the two-month period following
the 4th September paroxysm.
Oldoinyo Lengai is sending on and off its ash plumes high into the
atmosphere. There are entire days when the volcano appears calm,
resuming then at surprise with impressive explosions and ash plumes.
So happened when Jörg Keller was just leaving the north crater towards
the southern depression (after sampling in the ring plain and on the
slopes of the intra-crater ash-and-cinder cone).
An impressive bomb field with impacted blocks of up to one meter in
diameter extends along the crater rim, on the eastern ridge to the
summit and on the flank down into the south crater. This bomb field is
a strong expression of the intensity of the explosions from the
intra-crater vent. Given the witnessed suddenness of the onset of
explosive activity this is a clear indication of the possible dangers
in the summit area.
Fumarolic activity in the northern crater is strong in particular
along the northern rim. Also fumaroles are seen within the upper part
of the north flank.
Chronology of the activity:
5.12. pm: white steam cloud with light ash puffs
6.12. am: relatively quiet
6.12. pm: small ash eruptions
7.12. night time/early morning: explosion shocks felt during ascent
from the East side
7.12. 11:07 sudden and loud onset of heavy, dark, material-laden
explosion plumes (2000 m above summit) without any signs of warning.
Continuous feeding pulses 20 seconds. Lasting until 12:45. (Fig. 2,
3, 4)
7.12. 17:45ff dark vertical ash columns with lightning drifting
west. Ash-fall on the western slopes of Oldoinyo Lengai and on the
escarpment of the Gregory Rift. At about 2500 m above the crater the
ash plume hits the base of the clouds and develops into a white,
obviously condensation dominated vapor plume. (Fig. 5)
8.12.-11.12. Quiescence ("seemingly dormant")
11. 12. 7:30ff prominent ash plumes developed, rising in continuous
feeding pulses of 5-10 seconds to about 3000 m above the summit and
drifting NW. This continued for several hours, until Oldoinyo Lengai
came out of sight on return to Engaruka at 10:30. (Fig. 6)
Distribution of the 4th September ashes:
Profiles of the 4th September ashes have been identified, documented
and sampled in a number of places. Proximal accumulations occur in the
southern depression ("south crater"). Thickness of the 4th September
succession is here about 20 cm on the south side of the southern
depression and on the upper slopes of the south flank. It decreases to
1 cm at the eastern starting point of the new trail. This compares
with a thickness of about 5 cm in our profiles at the upper parking
site of the old western trail and the nearby abandoned Boma of the
Lesele family (Fig. 7), which was the Maasai boma closest to the
volcano in a horizontal distance of 4.2 km.
Towards Engare Sero village, relics of the ash fall are still locally
preserved and indicate an original thickness of about 1 cm. This
thickness is consistent with the eyewitness reports of ash fall over
the village during 4th of September.
Ascent route:
Access to the summit of Oldoinyo Lengai has become rather difficult
since the paroxysm of Sept. 4th. Tim Leach from Ngare Sero Lodge
explored and opened, together with local Maasai, a new route from the
east-southeast. We used this route with the help of the Maasai guides
Saringe, Tumba and Lekoko. The route follows a prominent steep ridge
from the east or slightly southeast ending at the south-eastern edge
of the south crater. The track is quite strenuous and takes much
longer than the old trail from the west, although being rather direct.
Our party took seven hours. However, with ongoing explosive activity,
the south crater is the only safe arrival place.
For a possibly easier descent, it was attempted to use the old western
route, but this idea had to be given up because the very cemented
surface of the lapilli beds provides no grip on the steep entrance
from above to the ascent chasm. Really dangerous!
Crater situation and composition of the recent magma(s) involved in
the explosive eruptions:
The summit was reached at 7 am on the 7th of December after seven
hours of climbing. During the stay in the summit area for about 5
hours Jörg Keller collected from the active cone fresh samples of
black lapilli, ash and bombs often with cores.
The large intra-crater cone, which is visible on photographs from
overflights (in particular from Benoit Wilhelmi, forwarded by Matthieu
Kervin, and by Maxime Le Goff) was inspected and sampled. This
cinder-and-ash cone (Fig. 8) now occupies more than half of the former
crater platform, with a crater diameter of about 200 m. Its location
coincides with the large collapse structure formed during the
March/April 2006 natrocarbonatite activity, which has also been the
area of strong lava emission prior to the explosive eruption of 4th
September. It has a slightly N-S elongated, oval shape and despite the
heavy fumes filling the crater conveyed the impression that 2 vents, a
more northerly and a more southerly are erupting. The cone is formed
by and covered by black to brown lapilli, cinders, angular blocks,
cored oval bombs and ashes. The magmatic lapilli show the macroscopic
phenocryst phases of nepheline, garnet and wollastonite. With time,
the black lapilli and bombs on the slopes of the cone and in the ring
plain around it are turning white by incrustations of its components.
Products of the active cone have covered almost all the old
natrocarbonatite structures. Only the spiny remnant of the T49B
hornito still stands out at the northern crater rim of the cone, and
the surface of a blocky flow is still recognizable at the foot of the
northern wall of Oldoinyo Lengai summit.
The magmatic material collected seems to confirm the suspicion,
derived from the earlier explosive events as in 1966/67 and backed by
the recent observations of R. Mitchell and B. Dawson, that with the
onset of the explosive eruptive period on September 4th the silicate
component of the Lengai evolution becomes involved in its eruptive
activity. Microscopic inspection and analyses of the material will
follow immediately in the New Year.
Acknowledgements:
We want to express our appreciation and compliments for the logistic
support of Tim and Mike Leach's Ngare Sero Lodge organisation. As
obvious from our description the climb to the active crater would have
hardly been successful without this support.
Roger Mitchell and Barry Dawson are thanked for the cooperative and
friendly exchange of observations from their stay from 23rd September
to 1st October. Our samples from the summit can hopefully add to the
petrogenetic model, which they elaborated on the 24 Sept. ash-fall.
Photographs by Benoit Wilhelmi, forwarded by Matthieu Kervin and Maxim
Le Goff have been of great value for the planning of our expedition.
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