Smithsonian Institution
Global Volcanism Network Bulletin v. 20, no. 10, October 1995

Ol Doinyo Lengai (Tanzania)  New hornitos and lava flows observed in July

Ol Doinyo Lengai
Northern Tanzania
2.75S, 35.90E; summit elev. 2,890 m
All times are local (= GMT + 3 hours)

Intermittent explosive activity and extrusion of
carbonititic lava on the crater floor began in January 1983
and continued for over ten years. Vigorous effusive and
explosive activity in June 1993, perhaps the strongest of
that eruptive episode, covered most of the crater floor and
upper W flank with fresh lava flows and deposited ash on the
flanks (Bulletin v. 18, nos. 7-10). In September 1994 a deep
central depression contained a hornito from which highly
vesicular brown lava was erupting (Bulletin v. 19, no. 9).

Activity observed in mid-July 1995 was the first reported
since September 1994, although the appearance of the recent
flows indicated that they were a few months old. Members of
the Societe de Volcanologie Geneve (SVG) visited the summit
on 15 July 1995. A visit to the summit crater was also made
by Celia Nyamweru on 19 July.

Activity on 15 July 1995. SVG observers reported a new
active hornito (T36), ~4 m high, close to the S foot of T20
(figure 2). Fluid carbonatitic lava flows were emitted from
its base through a channel in the direction of a rounded
collapsed new opening ~15 m in diamteer, close to T5/T9. The
lava in the channel was pale brown and frothy, with a
velocity estimated at 1.5 m/second; temperature was ~550
degrees C. At the end of the channel, the flow moved N
through different tubes. Lava breakouts from some downstream
openings were still very fluid and completely black. Both
small pahoehoe and aa lava fronts were observed. Ejecta were
rare from the summit vent of T36. The new lava field was
mainly directed N, with one branch passing W of T20 and the
other going through and filling the oval-shaped depression
first noted in October 1993 (see Bulletin v. 19, no. 4).

Activity on 19 July. At 1000 the crater was full of cloud,
hiding features on the crater floor, but frequent sharp
cracks, bangs, and thumps were heard, as well as bubbling
noises. Conditions improved so that activity could be
observed after 1115. White to brown steam was escaping
continuously from the top of T20, and a little from T5T9.
Sulfurous fumes were emitted from cracks on the E crater rim
and wall. The lower slopes of T23 were made up of many small
parallel pahoehoe flows, now soft and pale brown; T23 was
not emitting steam. The new cones, T34 to T37, lay W of the
depression that had been virtually filled by lava flows from
these centers. T34 was a double cone, pale gray, with an
open vent on its upper slope from which no steam or heat was
being emitted. T35 was light brown to white, with no sign of
fresh lava. T37 was a shallow circular crater W of and close
to the base of T5/T9; it appeared fresh but showed no
activity on 19 July.

T36 was a compound cone of which T36A was the largest
component; it was composed of cascades of pahoehoe lava,
some whitened and others black and very fresh. T36B was a
rounded dome with a small vent at its base from which lava
was emitted. T36C appeared to have a crack along its crest
that emitted gas-rich lava. T36B and T36C were ~5 m apart
and very close in elevation. Activity from hornito cluster
T36 (figure 3) consisted of clots of lava thrown ~1 m above
T36B, gas-rich lava escaping from the top of hornito T36C
and flowing down its N slope, and very fluid, black shiny
lava escaping from a small crack (T36E) on the lower slopes
of this feature and flowing N across very recent pahoehoe.
At 1137 a small spray of gas-rich lava escaped from hornito
T36D, on the W side of T36. Warm pahoehoe flows on the W
slope of T36,  < 100-m long, 2-m wide, and 4-cm thick, had
probably formed that morning.

Crater morphology. Features from June 1993 and earlier (see
map in Bulletin v. 19, no. 4) were still visible, but major
new cones had formed in the area between T5/T9, T20, and T23
(figure 2). T5/T9 remained a very prominent feature, and the
tops of the T8, T14, and T15 cones remained visible,
although all were surrounded by many younger lava flows.
T24, T26, and T30 were not inspected closely, but there
seemed to be no change in these large features in the S part
of the crater; they were gray and white, with no sign of
recent activity. West of T36 were two low lava domes with
pale brown open craters, now inactive. To the W of them, on
the edge of F34, was a low wide feature, possibly a
collapsed cone, probably the features identified as T22,
T31, and T32 in September 1993 (Bulletin v. 18, no. 9).
There was also a rather new hornito in this area.

Recent pahoehoe flows ~10 cm thick had reached the base of
the E, N, and NW walls. Crater walls appeared lowest to the
NW. The rugged F34 and F35 lava flows of June 1993 were
heavily weathered and beginning to soften and crumble. They
were quite dark gray; a great contrast to the flows that had
formed over the last several months (thin pahoehoe flows
that whiten within a few weeks of eruption). No recent ash
was observed on the outer slopes of the cone, the crater
rim, or the inner walls; the vegetation was green and
healthly. Brown vegetation was observed in a few areas near
the base of the inner wall, probably due to contact with hot
lava reaching the wall, and on part of the S wall below the
summit.

This symmetrical stratovolcano in the African Rift Valley
rises abruptly above the plain S of Lake Natron. It is the
only volcano known to have erupted carbonatite tephra and
lavas in historical time. The cone-building stage of Ol
Doinyo Lengai ended about 15,000 years ago and was followed
by periodic Holocene ejections. Historical eruptions have
consisted of smaller tephra ejections and emission of
numerous natrocarbonatite lava flows on the floor of the
summit crater.

Information Contacts: Celia Nyamweru, Department of
Anthropology, St. Lawrence University, Canton NY 13617, USA;
M. Vigny and P. Vetsch, Societe de Volcanologie Geneve, B.P.
298, CH-1225 Ch nebourg, Switzerland.