Our most recent report on Ol Doinyo Lengai discussed observations from several climbing groups and pilots during April 2008 through January 2009 (BGVN 34:05). This report reflects observations made during 11-12 June 2009 (figures 129-133) by a scientific team composed of Maarten deMoor (University of New Mexico), David Hilton and Peter Barry (Scripps Institution of Oceanography, UCSD), Fredrick Mangasini (University of Dar es Salaam), Carlos Ramirez (University of Costa Rica), and Tobias Fischer (University of New Mexico).

Figure 129. The ash cone at Ol Doinyo Lengai (approximately 30 m high) is located in the pre-September 2007 crater area. The first explosive eruption of the ash cone occurred on 10 September 2007. The currently actively degassing crater is on the left of this picture to the E. Photo taken looking S on 11-12 June 2009. Courtesy of T. Fischer.

Figure 130. View of active crater at Ol Doinyo Lengai, looking N on 11-12 June 2009. The crater is approximately 80 m deep. Note the collapsed hornitos on the crater floor. Courtesy of T. Fischer.

Figure 131. Active carbonatite lava lake in the NW part of the Ol Doinyo Lengai crater on 11-12 June 2009. Photo looking NW. The lava was convecting vigorously with occasional spill-over during the observations. Courtesy of T. Fischer.

Figure 132. Installation of the plume sampling equipment on 11 June at the edge of the Ol Doinyo Lengai crater; photo looking E. The equipment was used to collect aerosols and to sample sulfur dioxide, hydrogen sulfide, and carbon dioxide in the plume. Courtesy of T. Fischer.

Figure 133. Stratigraphic section of the S crater at Ol Doinyo Lengai, showing the complete sequence of the 2007-2008 explosive eruptions; photo looking W. The total thickness was 84 cm, including layers consisting of ash and lapilli. Courtesy of T. Fischer.

The team stated that on 11-12 June 2009, "carbonatite is currently erupting from hornitos and a lava lake [sits] at the bottom of the new (September 2007) crater" (figure 131). These conditions demonstrated both the establishment of a lava lake, less explosive activity, and more passive lava emissions on the crater floor. Comparison of satellite imagery from July 2004 (figure 134) and September 2009 (figure 135) provided by the NASA Earth Observatory website, showed the summit changes caused by the switch from generally effusive eruptions to a series of explosions beginning in 2007 (BGVN 32:11, 33:02). In September 2007, explosive eruptions began. sending ash thousands of meters into the air. Ash also covered the surrounding landscape, forcing local residents to flee with their livestock. Explosive eruptions continued into 2008, building a ring of fragmental material over 100 m high on the edge of the N crater. In satellite imagery acquired 12 September 2009 by the Advanced Land Imager (figure 135), the new cone and its deep concentric crater are clearly visible.

Figure 134. Satellite image acquired 16 July 2004 shows Ol Doinyo Lengai's summit after a long period of effusive eruptions. Beginning in 1983, lava began to fill the crater of an ash cone that formed during explosive eruptions in 1966-1967. Over time, the lava filled the crater and created a large flat platform. Dark areas on the crater floor are recent lava flows (days to weeks old), while the beige and white regions are older lava that have reacted with rain and moisture in the atmosphere. In 1998, lava began to spill over the rim of the crater to the north and east. These lava flows are visible as beige fingers radiating down the sides of the mountain. Dark green vegetation covers the upper slopes. Image is from the Advanced Land Imager (ALI) aboard the Earth Observing-1 satellite; courtesy of NASA Earth Observatory.

Figure 135. In this satellite image of Ol Doinyo Lengai, acquired 12 September 2009, the new cone and crater (essentially a ring-shaped structure) are clearly visible in the center of the image. The dark spot in the crater may be fresh lava erupted from a new volcanic vent. Gray ash covers the volcano and much of the surrounding landscape. Image is from the the Advanced Land Imager; courtesy of NASA Earth Observatory.

Reports on Fredrick Belton's website described the following visits during the summer of 2009. Table 23 is a continuation of the one in BGVN 35:05. A final statement on the website notes that, although activity appeared to have returned in August 2009 to the typical gentle eruptions of fluid natrocarbonatite lava, no samples of the new flows have been obtained for analysis due to their inaccessibility deep inside the steep-walled crater. Therefore, it remains uncertain how compositionally similar the new inaccessible lavas are compared to those produced prior to the 2007-2008 eruption.

Table 23. Summary of selected observations of Ol Doinyo Lengai (from a climb, aerial overflight, flank, or satellite) during June-August 2009. Courtesy of Frederick Belton.

Ben Wilhelmi, a commercial pilot working in the region, sent us some recent aerial photographs taken on 12 September 2009. We present two of those photos featuring overviews of the summit complex (figures 136 and 137).

Figure 136. Aerial photo of Ol Doinyo Lengai looking approximately SE. The active N crater and the ring of fragmental material deposited by the 2007-2008 eruptions is in the left center of the image. The summit appears to the right. Taken 12 September 2009 and provided courtesy of Ben Wilhelmi.

Figure 137. Aerial photo of Ol Doinyo Lengai looking approximately NNE. The active N crater is in the distance (to the left of the photo's center). Compared to the N crater, the quiet S crater is larger and more pan-shaped. The sharp peak of the summit is prominent between the two craters. Taken 12 September 2009; courtesy of Ben Wilhelmi.

Geological Summary. The symmetrical Ol Doinyo Lengai is the only volcano known to have erupted carbonatite tephras and lavas in historical time. The prominent stratovolcano, known to the Maasai as "The Mountain of God," rises abruptly above the broad plain south of Lake Natron in the Gregory Rift Valley. The cone-building stage ended about 15,000 years ago and was followed by periodic ejection of natrocarbonatitic and nephelinite tephra during the Holocene. Historical eruptions have consisted of smaller tephra ejections and emission of numerous natrocarbonatitic lava flows on the floor of the summit crater and occasionally down the upper flanks. The depth and morphology of the northern crater have changed dramatically during the course of historical eruptions, ranging from steep crater walls about 200 m deep in the mid-20th century to shallow platforms mostly filling the crater. Long-term lava effusion in the summit crater beginning in 1983 had by the turn of the century mostly filled the northern crater; by late 1998 lava had begun overflowing the crater rim.

Information Contacts: Tobias Fischer, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA; Frederick Belton, Developmental Studies Department, PO Box 16, Middle Tennessee State University, Murfreesboro, TN 37132, USA (URL: http://oldoinyolengai.pbworks.com/); NASA Earth Observatory (URL: http://earthobservatory.nasa.gov/); Ben Wilhelmi (URL: http://benwilhelmi.typepad.com/benwilhelmi/).