A French team spent three days at the summit during 4-6 April 1996. Abundant gas was emitted from three radial fractures oriented E-W and less gas came from three other fractures oriented NE-SW. Numerous fumaroles were present on the N crater rim. Hornito T34 (as numbered in BGVN 20:11/12) released large amounts of gas. Significant changes in the crater morphology were noted since the December 1995 description. Three hornitos in the N part of the crater (T8, T14, T15) had disappeared under lava flows from T37. Only the top 50 cm of T8 was exposed, but a chimney 30 m deep was observed below. In the S part of the crater, hornitos T27 and T30 were notably modified.

Magmatic activity was observed from three hornitos. The spatter cone T37 was hooded but open to the WSW. Inside, a 10-m-diameter lava pond exploded every 2 seconds, throwing lava 2-5 m high. As the group arrived at 1200 on 4 April the T37 lava pond overflowed, producing a lava flow that had a pahoehoe surface near the pond, and changed to an aa texture at a distance. Numerous such lava flows occurred until 0730 on 5 April; they traveled ~150 m SSW towards T23 and T30. The lava pond level then dropped, leaving solid lava "stalactites" on the walls. Explosions continued, but no further lava emission was observed through 1300 on 6 April. Because of sloshing lava sounds, another lava pond was thought to exist inside another hornito (between T5/T9 and T37), but it remained invisible. Hornito T36D became active early on 6 April, ejecting small lava fountains to an average height of 2 m. During one 2-minute episode there were 20-30 ejections. Episodes (up to 5 minutes long) were separated by repose periods of 15-20 minutes. Noisy gas emissions followed each episode. Activity increased near 1300 with continuous lava emission during each episode.

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.

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: Jacques-Marie Bardintzeff, Lab. Petrographie-Volcanologie, bat 504, Universite Paris-Sud, 91405 Orsay, France.