Ol Doinyo Lengai volcano, altitude 2886 meters (9524 feet), is a unique and extremely fascinating volcano that towers above the East African Rift Valley in Northern Tanzania, just south of Lake Natron.  It is the only volcano in the world that sometimes erupts natrocarbonatite lava, a highly fluid lava that contains almost no silicon. Natrocarbonatite lava is also much cooler than other lavas, being only about 950 degrees F (510 degrees C) compared to temperatures over 2000 degrees F (~1100 degrees C)  for basaltic lavas. Natrocarbonatite is the most fluid lava in the world.  Lava with a low gas content can flow like a whitewater stream, and actually has a viscosity near that of water. Natrocarbonatite lava glows orange at night, but is not nearly as bright as silicon-based lavas since it is not as hot.  During the day it is not incandescent; most flows look like very fluid black oil, or brown foam, depending on the gas content. In the past, some visitors to the crater believed they were seeing mud flows. Most newly solidified lava is black and contains crystals that sparkle brightly in the sun.  There are also sometimes small flows known as "squeeze-ups" that are light gray when they flow and harden.  Contact with moisture rapidly turns natrocarbonatite lava white because of chemical reactions that occur when the lava absorbs water.  Eventually the water absorption process turns lava flows into soft brown powder. During dry weather the whitening of flows happens over a period of a few days to a couple of weeks, depending on the thickness of the flow. In rainy weather the lava surface turns white immediately.  In parts of the crater that have been inactive for several months, the ground is light brown/white and so soft that one sinks into it when walking.

Ol Doinyo Lengai also has phases of explosive activity during which the composition of the lava may contain much more silicate material, reducing its viscosity.  In this type of eruption there is no fluid lava and activity takes the form of ash eruptions accompanied by ejection of blocks and bombs.  However, initial phases of an explosive eruption may include strong lava fountains.  The two most recent eruptions of this type occurred during 1966-1967 and 2007-2008.  As of June 2008, Ol Doinyo Lengai seems to be in the late stages of explosive activity.

Since the mid 1980's, lava flows, and to a much lesser extent, explosive eruptions, have been witnessed by many observers of Ol Doinyo Lengai.   Natrocarbonatite lava eruptions are usually centered in one or more small cones that have been formed on the crater floor by previous eruptions of lava. These eruptions have typically taken the form of open lava pools or lakes that may or may not be overflowing, lava flows issuing from holes or cracks inside or near the base of the cones, or lava splashes or fountains from the summit vents of the cones.  The activity is not well understood but is thought to be a function of the plumbing of the crater, the level of the lava within the plumbing, and the gas content of the lava.  No one knows what causes the lava to flow out at any particular time or how the various vents in the crater are interconnected.  Mineralogists would like to understand how the lava evolves under the surface and why it has its unusual chemical composition, but that also is unknown, although there are several theories.  Finally, no one has any idea why the pattern of frequent small lava flows gave way to explosive activity at the end of August 2007 and how that may have related to earthquakes in the region during the previous two months.

I have climbed about 50 active volcanoes in various parts of the world, but Ol Doinyo Lengai has captured my interest like no other volcano ever has.  As of 2008 I have climbed Ol Doinyo Lengai 12 times and have spent 104 nights camping at the summit craters.  During my first visit on July 17, 1997, I went up and down in one day and spent about 4 hours in the crater.  There was some very minor activity, but four hours is not long enough to have a very good chance of seeing an eruption.  I decided to spend several nights there to increase my chances of seeing some flowing lava, which is what I did twice in 1998 and once in 1999.  The more time I spent on Ol Doinyo Lengai, the more fascinated I became. In July 2000 and July 2001 I organized camping expeditions to the crater for small groups of clients.  In August 2002 my expedition, which included photographers, a film team, and a volcanologist, encountered hazardous camping conditions due to violent lava fountains and extremely rapid lava flows.  Part of our camp was destroyed by lava and a Tanzanian guide was injured.