Ol Doinyo Lengai is an extremely fascinating volcano: it is the only active volcano known to erupt carbonatite lava, a sensational discovery scientists made as recently as in the 1960s: the lavas it erupts are NOT melts based on silica, but on natroncarbonate!
Thus, the temperatures of these lavas are much lower, "only" about 600 deg. C., and Lengai's lava does not emit enough light to glow during day,- only at night, a dull reddish glow that does not illuminate anything is visible. Also because of its peculiar chemical composition, the lava is extremely fluid and behaves very much like water, with the exception that it is black like oil. After it is cooled down it quickly alters and becomes a whitish powder.
Geologically, the present-day cone of the volcano was constructed about 15,000 years ago. Historical eruptions have been moderate to small explosive events. In addition to its intermittent explosive activity at intervals of typically years or decades, numerous natrocarbonitite lava flows have been erupted from vents on the floor of the active summit crater.
The depth and morphology of the active (northern) crater have changed dramatically during the course of historical eruptions, ranging from steep craters 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 uptill today lava had begun overflowing the crater rim.