The symmetrical Ol Doinyo Lengai stratovolcano is the only volcano known to have erupted carbonatite tephras and lavas in historical time. The prominent volcano, 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 of the volcano 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 eruptions 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.

Ol Doinyo Lengai Volcano has the coolest lava of any volcano. It erupts at less than 600 deg C. Most eruptions at the volcano are small. Larger than normal eruptions occurred in 1917 when vegetation was destroyed on the volcano and ashfall caused destruction of grazing land and death of herds of masai cattle. An eruption in 1940-41 lasted for six months and ash fell 100 km from the volcano.

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.

During July 2007 there were a series of shallow focus earthquakes around the volcano followed by a significant ash eruptions in September and October 2007.
Source:http://www.volcanodiscovery.com