Ol Doinyo Langai is a volcano in Tanzania that erupts with natrocarbonatite lava, which is rich in calcium, sodium, and potassium, and low in silica. Because this volcano is low in silica the lava is cool. It alternates between effusive eruptions and explosive eruptions that build large cinder and ash cones.
For the effusive erutpions, the 1983 lava filled the crater of an ash cone that formed during an eruption in 1966. Since then the lava filled the crater. 
The dark areas on the previous picture show recent lava flows while the white color shows older lava flows that have reacted with rain and moisture.
For the explosive erutpions, in September 2007 an eruption threw ash thousands of meters into the air. Ash also covered the landscape which forced residents to leave. The eruption built a cone over 300 feet high with a steep walled crater, which can be seen in the following picture:
