Volcanic activity in the mountain caused daily earth tremors in Kenya and Tanzania from 12 July 2007 until 18 July 2007 at 8.30pm in Nairobi. The strongest tremor measured 6.0 on the Richter scale. Geologists suspected that the sudden increase of tremors was indicative of the movement of magma through the Ol Doinyo Lengai. The volcano erupted on 4 September 2007, sending a plume of ash and steam at least downwind and covering the north and west flanks in fresh lava flows.