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Update submitted by Tobias Fischer, October 2015
As of July 2014, the top of Oldoinyo Lengai ash cone remains extremely steep and sloping at ~50° into the pit crater and ~60° to the other side. Only a very narrow (80 cm wide) path allows visitors to walk around the crater. The pit crater, formed in 2007, remains about 80 m deep with carbonatite erupting in the crater. The ash cone is made entirely of ash and bombs presumably from the 2007 and more recent (?) eruptions.
The activity observed in July 2014 is characterized by violent carbonatite lava fountaining inside the pit crater. The carbonatite fountaining is focused on a cone located on the north-western side of the pit crater. The active carbonatite cone is approximately 30 meters high. Carbonatite lava was observed ponding inside the cone and violently fountaining out of the cone. Fountaining activity is sporadic and lasted approximately 15 minutes with 10 minute quiescence intervals. Fountains up to about 20 m high were observed. Importantly, the carbonatite activity is eroding or dissolving the pit crater wall, exactly below the area that is first reached by climbers.