 It has been partly glaciated and Ar-Ar geochronology shows that is it about 730,000 years old.It mainly comprises palagonitized tephra (tuff, lapilli tuff, tuff-breccia) but also has a few dykes and jointed lava flows on its flanks.The top still has flat-lying lava flows erupted after the tephra pile grew above the surface of the enclosing lake.