The Igwisi hills are formed by three tuff cones formed in the middle of the Tanzania craton. They are above the landscape with a karst morphology and craters covered with grass, on a low ridge that may be the product of early eruptive stages. The northeastern hill has two craters, one with a breach from which a long lava flow originates, probably formed when a lava lake in the crater escaped through a breach. The central volcano has a lava coulee and a tephra cone in its crater.:72, 73 Craters have diameters of . The total volume of these cones is less than . Weak pyroclastic activity probably accompanied the eruptive activity. Presumably, low intensity explosive activity built the cones, starting from the northeast cone and ending with the southwest cone. Afterwards, lava flows were generated.