The lake is a maar lake, formed by an explosion about 4,000 years ago, leaving cliffs of up to 90 m (300 ft) around the lake. The airfall eruptive volume was 0.7 km (0.17 cu mi) and covered an area with tephra of 70 km (27 sq mi) with a lava eruptive volume of 0.5 km (0.12 cu mi). While the local rocks are rhyolite and pumice, with areas of alluvial sand and silt the formative eruption is one of only two, in a locality dominated by rhyolitic eruptions, recent basaltic fissure eruptions (the other is the 1886 eruption of Mount Tarawera). Tephra from the eruption was dated at 3440 ± 70 years before 1950. The eruption had the characteristics of a basaltic dike as there is a line of eruptive vents almost at right angles to the usual vent alignments of the recently active Ōkataina Caldera. The maar crater is located almost exactly where this alignment intercepts a postulated continuation of the known fault defining the southern limit of the Tikitere Graben. The lake has one major inflow, the Waimata Stream, on the south side and also small geothermal springs on the north-west shore. The lake discharges via groundwater to the Waiohewa Stream, near Tikitere.