Matahina Dam Ignimbrite

About 320 thousand years ago this area was inundated with pumaceous pyroclastic ash flows from a huge eruption of the Ōkataina Volcanic Complex in the Taupō Volcanic Zone (TVZ). The eruption emitted about 120 cubic kilometres of volcanic material in three eruptive pulses which took place over the course of 40 to 120 days. The temperature of the pyroclastic flow when it erupted would have ranged from 620 to 680 degrees Celsius. The same ignimbrite which is exposed around Lake Matahina can also be seen along the southern side of Lake Rotomā where there are large exposed bluffs. The thickness of the ignimbrite ranges from 5 to 200 metres from as far south as Ruapehu to as far north as White Island, covering an area of roughly 2000 square km.

From the lookout point on the eastern side of the dam look across the lake to the exposed bluffs. Note how thick the ignimbrite is here, perhaps 20 - 25 km from the location of the eruption. This gives some idea of the total volume of material erupted. The vertical lines (joints) you can see were formed due to the extremely hot rhyolitic flow cooling and shrinking. This has resulted in the pillar like formations you can see exposed around the lake.