Potassium argon dating problems
Numerous pyroclastic avalanches were also generated by fallback from the continuous eruption column, the avalanches consisting of a turbulent mixture of ash, bombs, and larger blocks which rolled swiftly down Ngauruhoe’s sides at about 60 km per hour.6566 The deposits from these avalanches and the later explosions accumulated as sheets of debris in the valley at the base of the cone, but did not extend beyond 2 km from the summit. A plume of steam or gas is still often seen above the summit of the volcano, as powerful fumaroles in the bottom of the crater discharge hot gases.
The series of nine cannon-like, individual eruptions followed a 1.5 hour period of voluminous gas-streaming emission, which formed a convecting eruption plume between 11 km and 13 km high.6264 The explosions took place at 20–60 minute intervals for more than five hours.
The vent lineation lies above this axis, which is considered to mark a major basement fracture that allows the intrusion of andesite dikes.1214 The Tongariro volcanics unconformably overlie late Miocene marine siltstones beneath Hauhungatahi, and a minimum age for the onset of volcanism is measured by the influx of andesite pebbles in early Pleistocene conglomerates of the Wanganui Basin to the south.1617 The oldest dated lavas from the Tongariro massif are hornblende andesites exposed at Tama Lakes between Ngauruhoe and Ruapehu, at 0.26±0.003 Ma; from Ruapehu, 0.23±0.006 Ma; and from Kakoramea, 0.22±0.001 Ma (potassium-argon dates).18 Tongariro itself is a large volcanic massif that consists of at least twelve composite cones, the youngest and most active of which is Ngauruhoe.
A broad division has been made into older (20 There is a north-northeast alignment of the younger vents of Tongariro, particularly evident between Te Mari and Ngauruhoe.
3 shows the distribution of those 1954 lava flows that are still able to be distinguished on the northwestern and western slopes of Ngauruhoe.