Potassium argon dating range
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At the AMDEL Laboratory each sample was crushed and pulverized.Whole-rock analyses were undertaken by total fusion of each powdered sample and then digesting them before ICP-OES for major and minor elements, and ICP-MS for trace and rare earth elements.
Potassium-argon “dating” of five of these flows and deposits yielded K-Ar model “ages” from Ar* content.
Intermittent explosive eruptions and spectacular lava fountaining during June and July 1954 built a spatter-and-cinder cone around the south sub-crater, modifying the western summit of the mountain.
Activity decreased for two months after the last of the lava flows on September 26, but increased again during December 1954 and January 1955 with lava fountaining and many highly explosive pyroclastic (ash) eruptions.
1) and is up to 50 km wide in the central part, narrowing northward and southward. Ngauruhoe in the Taupo Volcanic Zone (TVZ), New Zealand, showing the main structural features.
This volcanotectonic depression (Taupo-Rotorua depression6) comprises four rhyolitic centers (Rotorua, Okataina, Maroa, and Taupo), plus the calc-alkaline Tongariro Volcanic Center, part of a young ( Fig. The shaded area is the andesite arc, and the inset shows the major components of the boundary between the Australian and Pacific Plates in the New Zealand region (arrows indicate relative motions).
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.
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.A set of representative pieces from each sample (approximately 100 g) was then dispatched to the AMDEL Laboratory in Adelaide, South Australia, for whole-rock major, minor, and trace element analyses.A second representative set (50–100 g from each sample) was sent progressively to Geochron Laboratories in Cambridge (Boston), Massachusetts, for whole-rock potassium-argon (K-Ar) dating—first a split from one sample from each flow, then a split from the second sample from each flow after the first set of results was received, and finally, the split from the third sample from the June 30, 1954, flow.The Ngauruhoe area was accessed from State Highway 47 via Mangateopopo Road. Care was taken to ensure correct identification of each lava flow and that the samples collected were representative of each flow and any variations in textures and phenocrysts in the lavas.From the parking area at the end of the road, the Mangateopopo Valley walking trail was followed to the base of the Ngauruhoe cone, from where the darker-colored recent lava flows were clearly visible and each one easily identified on the northwestern slopes against the lighter-colored older portions of the cone (fig. Eleven 2–3 kg samples were collected: two each from the February 11, 1949, June 4, 1954, and July 14, 1954, lava flows and from the February 19, 1975, avalanche deposits; and three from the June 30, 1954 lava flows. All samples were sent first for sectioning one thin section from each sample for petrographic analysis.It has been interpreted as oblique subduction of the Pacific plate beneath the Australian plate.