Standing roughly in the centre of New Zealand’s North Island, Mt Ngauruhoe is New Zealand’s newest volcano and one of the most active (Figures 1 and 2).
Of course, no geologist was present to test this assumption by observing ancient lavas when they cooled, but we can study modern lava flows. The samples were sent progressively in batches to Geochron Laboratories in Cambridge, Boston (USA), for whole-rock potassium-argon (K–Ar) dating—first a piece of one sample from each flow, then a piece of the second sample from each flow after the first set of results was received, and finally, a piece of the third sample from the 30 June 1954 flow.15 To also test the consistency of results within samples, second pieces of two of the 30 June 1954 lava samples were also sent for analysis. No specific location or expected age information was supplied to the laboratory.
Blocks weighing up to 1,000 tonnes were hurled 100 m (330 feet).
However, the most violent explosions occurred on 19 February 1975, accompanied by what eye-witnesses described as atmospheric shock waves.9 Blocks up to 30 m (100 ft) across were catapulted up to 3 km (almost 2 miles). Turbulent avalanches of ash and blocks swept down Ngauruhoe’s sides at about 60 km (35 miles) per hour.10 It is estimated that at least 3.4 million cubic metres (120 million cubic feet) of ash and blocks were ejected in 7 hours.11up If any of these assumptions are violated, then the technique fails and any “dates” are false.
The K–Ar method works on the assumption that the “clock” begins to “tick” the moment that the rock hardens.
That is, it assumes that no argon derived by radioactive decay was present initially, but after the lava cooled and solidified, the argon from radioactive decay was unable to escape and started to accumulate.
Argon gas, brought up from deep inside the earth within the molten rock, was already present in the lavas when they cooled.