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The potassium-argon (K–Ar) dating method is often used to date volcanic rocks (and by extension, nearby fossils). Eleven samples were collected from five recent lava flows during field work in January 1996—two each from the 11 February 1949, 4 June 1954, and 14 July 1954 flows and from the 19 February 1975 avalanche deposits, and three from the 30 June 1954 flow14 (Figure 6).
In using this method, it is assumed that there was no daughter radiogenic argon (Ar*) in rocks when they formed.13 For volcanic rocks which cool from molten lavas, this would seem to be a reasonable assumption. Inset: Andesite of the June 30, 1954 flow, Mt Ngauruhoe, seen at 60 times magnification under a geological microscope. The darker recent lavas were clearly visible and each one easily identified (with the aid of maps) on the northwestern slopes against the lighter-coloured older portions of the cone (Figures 4 and 7).
Because argon is a gas, it should escape to the atmosphere due to the intense heat of the lavas. All flows were typically made up of jumbled blocks of congealed lava, resulting in rough, jagged, clinkery surfaces (Figure 8).
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.
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When living things die, they stop taking in carbon-14, and the radioactive clock is "set"!
This same failure is also known to occur in many other rocks, including both recent volcanics18and ancient crustal rocks.19 The radioactive potassium-argon dating method has been demonstrated to fail on 1949, 1954, and 1975 lava flows at Mt Ngauruhoe, New Zealand, in spite of the quality of the laboratory’s K–Ar analytical work.
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The 18 August flow was more than 18 m (55 feet) thick and still warm almost a year after congealing.
Explosions of ash completed this long eruptive period.
However, Mt Ngauruhoe is an imposing, almost perfect cone that rises more than 1,000 metres (3,300 feet) above the surrounding landscape to an elevation of 2,291 m (7,500 feet) above sea level1 (Figure 3).