Over the second half-life, of the atoms remaining decay, which leaves of the original quantity, and so on.
Mt Ngauruhoe is thought to have been active for at least 2,500 years, with more than 70 eruptive periods since 1839, when European settlers first recorded a steam eruption.2 Of course, before that, the Maoris witnessed many eruptions from the mountain.
The first lava eruption seen by Europeans occurred in 1870.3 Then there were ash eruptions every few years until a major explosive eruption in April–May 1948, followed by lava flowing down the northwestern slopes in February 1949.4 These flows are still distinguishable today on the northwestern and western slopes of Ngauruhoe (Figure 4).
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).
Eruptions from a central 400 m (1,300 foot) wide crater have constructed the cone’s steep (33°) outer slopes.
Argon gas, brought up from deep inside the earth within the molten rock, was already present in the lavas when they cooled.
We know the true ages of the rocks because they were observed to form less than 50 years ago.
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).
Yet they yield “ages” up to 3.5 million years which are thus false.