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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.
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).
To me it has been a real eye opener to see all the processes that are taking place and their potential influence on radiometric dating.
Radiometric dating is largely done on rock that has formed from solidified lava.
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.