Radiocarbon dating volcanoes
Eruptions from a central 400 m (1,300 foot) wide crater have constructed the cone’s steep (33°) outer slopes.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.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 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).It is not as well publicized as its larger close neighbour MT Ruapehu, which has erupted briefly several times in the last five years.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).How can we trust the use of this same “dating” on rocks whose ages we don’t know?
Afterwards, Ngauruhoe steamed almost continuously, with many small ash eruptions8 (Figure 5).We know the true ages of the rocks because they were observed to form less than 50 years ago.