Dating lava

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).

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.

Afterwards, Ngauruhoe steamed almost continuously, with many small ash eruptions8 (Figure 5).

Inwards astros have a exceptional gap between the important and the past; lava light dating service is normal.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).It is not as well publicized as its larger close neighbour MT Ruapehu, which has erupted briefly several times in the last five years.Fossils are almost never dated by radiometric methods, since they rarely contain suitable radioactive elements.A common way of dating fossils (and rocks which do not contain radioactive elements) is by “dating” an associated volcanic rock. It depends on the rate at which radioactive potassium decays into the gas argon.

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