How do you date a New Zealand volcano?
Among impressive volcanic scenery in northern New Zealand lies the city of Auckland. The district is known for its volcanic cones. In fact, there are more than 50 recognized small volcanoes in the city and surrounding areas. But the largest volcano by far in Auckland is also the youngest. It is called Rangitoto. How young is this youngest volcano? Now your problem starts. Rangitoto is generally regarded as young for several reasons. Evidence based on botany and geomorphology, and a hint from Maori legend that the name can mean ‘red sky’, contribute to a common acceptance that Rangitoto is youthful. Some of the lavas (scoria) have no vegetation, and seem to be no more than a few hundred years old.
In the late 1960s, scientists from the Australian National University in Canberra dated numerous volcanoes in Auckland using the potassium-argon method.1 Ten samples from both vegetated and unvegetated lava on Rangitoto were dated. Results seemed to show that Rangitoto was not a few hundred years old as it appeared to be. Ages from the 10 samples ranged from 146,000 years up to almost half a million years! So how old is Rangitoto? A couple of hundred years? Or half a million? The scientists took a sample of wood from beneath some Rangitoto lava and dated it by the carbon- 14 method.2 The wood gave an age of only 225 years (plus or minus 110 years)—which potentially puts it in the lifetime of George Washington and German composer Johann Sebastian Bach. This is about the age all evidence points to except potassium-argon dating. If lava which is little more than 200 years old can be wrongly dated at up to 465,000 years by the potassium-argon method, could potassium-argon dating always be wrong?
Wrong every time
The scientists who did the Rangitoto tests dated 16 volcanoes in all. Eleven of these were able to be compared with carbon-14 dates. In every case the potassium-argon dates were clearly wrong to a huge extent. Similar conflict was found by researchers in Hawaii. A lava flow which is known to have taken place in 1800-1801—less than 200 years ago—was dated by potassium-argon as being 2,960 million years old.3 If the real dates were not fairly well established by other means, who could have proved that the potassium-argon dates were so wrong? So how do you date a volcano? The lesson seems to be that how ever you date it, don’t count on the potassium-argon method.
- Ian McDougall, H. A. Polach and J.J. Stipp, ‘Excess radiogenic argon in young subaerial basalts from the Auckland volcanic field, New Zealand’, Geochimica et Cosmochimica Acta, Vol.33. 1969, pp. 1485-1520. Return to text.
- Carbon-14 dating is regarded by creationists as reasonably reliable for recent objects. For explanation see The Answers Book, pp.43-SO. (See ad p.40 this issue.) Return to text.
- J. G. Punkhouser and J.J. Naughton, ‘He and Ar in ultramafic inclusions’, Journal of Geophysical Research, Vol.73,1968, pp. 4601-4607. Return to text.
Many geological layers have actually been assigned vast radiometric ages on the basis of potassium-argon dating of volcanic intrusions into the layers. Well-known fossil hunter Richard Leakey’s Skull 1470 was ‘dated’ by the same method used on surrounding volcanic material.