There are more than 50 small volcanoes in the city and surrounding areas of Auckland, New Zealand. But the largest volcano of these 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 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 had 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 roughly the age that all the evidence points to except potassium-argon dating.
If lava that 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?
The answer seems to be "yes." The scientists who did the Rangitoto tests dated 16 volcanoes in all. Eleven of these could be compared with carbon-14 dates. In every case the potassium-argon dates were clearly wrong to a huge extent.
Researchers found similar conflict in Hawaii. A lava flow that 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 had not been reasonably 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.