Peter Litwin, asks :-

Can you find the age of rocks by using radiocarbon dating or are they generally too old? If a rock was shot from a volcano and isn't that old, can we use radiocarbon dating?

Tom Higham, a radiochemist at Waikato University's Radiocarbon Dating Laboratory, and who receives many questions via the web page, responded.

Samples of rock are not able to be dated using radiocarbon, because rocks contain no organic carbon from living organisms that are of recent enough age. Most rocks formed hundreds of thousands if not millions of years ago. Geologic deposits of coal and lignite formed from the compressed remains of plants contain no remaining radiocarbon so they cannot be dated (radiocarbon dating is limited to the period 0-60 000 years, because the 'half-life' of radiocarbon is about 5700 years).

To date rocks scientists must therefore use other methods. We can date volcanic rocks using a method called argon-argon dating for instance. This method uses principles of isotopic decay like radiocarbon, but different isotopes (argon-39 and argon 40) which have a longer halflife (1250 million years). This means scientists can date rock which is many millions of years old (the method is used to date the oldest rocks on Earth, about 4.5 billion years old). The technique can date materials the size of one grain of volcanic ash, using a laser.

There are other methods which can be used as well which operate using different radiochemistries, such as Fission Track dating, which is being used to date the history of movement in the Alpine fault in the South Island.

The only way to date a volcanic ash layer using radiocarbon dating is to find ash within a lake sediment or peat layer and then date the organic carbon from above and below it, and therefore fix an age for the ash event. This is a commonly used approach to date volcanic events over the past 60,000 years, both here in New Zealand and around the world.