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They turned to a much older dating method: dendrochronology, the use of tree rings for mapping intervals of time. (From ACT English)
Why in this case has it to be a colon, but not a comma? I think since "dendrochronology" is in apposition with "a much older dating," both punctuations should be acceptable.
One of the functions of a colon is to introduce something that answers or illustrates what precedes it.
I'll tell you how I knew that: I looked it up.
Then I realized what I needed: air.
Here's what I do: I play golf
deadrat's answer to a similar question may be helpful.
A colon is use to provide explanation or example of the sentence that precedes it.
In the above example, we are providing example of an older dating method. Therefore, a colon has been used, whereas a comma is use to list things or for connecting two complete sentences using a linking word.
Example:
I bought vegetables, beverages and fruits. (listing things)
I went to supermarket, and I bought vegetables, beverages and fruits.
(connecting two complete sentences)
for reference:
