Argon is actually used to determine the age. It's called potassium-argon dating b/c when volcano erupts the rocks in it contain potassium. The potassium slowly decays and produces argon. Scientists measure how much argon is in a given volcanic rock and do math backwards to figure out how many potassium atoms were needed to produce the given amount of argon. We already know the rate at which potassium decays so then they put two and two together to find the age.
