What method should a geologist use to determine the exact age of a volcanic ash layer?

• A. Radiometric dating
• B. Relative dating
• C. Dendrochronology
• D. Astronomical dating

Answer: (A)

Radiometric dating is used to determine an actual numeric age by measuring the ratio of a parent isotope to its decay product in materials formed at the time of the event. For volcanic ash, minerals or glass within the ash can be dated with methods like potassium-argon or argon-argon dating, which lock in the time when the melt crystallized and the eruption occurred. This provides a specific age in years with stated uncertainties.

Relative dating, by contrast, places events in order but doesn’t give an exact age. Dendrochronology relies on preserved wood and tree-ring sequences, not the ash itself, so it doesn’t always apply. Astronomical dating isn’t a standard tool for dating volcanic ash layers.