The potassium-argon dating method is applied to dating potassium feldspar in rocks. But it is important to keep in mind just exactly what the resulting numbers mean. For each example, indicate which geologic process or event is being dated:

Question

• A potassium-argon date of 2.3 million years on potassium feldspar in a volcanic rock
• A potassium-argon date of 535 million years on potassium feldspar in a metamorphosed volcanic rock
• A potassium-argon date of 164 million years on potassium feldspar in a clastic sedimentary rock
What will be an ideal response?

Answer 1

A K-Ar date of 2.3 million years on potassium feldspar in a volcanic rock: The date reveals that a volcanic eruption occurred 2.3 million years ago to form this rock.
A K-Ar date of 535 million years on potassium feldspar in a metamorphosed volcanic rock: The date reveals that metamorphism occurred in this region 535 million years ago, and that volcanic activity occurred at some earlier time.
A K-Ar date of 164 million years on potassium feldspar in a clastic sedimentary rock: The date reveals that the sedimentary rock was deposited sometime no earlier than 164 million years ago and may have been deposited much later than 164 million years. The igneous or metamorphic rocks in the source area where the clastic sediment eroded from formed 164 million years ago.