Question

which unstable element is used to determine the age of volcanic rock? argon carbon-12 potassium carbon-14

Explanation:

Potassium - argon dating is a common method for determining the age of volcanic rock. Potassium - 40 is an unstable isotope that decays to argon - 40 over time. Carbon - 14 is used for dating organic materials, carbon - 12 is a stable isotope, and argon is a decay product in potassium - argon dating but not the unstable element used for the initial dating process.

Answer:

potassium