Which dating method is typically used to date very old igneous and metamorphic rocks?

• A. Rubidium-Strontium
• B. Potassium-Argon
• C. Uranium-Lead
• D. Carbon-14

Answer: (C)

Uranium-Lead dating is used to date very old igneous and metamorphic rocks because uranium isotopes decay slowly enough to remain present over billions of years, producing lead at a predictable rate. In minerals like zircon, uranium is incorporated during crystallization while lead is largely excluded, effectively locking in the formation age. Since there are two uranium-to-lead decay chains (U-238 to Pb-206 and U-235 to Pb-207), you have two independent clocks that should agree, providing a robust age even if some mineral parts experience disturbance. The high closure temperature of this system in zircons also helps preserve the isotopic signature through metamorphic events, making it ideal for ancient rocks.

Other methods exist, but they’re less reliable for the oldest rocks in different situations. Potassium-Argon dating is great for many volcanic rocks but can be reset by heating events. Rubidium-Strontium dating can also date old rocks but is more susceptible to alteration and requires careful mineral selection. Carbon-14 is only useful for recent materials and organic matter, not for dating rocks directly, especially those billions of years old.