Geochronology — Dating the Earth’s Oldest Rocks The age of the most ancient geological formations is estimated by which method?

Introduction / Context:
Geologists use radiometric dating to determine the absolute ages of rocks. Different isotopic systems work over different timescales. The question targets the method suitable for dating the most ancient (billions of years old) formations, such as crustal zircons and ancient igneous bodies.

Given Data / Assumptions:

• We must choose among several radiometric methods.
• Target materials include very old minerals (e.g., zircon) and rocks.
• We need a method that remains reliable over billions of years and resists alteration.

Concept / Approach:
Uranium–lead (U–Pb) dating uses the decay of U-238 to Pb-206 and U-235 to Pb-207. Zircon (ZrSiO4) incorporates uranium but rejects lead during crystallization. Any lead present later is radiogenic, making U–Pb in zircon one of the most robust geochronometers for the Archean and Hadean eons. By contrast, radiocarbon (C-14) is limited to recent organic remains (< 50,000 years). Potassium–argon is useful for volcanic rocks but is generally less precise than U–Pb for the very oldest terrestrial materials.

Step-by-Step Solution:

Verification / Alternative check:
Oldest known zircons (~4.3–4.4 billion years) are dated by U–Pb concordia methods, confirming its suitability for the most ancient materials.

Why Other Options Are Wrong:

• Ra–Si method: Not a standard geochronometer.
• Potassium–argon method: Useful but not the premier choice for the very oldest zircons.
• C14 method: Time window is only tens of thousands of years.

Common Pitfalls:
Assuming any radiometric method works for all ages. Always match the half-life and mineral system to the age range and rock type.

Final Answer:
Uranium - lead method