What is used to date the oldest rocks on Earth?

• A. Zircons
• B. Uraninite
• C. Fossils
• D. Chlorite

Answer: (A)

The ability to date Earth’s oldest rocks comes from radiometric dating with zircon crystals. Zircon forms early in crustal rocks and is incredibly durable, often surviving intense metamorphism and weathering that destroy other minerals. When a zircon crystallizes, it takes in uranium but rejects daughter lead, so the radioactive decay of uranium to lead over time creates a reliable clock. By measuring the ratios of uranium to lead in a single zircon grain, you can determine when the crystal formed, which marks the crystallization age of the host rock. Some zircons found in ancient crustal material date to around 4.4 billion years, providing a minimum limit for the age of Earth’s crust. Fossils can’t date the oldest rocks because they only tell you the age of the sediment they’re found in, not when the rock itself crystallized. Chlorite is used to infer metamorphic conditions rather than to establish an absolute age. Uraninite can date rocks, but it’s less common and more susceptible to alteration on Earth, making zircons the most reliable source for the oldest-rock ages.