Predict what would happen to global sea levels if the oceans were the same size but the oldest ocean floor was 300 million years old (rather than 180 million years old). Sea level would be (lower/higher) than today.

• A. lower than today
• B. higher than today
• C. the same as today
• D. uncertain

Answer: (A)

The key idea is how ocean basin depth controls global sea level. Ocean depth grows as the oceanic lithosphere cools and contracts with age, making older seafloor denser and the basins deeper. If the oldest ocean floor were 300 million years old instead of 180 million, the average depth of the oceans would be greater. With the same total amount of seawater, filling a larger, deeper basin would require a lower surface level, so global sea level would be lower than today. It wouldn’t rise or stay the same, and the outcome is not uncertain given the cooling/contracting behavior of old oceanic crust.