Why does the geological record contain fewer older rocks than younger rocks?

• A. Older rocks have been destroyed by geological processes such as plate tectonics and erosion
• B. Old rocks are more resistant to weathering
• C. Older rocks never formed in the first place
• D. Younger rocks are more likely to be meteorite-hosted

Answer: (A)

Rocks are continually recycled through the rock cycle, so the geological record shows fewer very old rocks than younger ones. Exposed rocks are worn away by weathering and erosion, and their material is carried off as sediment. Much of the crust is also subducted back into the mantle, where it melts and ceases to exist as solid rock at the surface for long times. Even rocks that aren’t subducted can be buried and metamorphosed, effectively erasing their original form. Only in a few very stable regions do ancient rocks persist, so the overall pattern is an abundance of younger rocks and a scarcity of ancient ones. The other ideas don’t fit because old rocks aren’t intrinsically more resistant to weathering, they did form, and meteorite-related processes don’t drive the global pattern.