What happens to the crust when erosion removes load?

• A. Crust rebounds and margins adjust.
• B. Plates detach.
• C. Crust sinks with erosion.
• D. Magma forms.

Answer: (A)

Removing surface load from the crust reduces the downward pressure that keeps it pressed into the mantle. The crust floats on the more ductile mantle like a buoyant shell, so when weight is taken away by erosion, it rises to reestablish balance with the mantle. This rebound causes uplift and the margins of the crust to adjust to the new isostatic equilibrium. Timescales can be long, from thousands to millions of years, as the mantle slowly flows to re-balance the system. The other ideas don’t fit because erosion doesn’t cause plates to detach, the crust wouldn’t sink when load is removed (it rises instead), and magma formation isn’t a direct result of removing surface load.