What is isostasy and how does it explain mountain elevation?

• A. Isostasy is buoyant equilibrium between crust and mantle; thicker crust floats higher, and erosion causes crustal rebound and marginal adjustments.
• B. Isostasy is the idea that continents and oceans are at the same depth due to sea level changes.
• C. Isostasy says crust is fixed and cannot adjust to erosion.
• D. Isostasy only applies to ocean crust and not continents.

Answer: (A)

Isostasy is buoyant equilibrium between the crust and the mantle. Think of the crust as floating blocks in the underlying, viscous mantle. When the crust is thicker, it has a deeper root that extends into the mantle, and this deeper rooting allows a taller surface expression to be supported by the mantle. In other words, mountains stay high because their extra crustal mass is balanced by a deeper, buoyant lower portion.

Erosion changes the balance too. When the top of a mountain is worn away, the load on the crust decreases. To restore equilibrium, the crust subtly rises or uplifts—isostatic rebound—so the overall mass balance with the mantle is maintained. Over long timescales, this adjustment helps maintain or even enhance surface elevation as mountains respond to ongoing erosion and tectonic processes.

That description captures how buoyant balance and deep crustal roots explain high elevations, and why removal of surface material can trigger uplift. The other statements don’t fit: sea level changes don’t fix depth in isostatic terms, the crust isn’t rigidly fixed and cannot adjust, and isostasy applies to both continental and oceanic regions.