15. refers to the idea that solid portions of Earth's crust float on a fluid layer.

A. Theory of isostasy

B. Fault block formation

C. Subduction

D. Orogenic processes

A. Theory of isostasy

Explanation:

Isostasy is a fundamental concept in Geography and Geology. It is the idea that the lighter crust must be floating on the denser underlying mantle. It is invoked to explain how different topographic heights can exists on the Earth's surface.

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