The Theory of Isostasy

The theory of isostasy explains how Earth's crust floats on the denser, semi-fluid mantle beneath it. Imagine the crust as blocks floating in a liquid; just as icebergs float with more or less of their volume submerged depending on their density and weight, Earth's crust adjusts its position—thickening or thinning—so that the gravitational force is balanced. This balancing act causes higher landforms like mountains to have deep “roots” extending into the mantle, while lowlands have thinner crust. Isostasy maintains this equilibrium, allowing Earth's surface to adapt over time to changing loads such as erosion or sediment buildup.