Isostasy is the concept that explains how the Earth's crust floats on the denser, semi-fluid mantle beneath it. This balance occurs because the crust is less dense than the mantle, allowing it to rise or sink depending on the weight of the material above it. For example, when glaciers form, they add weight to the crust, causing it to sink slightly. When the glaciers melt, the crust begins to rise again, a process known as glacial rebound. Isostasy helps to maintain equilibrium in the Earth's surface, influencing landforms and geological processes over time.