Isostasy is the concept that explains how the Earth's crust maintains balance or equilibrium on the more fluid asthenosphere beneath it. This balance occurs because the crust floats on the denser mantle, similar to how an iceberg floats on water. When weight is added or removed from the crust, such as through the accumulation of ice or erosion, the crust adjusts its position to maintain stability. The principle of isostasy helps to understand various geological processes, including mountain formation and the effects of glacial melting. As glaciers retreat, the land beneath them can slowly rise, a process known as glacial rebound, demonstrating the dynamic nature of Earth's crust.