Isostasy

Isostasy, or isostatic equilibrium, describes the tendency of large regions of Earth's lithosphere to approach gravitational balance. The lithosphere—the rigid outer shell comprising the crust and uppermost mantle—rests over warmer mantle material that can deform and flow over geological timescales. The familiar image of crust 'floating' is an analogy: the mantle is solid rock, not a global ocean of liquid. In simple models, columns of rock above a compensation depth have approximately balanced masses or pressures, even when their surface heights differ. Real Earth structure and motion are more complex, so the idealized balance is not exact everywhere or at every time.

Two classic model families illustrate how topography may be compensated at depth. In the Airy model, crust has broadly similar density but varies in thickness: elevated terrain is supported by a deeper, relatively low-density crustal root, much like an iceberg with more of its mass below water. In the Pratt model, columns have a similar compensation depth but different average densities, with lower-density columns standing higher. These are simplifying end members; actual regions may combine variations in crustal thickness, rock density, temperature and strength. Isostasy therefore helps explain regional elevation, but it does not by itself account for every landform or replace tectonic processes.

Adding or removing mass can disturb isostatic balance. A thick ice sheet presses the underlying lithosphere downward and can raise a broad peripheral bulge. When ice melts, the depressed region begins to rise and the bulge may subside. This continuing vertical motion is called glacial isostatic adjustment, or postglacial rebound in common usage. Because mantle material responds slowly, adjustment continues long after the ice has retreated. Sediment accumulation, erosion, volcanic loads and changes in water or ice mass can also redistribute loads and contribute to vertical crustal motion.

Raised ancient shorelines and beaches, changing relative sea levels, and geodetic measurements of present-day vertical motion provide evidence of adjustment to past loading. Around Hudson Bay, the former weight of the Laurentide Ice Sheet depressed the land; as the ice melted, the land rebounded, leaving former beach ridges inland and above today's shore. Such ongoing motion matters for interpreting geological records and regional sea-level change. Isostatic effects are one part of the picture: tectonic deformation, sediment compaction, ocean-volume change and local subsidence can also influence land and relative sea level.