M5 Isostasy and Glacioisostatic Adjustment

Topic

Isostasy is the state of equilibrium maintained by the lithosphere: elevated topography is compensated at depth either by a less dense crustal root extending beneath that topography or by a lateral variation in density within the crust itself. Any disturbance to the load borne by the lithosphere—whether through ice accumulation or melting, erosion, sedimentation, or even flooding—displaces the lithosphere from its equilibrium, simultaneously triggering both an immediate elastic response and a much slower viscous response.

Glacio-isostatic adjustment is the process by which the lithosphere and mantle respond to the loading and unloading cycles imposed by ice sheets. During the last glaciation, approximately twenty-one thousand years ago, an ice mass up to three or four kilometers thick depressed the lithosphere several hundred meters below its equilibrium position.

Since deglaciation occurred—between twelve thousand and seven thousand years ago—isostatic rebound has uplifted Scandinavia by about three hundred meters; this rebound continues today at a rate of five to eight millimeters per year, progressively lowering the relative sea level in the region.

The response of this entire system involves two distinct characteristic timescales: the elastic response of the lithosphere, which occurs almost instantaneously on a geological timescale, and the much slower viscous relaxation of the mantle. The characteristic time for the latter depends on the viscosity of the mantle itself, the wavelength of the relaxing deformation, and the density contrast between the mantle and the removed load (whether ice or water). The expected behavior of this glacio-isostatic adjustment includes sustained uplift in areas formerly covered by ice and subsidence in the peripheral ring surrounding those glaciated zones—resulting from the collapse of the bulge that the ice itself had displaced outward while present—with rates that decay exponentially as the system returns to equilibrium, thereby closing the link between the initial load perturbation and the lithosphere's slow return to its prior isostatic state.

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