Though the Earth often seems solid and fixed, it is not. You’ve probably heard of continental drift—the horizontal movement of continent-sized bodies of rock—but fewer of you may appreciate that the earth can move vertically as well. Studies have shown that North America and Europe are rebounding, slowly but steadily, due to the removal of thick ice sheets which once covered them during the last ice age, which ended about 21,000 years ago.
This process of postglacial upward movement is called glacial isostatic adjustment (GIA). Researchers have established that some materials have a viscous response when a surface load is placed on them, flowing like slow-moving honey, and remaining deformed when the load is removed; others have an elastic response, stretching like rubber and bouncing back to their original form. The substances that compose the upper sections of the earth are somewhere between these extremes, and have what is termed a viscoelastic response. As a result, when a mass of an icesheet is removed, the solid Earth underneath may display some degree of rebound. It was observed that the uplift rate in North America and Europe can reach 1 cm/yr.
Researchers have established that the formation of icesheets generated pressure on the underlying rocks, pushing them downward. In addition to this downward dislocation of the crust, the mantle beneath might be compressed as well. Previous studies on GIA have seldom included this compressibility of the Earth in their calculations, because of the complexities and uncertainties that it would introduce into quantitative models. But a paper published by Tanaka et al. earlier this year in the Journal of Geodynamics established a model which includes compressibility for the GIA in southeast Alaska and compared this model to another which did not include compressibility.
Southeast Alaska, which is also referred to as the Alaska Panhandle, lies west of the Canadian province of British Columbia. This region is known to have the largest GIA rate in North America, approximately 30 mm/yr. The reseachers anticipated that the compressibility effects would be larger and easier to detect in this region. In this region, models of GIA integrate the effect of ice sheet mass variations over three periods: the Last Glacial Maximum (LGM) about 20,000 years ago, the Little Ice Age a few centuries ago (LIA) and present-day (PD).
Measurements of rebound at different locations can serve to test these models, since information is available on the extent of icesheets in different periods. It is known, for example, that icesheets retreated earlier at lower elevations, so effects from earlier periods will be stronger there. In the case of southeast Alaska, rebound results primarily from post-LIA and PD ice melting; the former, larger in magnitude, was incorporated into the compressibility model. This model examined the rheological properties of the Earth’s mantle—the geological processes which allow rocks to flow on long time scales, and a second set of properties, called flexural rigidity, which determine the capacity of the earth’s crust to bend.
The authors conclude that their modeling efforts demonstrate the value of including compressibility. Without this element, the current uplift rate in southeast Alaska would be 27% (4 mm/yr) slower, and as a result would not match field measurements as well. Phrased in simpler language, they show that the vast ice sheets of the past not only pushed the mantle down, but squeezed it as well. This study demonstrates the great power of ice to alter our planet’s surface, and indicates that it can have measurable effects centuries, or millennia, after it melts.