Earth appears ‘as hard as rock,’ yet its surface deforms when subjected to enough weight. The Earth’s crust is tough and pliable, like the skin that forms on top of cooling pudding. Just as a spoon placed onto the pudding depresses the smooth, flexible surface into the creamy confection, so too does Earth’s crust sag under the weight of an ice sheet or ocean water. A deep viscous layer of rock called the mantle is analogous to the pudding’s warmer more-fluid interior. Though the crust and mantle move exceedingly slowly on human time scales, Earth’s surface is in constant motion on geologic time scales. When an ice sheet advances, the land underneath subsides. When an ice sheet retreats, the land rebounds, reversing the process. The effects of an ice sheet on Earth’s surface extend far beyond the boundaries of the ice. When a great mass depresses Earth’s crust anywhere, the surface must rise up somewhere else. The planet’s fixed volume requires that regions of depression be matched by regions of uplift. Elevated regions generally rise up just beyond the depression created by the mass, creating an area that looks like the rim around a crater. This region, which can extend for hundreds of miles beyond an ice sheet, is called a peripheral bulge.
The Pliomax team must take into account the effect of long-gone ice sheets on their research sites. Land covered by an ice sheet during the ice age that ended 12,000 years ago is still rebounding. Land on the peripheral bulge of this long-melted ice sheet is still relaxing back to lower elevations. With help from ice sheet modelers Rob DeConto and David Pollard, Mitrovica calculates how much the height of each site has changed, due to these effects, since the Pliocene. To obtain an accurate estimate of the change in global sea level due to changes in the amount of water in the ocean, the figure he calculates is subtracted from (or added to) the measured elevation of the field site.
Learn about other adjustments the Pliomax team makes to its field measurements: