Interesting follow-up from Harvard on how glaciers move the crust of the earth horizontally after being liberated of the burden of the weight of glacial ice during melting.
As the continents’ frozen burden dissipates, the ground deforms — not only in the immediate area, but also in far-flung locations.
The loss of melting ice from land masses such as Greenland and Antarctica is causing the planet’s crust to warp slightly, even in spots more than 1,000 kilometres from the ice loss.
Ice melt removes mass from Earth’s continents. Liberated from the overlying weight, land that was once covered by ice lifts up. This vertical response has been much studied, but Sophie Coulson at Harvard University in Cambridge, Massachusetts, and her colleagues wanted to analyse how the ground shifts horizontally. They gathered satellite data on ice loss from Greenland, Antarctica, mountain glaciers and ice caps, and combined them with a model of how Earth’s crust responds to changes in mass.
Between 2003 and 2018, ice melting from Greenland and from Arctic glaciers caused the ground to shift horizontally across much of the Northern Hemisphere, and by as much as 0.3 millimetres a year in much of Canada and the United States. In some areas, even far from the melting ice, the horizontal movement was greater than the vertical movement.
The study -Geophys. Res. Lett. (2021)
NOAA describes the process thusly— What is glacial isostatic adjustment?
Earth is always on the move, constantly, if slowly, changing. Temperatures rise and fall in cycles over millions of years. The last ice age occurred just 16,000 years ago, when great sheets of ice, two miles thick, covered much of Earth's Northern Hemisphere. Though the ice melted long ago, the land once under and around the ice is still rising and falling in reaction to its ice-age burden.
This ongoing movement of land is called glacial isostatic adjustment. Here's how it works: Imagine lying down on a soft mattress and then getting up from the same spot. You see an indentation in the mattress where your body had been, and a puffed-up area around the indentation where the mattress rose. Once you get up, the mattress takes a little time before it relaxes back to its original shape.
Even the strongest materials (including the Earth's crust) move, or deform, when enough pressure is applied. So when ice by the megaton settled on parts of the Earth for several thousand years, the ice bore down on the land beneath it, and the land rose up beyond the ice's perimeter—just like the mattress did when you lay down on and then got up off of it.
That's what happened over large portions of the Northern Hemisphere during the last ice age, when ice covered the Midwest and Northeast United States as well as much of Canada. Even though the ice retreated long ago, North America is still rising where the massive layers of ice pushed it down. The U.S. East Coast and Great Lakes regions—once on the bulging edges, or forebulge, of those ancient ice layers—are still slowly sinking from forebulge collapse.
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