The coastline of Southeast Greenland is uplifting more rapidly than other parts of the island. This is caused by weak rocks beneath this part of Greenland, combined with the melting of the ice sheet, according to researchers.

Greenland, the world’s largest island, has an area of 2,166,086 km², and 81% of the island is covered by ice. This ice is mostly contained within the Greenland Ice Sheet, which in some places is more than 3 km thick. The ice sheet loses ice mass to runoff and meltwater every year.

Data from satellite-based navigation and positioning systems (GNSS) show that Greenland’s southeastern coastline has faster ground uplift than the rest of the island. Ground uplift refers to changes in the height of the ground relative to sea level, and in southeast Greenland the ground is rising by more than 12 millimeters annually.

In an article published in Communications Earth & Environment, two researchers from the University of Oslo have investigated why ground uplift is faster in the southeast of the island. The two are former CEED Ph.D. student Maaike Weerdesteijn, and Clint Conrad from PHAB and the Department of Geosciences.

All of Greenland is rising, but at different rates

Satellite measurements show that the entire coastline of Greenland is rising. One reason for this is the ongoing melting of the ice sheet as a result of a warmer climate. When the ice mass is reduced, the rocks beneath the ice become decompressed, leading to ground uplift of a few millimeters per year.

However, the research duo also found that the speed of ground uplift during deglaciation varies depending on where on the island it is measured.

“In southeastern Greenland, ground uplift is occurring unusually quickly, at rates of more than 15 mm/year. Numerical models of glacial isostatic adjustment (abbreviated GIA), which compute the response of the solid Earth to melting, were unable to explain this rapid uplift,” says Clint Conrad, professor of geophysics and one of the researchers behind the study.

Properties of the crust and mantle

Melting of the ice sheet is only one factor that affects the rate of ground uplift. The properties of the mantle below the crust also play a major role. Viscosity is especially important because it affects how the rocks in the Earth deform, according to the researchers.

Previous GIA models have mainly taken into account changes in viscosity with depth below the subsurface. However, Weerdesteijn and Conrad used a new GIA code that can take into account side-to-side variations in viscosity.

Rocks weakened by a hot plume

Today, a hot “plume” of rock is rising from the deep interior of the Earth beneath Iceland, where it generates active volcanism and hot springs. The researchers noted that because of continental drift, Greenland must have passed over this hot plume more than 40 million years ago.

Weerdesteijn and Conrad hypothesized that heat from this plume may have weakened the rocks beneath southeast Greenland, as this part of Greenland is closed to Iceland. Using the adjusted GIA model, they reduced the viscosity to the upper mantle rocks along the path of the plume across Greenland.

The new model simulations showed that the rate of ground uplift was significantly faster along the track of the hot plume. Uplift that would normally have happened over thousands of years instead occurs in only centuries or decades above the weakened rocks.

This explains why the southeastern part of Greenland is rising particularly quickly. Here, rapid ice loss is positioned directly above rocks that were weakened by the hot Iceland plume.

Rapid uplift also happened in the Holocene

The researchers also found that a similar historical uplift event may have occurred about 10,000 years ago. Sea level indicators from the early Holocene (from 11,700 years ago to the present) show that the coastline of southeastern Greenland rose rapidly after extensive melting of the ice sheet at the end of the last ice age.

“Uplift rates in the Holocene period may have been almost twice as high as those we see today. This indicates that there is potential for even faster land uplift in the future as melting in Greenland accelerates. Such uplift is important because it changes the elevation of the glaciers as they enter the ocean. This can affect the rate of ice mass loss,” says Weerdesteijn, first author of the study and currently affiliated with UNIS in Svalbard.