Hidden giant granite discovered beneath West Antarctic ice sheet

Scattered pink granite boulders across the dark volcanic peaks of West Antarctica’s Hudson Mountains have revealed the presence of a vast buried body of granite, almost 100km in diameter and 7km thick – about half the size of Wales in the UK – beneath the Pine Island Glacier.

These unusual rocks, perched high in the mountains, have intrigued scientists for decades. Where do they come from and what could they reveal about the past and future of the ice sheet?

A team of researchers, led by the British Antarctic Survey (BAS), dated the granites using the radioactive decay of elements encased in microscopic crystals, finding that the rocks formed around 175 million years ago, during the Jurassic period. But how the rocks came to rest in these mountains remained a mystery until new evidence came from airborne surveys. The study is published in the journal Earth and Environment Communications.

Precise gravity measurements collected by BAS’s Twin Otter and other aircraft flying over the region revealed an unusual geological signal coming from beneath the glacier, matching the expected signature of buried granite.

Connecting the scattered boulders to this hidden giant granite provided a breakthrough. Not only does it solve a long-standing geological puzzle, it also offers vital clues about the past behavior of the Pine Island Glacier, tearing rocks from the bed and depositing them on the mountains at a time when the ice sheet was much thicker. Understanding ice thickness and flow regimes during the last ice age (about 20,000 years ago) helps scientists refine computer models of the ice sheet, which are key to predicting how Antarctica will respond to future climate change.

Dr Tom Jordan, lead author and geophysicist at BAS, analyzed the airborne survey data. He said: “It is remarkable that pink granite boulders spotted on the surface led us to a giant hidden beneath the ice. By combining geological dating with gravity studies, we not only solved a mystery about the origin of these rocks, but also discovered new information about how the ice sheet flowed in the past and how it might change in the future.

The discovery also sheds light on current processes. Beneath Pine Island Glacier, a region that has seen some of Antarctica’s fastest ice loss in recent decades, geology strongly influences how ice slides across the bed and how meltwater flows underneath. The new findings will help improve computer models of ice flow used to project sea level rise.

Dr. Joanne Johnson, co-author of the study and a geologist at BAS, collected the rocks during fieldwork around the Hudson Mountains as part of the International Thwaites Glacier Collaboration. She says: “The rocks provide an astonishing record of how our planet has changed over time, particularly how ice has eroded and altered the landscape of Antarctica. Rocks like these are a treasure trove of information about what lies deep beneath the ice sheet, far from its reach.

“By identifying their source, we were able to understand how they got to where they are today, giving us clues about how the West Antarctic ice sheet might change in the future – vital information for determining the impact of sea level rise on coastal populations around the world.”

This study highlights how combining different scientific fields, in this case geology and geophysics, can provide new insights into the hidden processes that shape our planet.