Pink granite boulders lead scientists to massive hidden rock formation under Pine Island Glacier

A geological puzzle sitting on the surface of Antarctica's Hudson Mountains has pointed researchers toward something much larger beneath the ice. Pink granite boulders, visually distinct from the volcanic rock that dominates the Hudson Mountains, had no obvious source in the surrounding geology. Following the trail of that mismatch, scientists have now identified a previously unknown granite mass buried under Pine Island Glacier, roughly 100 kilometres wide and 7 kilometres thick.

The findings were published in the journal Nature Communications. Pine Island Glacier is one of the fastest-retreating glaciers on Earth. It drains a significant portion of the West Antarctic Ice Sheet and has been losing mass at an accelerating rate over the past three decades. Understanding what lies beneath it, including the composition and shape of the bedrock, directly affects how scientists model its future behaviour and how much sea level rise it might contribute.

Why pink granite boulders were so out of place

The Hudson Mountains are composed primarily of volcanic rocks formed by relatively recent volcanic activity. Granite is an entirely different rock type. It forms deep underground through the slow cooling of magma over millions to hundreds of millions of years, not through surface volcanic processes. Finding granite boulders sitting on top of a volcanic mountain range with no nearby granite outcrop visible meant the glacier had carried them there from somewhere else.

Glaciers are highly effective at transporting rock. As ice flows over bedrock, it picks up fragments and carries them, sometimes hundreds of kilometres, before depositing them when the ice melts or the glacier retreats. This process is called glacial erratic transport, and the boulders are called erratics. The pink granite erratics on the Hudson Mountains were telling geologists that a granite source existed somewhere upstream in the direction of Pine Island Glacier's flow.

How the buried granite mass was mapped

The research team used a combination of airborne radar surveys and geophysical data to image the bedrock beneath the glacier. Radar waves penetrate ice and reflect off the rock surface below, allowing scientists to map bedrock topography and, to some degree, rock composition. The granite formation shows up as a distinct anomaly in the radar and gravity data, sitting beneath approximately 1.7 kilometres of ice at its deepest point.

The team also dated samples from the surface granite boulders using cosmogenic nuclide dating, a technique that measures how long a rock surface has been exposed to cosmic radiation. The results showed that some boulders were deposited on the Hudson Mountains surface approximately 8,000 years ago, during a period when Pine Island Glacier was retreating after the last glacial maximum. That timing tells researchers something about when the glacier was thin enough or short enough to expose the buried granite formation and allow erosion to carry material to the surface.

What the granite mass means for glacier stability

Bedrock geology matters for glacier dynamics in a direct and practical way. The friction between a glacier's base and the underlying rock affects how fast the ice slides toward the ocean. Granite is generally harder and less deformable than the softer sedimentary or volcanic rocks found under many parts of the West Antarctic Ice Sheet. A hard granite base can, depending on its topography, either anchor ice in place or create channels that accelerate flow.

The specific geometry of the newly identified granite formation includes elevated ridges within the mass that may act as a partial pinning point for overlying ice. The research team's modelling suggests that this ridge structure could be slowing the landward retreat of Pine Island Glacier's grounding line, the point where the glacier transitions from resting on bedrock to floating on ocean water. If the granite ridges are providing even a modest restraining effect, their presence needs to be incorporated into ice sheet models used for sea level projections.

The sea level implications of getting the bedrock wrong

Pine Island Glacier and its neighbour Thwaites Glacier together hold enough ice to raise global sea levels by approximately 1.2 metres if they were to fully collapse. Current IPCC projections for West Antarctic contributions to sea level rise carry wide uncertainty ranges partly because the bedrock beneath these glaciers is not fully mapped. The 2021 IPCC Sixth Assessment Report noted that West Antarctic instability is the primary source of deep uncertainty in sea level projections beyond 2100.

The newly identified granite mass covers an area that was previously assumed to have the same soft sedimentary character as much of the surrounding bed. That assumption fed into numerical ice sheet models used to project how quickly Pine Island Glacier might lose mass under different warming scenarios. Revising those models to include a hard granite formation with internal ridges will likely change the projected timelines for grounding line retreat, though whether the revision makes the projections more or less alarming depends on the detailed geometry of the ridges and how the glacier responds dynamically.

The British Antarctic Survey team behind the study is planning follow-up fieldwork to collect additional rock samples from the Hudson Mountains erratics and to refine the radar imaging of the granite formation's boundaries. That work is scheduled for the 2026 to 2027 Antarctic field season.