A 76-Square-Kilometer Slab of Greenland Hit an Island at Speed and Did Not Break

On August 4, a tabular slab of ice covering 76 square kilometers broke away from Greenland's Petermann Glacier — the largest calving event by any Arctic glacier since 2020, and a piece of ice roughly the size of Manhattan. Three weeks later it ran into something.

The Operational Land Imager aboard Landsat 9 photographed the encounter on August 23 and again on August 24, capturing the ice island as it drifted down Petermann Fjord and made contact with Joe Island in northwestern Greenland. Slabs this large tend to fracture when they hit fixed obstacles; the internal stresses at the point of contact are enormous, and a tabular berg is essentially a brittle plate. This one did not fracture. It struck the island, held together, and carried on into Nares Strait, the channel separating Greenland from Canada's Ellesmere Island.

What makes the survival interesting is what it says about the ice itself. A slab that absorbs a collision of that scale without splitting is telling researchers something about its internal structure — the density of existing crevasses, the temperature through its thickness, how the fractures it already carried were oriented relative to the impact. Those are properties that are otherwise very hard to measure from orbit, and the collision functioned as an unplanned stress test.

Petermann is one of Greenland's largest marine-terminating glaciers, and it does a specific job in the ice sheet's mass balance. Its floating ice tongue extends far out into the fjord and acts as a brake on the grounded ice behind it. Removing large sections of that tongue reduces the buttressing, and the ice upstream can accelerate toward the ocean. The glacier is a gatekeeper, and calving events of this size are how the gate widens.

The floating ice already sits in the water, so its melting adds essentially nothing to sea level directly. The consequence runs through what it was holding back. Ice that was resting on bedrock and is now free to move faster into the sea does raise sea level, which is why glaciologists track the tongue rather than the berg.

The berg itself is now a moving experiment. It will drift south through Nares Strait, into Baffin Bay and the Labrador Sea, thinning and eventually fragmenting over months or years, all of it visible to satellites. Ice islands this large are rare enough that each one is a chance to watch a full life cycle — formation, transit, decay — at a scale that models normally have to assume rather than observe. Petermann produced comparably sized islands in 2010 and 2012, and the records from those events are still being used to calibrate how the Arctic sheds its ice.