An iceberg roughly the size of St. Thomas in the U.S. Virgin Islands has done something the scientists watching it did not expect. It hit a small rocky island at the mouth of its home fjord and stayed in one piece.

The berg broke from Greenland's Petermann Glacier in early August and measured just over 76 square kilometers (29 square miles) when it calved, making it the largest calving event by any Arctic glacier since 2020 and the largest from Petermann itself since 2012. Satellite images from Landsat 9 captured the encounter with Joe Island on August 23 and August 24, and NASA Earth Observatory published the sequence on September 1. The ice island has since pivoted away and is continuing southwest through Nares Strait.

Collisions with Joe Island often mark the beginning of the end for a Petermann berg. The much larger ice island that split in two there in 2010 is the reference case. This one did not break up.

Tracking a Berg the Size of St. Thomas

The calving was first spotted on August 4 by Adam Garbo, a doctoral student in glaciology at the University of Ottawa, in imagery from the European Space Agency's Sentinel-1 mission. Garbo and an international team drawn from the University of Ottawa, the University of Stirling, Environment and Climate Change Canada, Lancaster University and the University of Leeds have been using remote sensing to monitor Petermann's floating ice tongue.

The break was smaller than the team had been anticipating. They had expected a major calving once one of the large rifts they were watching finally cut all the way across the tongue. Garbo said the calving instead "followed a different fracture," producing a smaller ice island than they had originally anticipated. The University of Ottawa announcement of the calving describes the collaboration behind the monitoring work.

In the week after it calved, the berg moved down Petermann Fjord at an average of about 3 kilometers (1.9 miles) per day. Glaciologist Mauri Pelto of Nichols College has been tracking its progress separately using imagery from the NASA and USGS Landsat satellites.

Then it reached Joe Island, a small rocky outcrop at the fjord mouth and one of the first obstacles any departing ice island meets. Garbo said the team was "impressed that it survived the interaction" without further fragmentation. In the NASA Earth Observatory account of the encounter, Pelto notes that Petermann bergs tend to be thinner and more fragile than those from glaciers such as Jakobshavn and Helheim, and thinner still than Antarctic tabular bergs. This one was estimated at less than 150 meters (about 490 feet) thick when it broke away.

Two Rifts Still Waiting on the Ice Tongue

The forward-looking part of the story sits back at the glacier. As of late August, two large rifts remained on Petermann's ice tongue, and the research team expects them to eventually produce ice islands of roughly 94 square kilometers and 84 square kilometers. Both would be larger than the berg now drifting down Nares Strait.

Timing is the open question, and the researchers have been careful not to answer it. The team has said the timing remains uncertain. That is a meaningful limitation. Rifts on floating ice tongues can widen for years before they cut all the way across, and August demonstrated that a fracture can propagate along an unexpected path.

Some context is essential here. Iceberg calving is a routine part of an outlet glacier's life cycle, and a single calving event is not evidence of collapse. NASA's account describes calving as normal and notes that scientists watch the process alongside many other observations for longer-term signs of instability. Petermann's earlier major events came in 2008 at 31 square kilometers, 2010 at just over 250 square kilometers, and 2012 at 130 square kilometers. By that record, this summer's break is substantial but not extraordinary.

Reasons Scientists Watch Petermann Specifically

Petermann is one of Greenland's largest marine-terminating glaciers, and it acts as a gatekeeper for ice moving off the ice sheet and into the ocean. That is why a routine event on this particular glacier gets attention that a comparable event elsewhere would not.

The floating ice tongue does not raise sea level when it calves, because it is already displacing seawater. What matters over longer timescales is whether the grounded ice behind it speeds up. NASA links Petermann's future stability to sea level rise research for that reason, and a 2022 Nature Communications study on whether the shelf could recoverconcluded that the Petermann ice shelf may not recover after a future breakup.

That is a modeling result, not an observation of what this berg has done, and it should not be read as a forecast for the glacier. Nothing in the current observations establishes that this calving changes the glacier's trajectory. The honest summary is that scientists are collecting evidence, not announcing a conclusion.

Drift Hazards and Freshwater in the Water Column

Ice islands and their fragments travel a long way. Garbo and colleagues have noted that these features can pose hazards to marine activities and infrastructure while distributing freshwater through the ocean as they melt. Many past ice islands have grounded off the coasts of Coburg and Baffin islands, well down the drift path toward Baffin Bay.

Bergs of this size are tracked, but the fragments they shed as tides, winds, currents and melting work on them are the more common navigation problem.

There is no coastal risk to U.S. communities from this event, and no reason for anyone onshore to take action. Readers who want to follow the drift can watch NASA Earth Observatory's image of the day feed and the Landsat archive maintained by the U.S. Geological Survey, which is where much of the tracking imagery originates.

What happens next depends on physics that nobody controls and nobody can schedule. The berg is expected to fracture into smaller pieces as it moves southwest, with tides, winds, currents, and melting all working on ice that was thin to begin with. Whether either remaining rift on Petermann breaks through this year, next year or later is unknown, and no responsible forecast exists. The most useful thing to watch is not a countdown but the satellite record itself, which now updates every few days and is public.

What Readers Want to Know

How big is this iceberg?

It measured just over 76 square kilometers (29 square miles) when it calved in August, roughly the size of St. Thomas in the U.S. Virgin Islands. It was estimated at less than 150 meters thick.

Is this the largest iceberg from Petermann Glacier?

No. Petermann released larger ice islands in 2010 at just over 250 square kilometers and in 2012 at 130 square kilometers. This is the largest Arctic calving since 2020 and the largest from Petermann since 2012.

Does this raise sea level?

Not directly. The ice tongue was already floating and displacing seawater. Scientists monitor Petermann because of what the grounded ice behind the tongue may do over longer timescales.

Was this caused by climate change?

No attribution analysis has been published for this specific event, and the researchers involved have not made that claim. Calving is a normal part of an outlet glacier's life cycle.

When will the next two ice islands break off?

The research team has said the timing remains uncertain. Two large rifts remain, expected to produce ice islands of roughly 94 and 84 square kilometers, but no date has been forecast.

Does the berg pose any danger?

Not to people on land. Ice islands and their fragments can be a hazard to shipping and offshore infrastructure as they drift, which is why they are tracked.