Glacier calving is the process where chunks of ice break off from the edge of a glacier, ice shelf, or ice sheet, typically falling into water to become icebergs. It happens because the forward motion of a glacier pushes its front end, called the terminus, into an increasingly unstable position until pieces crack away under their own weight. Calving is one of the primary ways glaciers and ice sheets lose mass, and it plays a growing role in global sea level rise as oceans warm.
How Calving Works
Glaciers are rivers of ice that flow slowly under their own gravity. As a glacier pushes forward, its terminus extends over water or past stable ground, creating stress fractures called crevasses. When those cracks grow deep enough, gravity and water pressure finish the job: a section of ice separates and falls or slides into the ocean, a lake, or a fjord. The result can be anything from a small splash to a building-sized block of ice crashing into the water with tremendous force.
Not all calving looks the same. When a glacier is grounded, meaning its base sits on solid rock beneath the water, it tends to shed a steady stream of smaller icebergs. But when a glacier transitions to floating on water, the style changes dramatically. Observations at Columbia Glacier in Alaska showed that once the ice began floating, calving shifted from frequent small releases to sudden, massive rifting events that produced very large icebergs, similar to how Antarctic ice shelves break apart. These larger events actually produce less seismic energy than grounded calving, making them harder to detect with traditional monitoring.
Why Warm Oceans Accelerate Calving
Calving is a natural process, but warming ocean water is making it happen faster. When relatively warm seawater reaches the submerged face of a glacier, it melts the ice below the waterline, carving out cavities and undercuts. These hollowed-out sections destabilize the ice above, triggering calving events that can be several times larger than the original melted cavity. In other words, the ocean doesn’t just melt ice directly. It weakens the structure so that much larger pieces collapse.
Research in Greenland shows that the rate of this underwater melting increases in proportion to water temperature and to the flow of meltwater running off the glacier’s surface and emerging at its base. Where meltwater exits beneath a glacier, it creates buoyant plumes that draw in warmer ocean water, intensifying the melting at the ice front. The shape of the channels beneath the glacier matters too: concentrated drainage points create deeply undercut zones that are especially prone to large calving events.
Greenland’s glaciers are losing mass at accelerating rates largely because of this increased heat from the ocean. Jakobshavn Glacier on Greenland’s west coast, one of the fastest-flowing glaciers on Earth, drains about 7% of the entire Greenland ice sheet. Between 2003 and 2016, its total thickness shrank by 500 feet. Though NASA researchers found a temporary slowdown linked to cooler local water, the long-term trend of thinning and retreat has been dramatic.
Contribution to Sea Level Rise
All glaciers lose mass through surface melting, but glaciers that terminate in the ocean also lose mass from calving and underwater melting. Marine-terminating glaciers make up about 40% of Earth’s total glacier-covered area, yet they contribute only around 26% of global glacier mass loss, because many of the world’s smaller mountain glaciers melt almost entirely from their surfaces.
Still, the numbers add up. Glacier melt worldwide has accounted for roughly 21% of global sea level rise over the past two decades. The great ice sheets contribute on top of that: Antarctica alone has been shedding approximately 135 billion tons of ice per year between 2002 and 2025, raising global sea levels by about 0.4 millimeters annually. Much of that loss comes from calving at the edges of ice shelves and outlet glaciers.
Sizes of Calved Ice
The ice that breaks off during calving ranges enormously in size, and the U.S. Coast Guard’s International Ice Patrol classifies it into standard categories. To officially count as an iceberg, a piece must be at least 15 meters (about 50 feet) long or 5 meters (about 16 feet) tall above the waterline. Anything smaller falls into two categories:
- Growlers: Less than 1 meter above water and under 5 meters long, roughly the size of a car.
- Bergy bits: 1 to 5 meters tall and 5 to 15 meters long, comparable to a small cottage.
True icebergs scale up from there. A “small” iceberg is the size of a small office building. Medium icebergs compare to a mid-size hotel. Large icebergs, 46 to 76 meters tall and up to 200 meters long, rival the footprint of a sports arena. Very large icebergs exceed 75 meters in height and 200 meters in length. Tabular icebergs, the flat-topped slabs that calve from ice shelves, have a length-to-height ratio greater than 5:1 and can stretch for kilometers. Only about one-fifth of an iceberg’s total mass sits above the waterline, so the underwater portion is roughly five times the visible height.
Calving Can Generate Tsunamis
When a massive block of ice drops into a confined body of water like a fjord or glacial lake, the displacement can create dangerous waves. These iceberg-generated tsunamis have reached up to 50 meters in amplitude, tall enough to pose serious threats to coastal infrastructure, boats, and people.
In 2014, a calving event at Eqip Sermia Glacier in Greenland produced a wave estimated at 45 to 50 meters. In 2011, an overturning iceberg at Tasman Glacier in New Zealand generated a 3.1-meter tsunami in the glacial lake, felt 3.5 kilometers away. Calving events have destroyed harbors and fishing boats in Greenland on multiple occasions. In 2018, an iceberg roughly 100 meters tall drifted close to the village of Innaarsuit in Greenland, forcing the evacuation of its 170 residents over fears that it could roll and generate a sudden wave.
Even at great distances, calving produces measurable effects. Sixteen calving events recorded at Helheim Glacier in Greenland during 2013 and 2014 still produced detectable waves 30 kilometers from the glacier front.
How Scientists Monitor Calving
Calving events are often remote and dangerous to observe directly, so scientists rely on a combination of satellite imagery, seismic stations, and underwater microphones called hydrophones. Before a major calving event at the Nansen Ice Shelf in Antarctica in 2016, hydrophones deployed in the water recorded hundreds of short, broadband cracking signals over several months as fractures propagated through the ice. A nearby seismic station picked up low-frequency tremors and harmonic vibrations as the shelf finally broke apart.
Satellite-based gravity measurements from the GRACE and GRACE Follow-On missions allow researchers to track total ice mass changes across entire ice sheets over time, capturing the cumulative effect of calving and melting year after year. These tools, combined with GPS sensors placed directly on glaciers and radar measurements of ice thickness, give scientists a detailed picture of how quickly glaciers are retreating and where the next major calving events are likely to occur.