An Unstable Front
Grey Glacier, a major outlet of the Southern Patagonian Ice Field, is a river of ice that terminates in three lobes in the proglacial Lago Grey. In 1996, the glacier covered 270 square kilometers, with a length of 28 kilometers. Its front end is approximately 6 kilometers wide and rises over 30 meters above the lake's surface. This ice wall is the site of dramatic calving events, where massive blocks of ice break away and become icebergs.
A particularly large rupture occurred in November 2017, when an iceberg measuring roughly 350 by 380 meters detached from the glacier's eastern front. This event was unusual for its scale, drawing significant attention from scientists. The frequency of these large calving events appears to be increasing. In early 2019, two more significant icebergs broke off within just two weeks of each other—one 8.8 hectares and the other 6 hectares. The glacier has been in a state of retreat for decades, losing about two kilometers in length over the last 30 years. Some estimates suggest an annual retreat rate between 100 and 200 meters.
The Science of the Calve
Glaciologists study calving to understand the mechanics of ice loss. The process is complex and is driven by multiple factors. The formation of a proglacial lake, like Lago Grey, fundamentally alters a glacier's dynamics. The lake water can lubricate the glacier's bed, increasing basal sliding and increasing its forward velocity. Near its terminus, Grey Glacier's surface velocity can reach up to 2.8 meters per day.
Water also exerts pressure on the ice front and can work its way into crevasses, which are fractures caused by the stress of flowing ice. This can weaken the glacier's structure. Warmer lake water can undercut the ice front at the waterline, creating an unstable overhang that eventually collapses. Scientists use a variety of tools to monitor these changes. Satellite imagery from sources like Landsat provides a long-term view of the glacier's retreat. Terrestrial camera systems and GPS stakes placed directly on the ice offer high-resolution data on ice flow velocities. Recently, researchers have deployed infrasound sensor arrays to detect the acoustic signals generated by calving events, allowing them to monitor collapses in real-time.