Unjammed ice is a precursor to calving

At the glacier–ocean boundary, a slushy mix of floating icebergs and sea ice often forms kilometer-scale conglomerates known as ice mélanges. In the photo above, the ice mélange is to the right of the vertical calving front of Greenland’s Jakobshavn Isbræ glacier. Mélanges often move across the landscape and through fjords and other coastal areas.

Ice mélanges can be viewed as soft matter because their granular composition responds collectively to various mechanical and thermodynamic effects. In 2015 Ryan Cassotto, now with the University of Colorado Boulder, and his colleagues showed that an ice mélange can mechanically resist the dynamic motion of calving events and may help control the amount of mass lost during them.

A better understanding of the dynamics of ice mélanges requires finely resolved data of the ice’s viscoplastic flow, but that data are difficult to collect in such a complex geophysical system. Cassotto and a different group of collaborators have now taken advantage of recent advances in terrestrial radar interferometry to observe ice-mélange dynamics at the granular scale both before and after calving events along a Greenland glacier. Their results may help enable a detection system for gathering data on calving events in real time.

Cassotto and some of his coauthors in August 2012 collected the observations by using a pair of portable terrestrial radar interferometers (TRIs)—one of which is pictured below. The TRIs were positioned about 2 km apart so that they had overlapping kilometer-scale views of the ice. The radar waves reflected off the ice, and the interfering electromagnetic phases of returned signals yielded millimeter-scale sensitivity of the speed of individual icebergs in the mélange. A single TRI can provide only line-of-sight measurements, but with two instruments collecting data every three minutes, Cassotto and his colleagues could construct a finely resolved two-dimensional flow field of the ice mélange.

The left-hand image below, acquired long before a calving event, shows a relatively coherent flow of the granular material. But in the right-hand image, taken immediately preceding an event, bits of ice separate from the bulk and the ice mélange’s flow becomes less coherent. All told, the researchers analyzed 14 calving events and found that flow speeds increased significantly about one hour before an event.

The researchers analyzed the events further with a particle dynamics model, which showed that small displacements of ice grains in the mélange could lead to a loss of flow coherence in the hour leading up to the event. More research needs to be done to understand how a mélange’s flow affects calving, but Cassotto and his colleagues suspect that the transition of the ice from a dense, jammed state to a disaggregated one lowers the mechanical force resisting calving events. (R. K. Cassotto et al., Nat. Geosci., 2021, doi:10.1038/s41561-021-00754-9 .)