Showing once again just how much we have left to learn about environmental processes, a team of researchers in the US have worked out the factors that determine when icebergs will calve off ice sheets.

Perhaps unsurprisingly, the biggest factor is how fast the ice sheets are spreading out over the oceans. The width and thickness of the sheet are also important, says a paper in Science (press release, paper).

“Fracture-mechanics problems are invariably difficult,” says paper author Richard Alley of Pennsylvania State University (Physics World). “Earthquake prediction comes to mind, or guessing whether a tea cup pushed off the table will break or bounce upon hitting the floor. With the tea cup, a drop from 1 mm high won’t break it, and a drop from 100 m almost surely will — one term, the height of the drop, explains a whole lot of the behaviour. Our hope was to find such a dominant term in calving of bergs from ice shelves.”

He adds, “Our first hypothesis was that spreading is required to open a crack that isolates a new iceberg, so the spreading tendency in the direction of ice and berg motion should play the role of the height of your tea cup. “Almost surprisingly, this simple hypothesis explains most of the variance in a data set we assembled to test [it].”

Alley and colleagues gathered data on a representative selection of ice shelves and worked out what factors were important for calving. “Although we have not learned the complete calving law, we suggest that the relations derived here from intercomparison of ice shelves may be more encouraging than any obtained previously for this vexing problem and so merit additional testing and cautious implementation in ice-flow models,” they write in their paper.

Scientists crack iceberg mystery – Reuters

Clue to break-up of ice shelves – BBC

Image: iceberg off Greenland / Richard Alley