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Glaciers that drain ice caps such as Antarctica or Greenland often throw themselves into the ocean and end in almost vertical cliffs. When the glacier empties into the sea, pieces of ice break off during calving. Although calving often occurs when the ocean melts the ice front and the ice cliff falls over, a new study presents another method of calving: collapse. And this process could break ice chunks much faster.
Research on the ice cliffs was stimulated by a helicopter ride over the Jakobshavn and Helheim Glaciers on the east coast of Greenland. Helheim ends abruptly in the ocean, in almost vertical ice cliffs reaching 30 storeys (100 meters). During the flight, scientists observed large cracks (called crevasses) above the ice that was heading towards the end of the glacier.
"Geologists have spent decades, if not centuries – worrying about subsidence," says Richard Alley, co-author of the new article on Geology. A subsidence occurs when the mass of rocks or sediments loses some of its strength, detaches itself from nearby lands, and slides on a slope. Typically, subsidence is marked by a steep escarpment where the material is broken, followed by a block of material moved down.
According to Alley, the research team noted that the characteristics of the Helheim Glacier are typical of what you might see in a subsurface-prone landscape and asked if the ice could undergo the same fate. "You have a crevice that serves as a scarf, then you have the stress [within the ice] maximized at the water level, "he says.
To test for subsidence on the ice cliffs, the team monitored the Helheim Glacier during a calving, using a real aperture radar interferometer. They measured the speed, position and movement of the ice at calving. The researchers observed an acceleration of the ice flow just prior to initial subsidence, followed by a rotary calving of the glacier throughout its thickness, including the entire remaining ice cliff, reaching both the top and the bottom of the ice. below the waterline.
By removing the weight from the top of the ice by collapse, the underlying ice rises. "Because he's still attached to the back, he's going to spin a bit," says Alley. The rotation causes the formation of a crack at the bottom of the glacier when the ice bends. In turn, the crack can weaken the ice, creating a significant calving event – all triggered by the initial slump at the top of the ice cliff.
After observing the calving phase triggered by subsidence, the team modeled when a fall was most likely to occur on an ice cliff. The modeling examined tensile, shear and compressive strength for ice cliffs, as well as ice characteristics. Scientists have discovered that cliffs reaching over 100 meters of ice over the water could collapse.
According to Alley, regular calving occurs relatively slowly, for example when the ice front melts with time, degrades the ice and weakens the cliff. "But it's not going to go really, really, really fast because we have to wait for the merger to melt the top," he says.
In case of sagging, calving occurs without waiting for melting. "We are going to collapse … in the basal crevice … in the ramp," he said, pointing out that when the calving takes place, the 100 meters of ice above the water and the 900 meters under the water will be very fast. .
And 1000 meters of ice calving at a time is not the limit. Alley says that in some places in Antarctica, the ice bed can be located between 1,500 and 2,000 meters below sea level, thus creating a much higher cliff above the water . He says the worry is that the highest cliffs are even more likely to sag. "What's scary is that if parts of the West Antarctic begin to do what Helheim is doing, then, over the next hundred years, the models indicate that sea level is rising rapidly at speeds higher than predicted, "says Alley.
According to Alley, understanding the economic downturn has been a collaborative effort and new investigations are planned in the near future. "We want to understand what are the rules for [ice] Alley adds that they hope to collect more observational data and refine their models to better understand the slowdown process. "There is still work to be done."
The research was funded by NASA, the National Science Foundation, and the Research Institute of New York University in Abu Dhabi.
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