Rushing Water
Schlindwein and her colleagues recorded several iceberg tremors lasting from minutes to hours. The most spectacular one lasted 16 hours on July 22, 2000.
The team speculates the tremors result from high-pressure water squeezing through iceberg tunnels and crevasses, causing the berg's walls to shake.
Schlindwein said an iceberg has to get stuck to create sufficient water pressure to "sing." This forces water that normally pushes the iceberg along to try to rush through it.
The scientists believe that a 20-by-30-mile (30-by-50-kilometer) iceberg got stuck on July 22, 2000 when it collided with an underwater peninsula, or escarpment. The collision caused a seismic shock with an estimated local magnitude of 3.6.
"It got stuck and this water flowing through the iceberg was strong enough to generate tremors," Schlindwein said.
The tremors continued for 16 hours as the iceberg slowly scraped around the escarpment, pushed by the coastal current. Chunks of ice falling off the berg may have accounted for certain features in the seismic signal, the researchers add.
Once the iceberg was free of the escarpment, the tremors stopped.
"We can't prove [this] of course," Schlindwein said. "Just like in volcano research, we make observations and try to draw conclusions of what the inner workings must look like."
Other Theories
Aster, the New Mexico-based geophysicist, says Schlindwein and her colleagues propose an interesting and testable hypothesis but adds that the research field is still open.
"There's more than one proposed way to get an iceberg to sing," he said. "This is a possible mechanism. But I think the jury is still out whether this is the only mechanism or to what extent it may be valid."
Aster and colleagues based in Illinois have studied similar iceberg tremors immediately following the collision of icebergs in the open ocean.
He says an alternative mechanism for the singing sounds is repeated stick and slip of ice—the same process that makes a door hinge squeak.
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