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"Singing" Iceberg Recorded in Antarctica

John Roach
for National Geographic News
November 29, 2005
 
They may not sound like Bing Crosby, but some icebergs can sing, scientists report.

"They are partly melodic, but not really melodic like singing, more like the screeching of a horror film in parts," said Vera Schlindwein, a scientist with the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany.

"Some are like an orchestra when all the violins start [playing] at once. Then, all of a sudden, it develops into harmonic sounds like a humming bird," she added. "It is a well-developed sound."

The sounds are too low to be heard by humans and are only audible when played at higher speeds, Schlindwein said.

She and her colleagues picked up the acoustic noise from seismic recordings made along Antarctica's South Atlantic coast in 2000.

The researchers believed the tremors were from volcanic activity. But when they tracked to their source, the scientists discovered the rumblings' true origin.

The team published their study last week in the journal Science.

Volcanic Tremors

Schlindwein notes that some types of volcanic tremors, when viewed on a seismograph, are "absolutely undistinguishable from these iceberg tremors."

Since icebergs are easier to study than rocky volcanoes, Schlindwein said examining the mechanisms that make the icebergs screech may help scientists better understand the mechanisms of volcanic tremors, which often precede an eruption.

Richard Aster, a geophysicist at the New Mexico Institute of Mining and Technology in Socorro, has measured similar iceberg tremors in the Ross Sea along Antarctica's Pacific Ocean coast.

He says that, generally speaking, iceberg tremors and volcanic tremors have similar characteristics. But he cautioned that "we're still at the early stage of trying to understand these sorts of signals."

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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