London, Jul 10: Some of Antarctica's largest icebergs can make eerie sounds, which are a result of huge lumps of ice scraping past each other and producing thousands of tiny "icequakes".
According to a report in New Scientist, these icebergs, which travel from Antarctica to Tahiti, can sound like laughing monkeys and barking dogs. Massive tabular icebergs break off the Antarctic ice shelf about every 50 years. Soon after the last "calving" event in 2000, unusual harmonic tremors were picked up by underwater hydrophones as far as Tahiti. Emile Okal of Northwestern University in Evanston, Illinois, and colleagues soon traced the sounds back to the new icebergs, including what was then the world's largest free-floating berg, B15A.
To find out how the icebergs produce the noises, Doug MacAyeal at the University of Chicago and colleagues constructed a network of seismographs on iceberg C16, which at the time was aground in the Ross Sea and adjoining to B15A.
They monitored the tremors, which are inaudible to the human ear, but can be heard if the recordings are speeded up - during the austral summer of 2003.
The team found that one particular song was repeated by the giants daily, and the timing matched that of the tides in the Ross Sea.
It began vigorously on the first surge, then slowly ground to a halt and began again when the tide reversed.
The researchers triangulated the source of the tremors and discovered that they came from an area where C16 was rammed by B15A in a collision zone only a few kilometres long. Pushed by the daily tides, the bergs scraped past each other in many brief, jerky movements.
Other theories have been put forward as to the source of the iceberg "songs", but the researchers say this is the first time a mechanism has been demonstrated.
"Each of these movements is only 0.6 millimetres, but they send booming broadcasts into the world oceans," MacAyeal told New Scientist. "A seismometer at the South Pole records each of these tremors as if an earthquake of magnitude 3.5 was occurring underneath the iceberg," he added.
According to the researchers, the icebergs could allow them to study earthquakes in a laboratory-like setting.
"It turns out that these icebergs are perfect analogies of plate tectonics. They float on the ocean like surface plates float on magma; and just like them, they occasionally collide and slide against each other," said MacAyeal.
"The iceberg tremors could also lead to ways to predict the strength of aftershocks," he added.