The song of the iceberg
September 21, 2010 1 Comment
Before the advent of radar and sonar navigation, sailors used to traverse iceberg-littered waters by listening for sounds to indicate how close or how far away an iceberg was. Since the wreck of the Titanic, people have moved beyond the basic human ear to track icebergs, but if we had whale ears, sound might still be sufficient.
That’s because some icebergs can sing. It’s not exactly an aria—more like a cacophonous symphony warm-up—but it still sounds like a song. Unfortunately, it’s not a song we can hear unaided with our unworthy human ears, which cannot detect the very low frequency—about 0.5 Hz—the icebergs emit. It is possible that whales might be able to pick up the sounds since they can detect very low frequencies.
A brief primer on sound
Sound travels in waves, and those waves have frequency—the speed at which they arrive—and amplitude—the size of the wave. Sound is detected when the waves vibrate something; in the case of our own ears, sound waves hit and vibrate the membranes of our inner ears, triggering hair cells that send nervous signals to our brains, where we process the sound. High-frequency waves produce high-pitched sounds, and low-frequency waves produce bass.
Because we can’t hear the low frequencies the singing icebergs emit, their song went undetected until a fluke discovery by a team of earthquake researchers. The team had been using seismic equipment to monitor earthquakes in the Antarctic region. The seismograph produces on paper the frequency and amplitude of vibrations of the earth as movement occurs. An earthquake pattern usually looks like a flurry of large-amplitude events that tapers off and ceases. But the researchers discovered one day a pattern that looked more like a comb—vibrations occurring at regular frequencies and being sustained for a fairly long period of time.
Earthquakes lead to iceberg song
Mystified, they tried to locate the source of the recordings, which seemed to ramble all along the continent’s edge. But it wasn’t until July 2000 that they had a breakthrough. Two small earthquakes triggered a signal that lasted for 16 hours. This event gave them the opportunity to use satellite tracking to pinpoint where the signals were being generated. The epicenter of the quakes turned out to be an iceberg over 1200 feet high that had lodged against an underwater peninsula jutting from the continent. The iceberg was slowly edging its way along the shelf, apparently singing as it went.
The researchers surmised that when the iceberg became lodged, water pressure built up within the crevasses and tunnels that crisscross these floating mountains of ice. The pressure, they speculate, caused water to rush through these deep gulleys and holes, vibrating the walls and producing the “song” of the iceberg. The scientists made a recording of the seismic noise and sped it up, increasing the frequency to a level that human ears could detect. The recording sounds like something between a screech and the playing of a lot of untuned violins. But eventually, it merges into a smooth sound that evokes many stringed instruments playing the same note simultaneously. (Click on the video above to hear.)
From icebergs to volcanoes
The comb-like signal that the iceberg’s vibrations produced is very like what volcanoes produce when they tremble and vibrate. Now that researchers have hypothesized a mechanism to explain the vibrations, they hope to try to apply the concept to volcano models, which are difficult to study because of the heat factor. An iceberg is not the only thing on Earth that consists of a solid substance riddled with tunnels and crevices through which liquid flows, and may serve as a good volcano model.