Predicting eruptions
during the last three decades, a volcano has been erupting regularly in Arenal, Costa Rica. In 1992, geophysicist Milton Garces, based at the University of Hawaii at Manoa, usa, took a step no one had ever dared to even dream of. He climbed up Arenal's rainforest slopes to listen to the sound emanating from the volcano. "I sat there in the fog and the rain and listened for about half a week,' said Garces. "I heard explosions, I heard hissing, I heard this remarkable chugging sound, like a train. While listening, it occured to me that I could use these sounds to find out what is happening inside the volcano.' For him, the sounds could lead to an entirely new way of studying the volcano. Garces teamed up with Michael Buckingham, an expert in the underwater acoustics at the Scripps Institution of Oceanography in San Diego, California, usa , and got down to working on a computer model that could help answer those questions.
They are working on manufacturing equipment for volcanoes similar to what the stethoscope is to human beings. Through this equipment, they intend to study subtle variations in the boiling hearts of volcanoes that show, for example, whether a river of fresh magma charged with explosive gases is surging up from the depths.
Since 1989, Garces has been involved in research aimed at detecting volcanic eruptions on the seabed. He observed that underwater volcano hunters wanted to distinguish these eruptions from the confusion of background noise in the ocean. However, none of them could understand: what is it that makes a volcano sound like a volcano, and what these noises could reveal. Garces was intrigued by the question, but realised that studying the acoustics of underwater volcanoes was not simple because most eruptions occur kilometres beneath the seabed. Studying them would be difficult.
On the other hand, volcanoes on dry land produce a cacophony of noises, ranging from high pitched whoosing and whining to low rumbles of infrasound with frequencies of less than 20 hertz, outside the range of human hearing. "These are the waves that physically shake you,' says Garces. He found that infrasound can reveal volcano's innermost secrets. Infrasound is not only generated near the surface from exploding bubbles of gas and rock, but also comes from the very heart of a volcano, produced by the turbulent fluid dynamics of magma flows deep beneath the surface.
Geophysicists believe that oscillating streams of magma or gas bubbles expanding and contracting with changes in pressure may give out deep growls and bellows. Since the magma has a different density compared to its surroundings, the molten rock traps the sound and acts like a wave-guide, channeling them up through the magma-filled passage, called the conduit, to the volcano's vent.
These waves of infrasound force the surface of the magma to vibrate. It is a complex mixture of low frequencies. And because some of it comes from deep inside a volcano, Garces believes that it contains vital information about the processes that trigger eruptions. Perhaps it may even tell us how likely a volcano is to blow its top. "There is an ocean of information in these low frequencies,' said Garces. The first step in deciphering the complex language of a volcano was to model a physical structure mathematically, using parameters such as size of the conduit, the rigidity of the bedrock and the gas content of the magma. The model generated an infrasound spectrum that they could compare with the real thing: tweak the parameters until the two spectra matched, and they should have a good idea of what goes on inside the volcano.
The model was based on simple physics. To start with, they decided to recreate the short, sharp explosive blasts that occur at the summit of an erupting volcano. The first moment after the blast, a strong shock wave radiates in all directions, like the sonic boom of the jet. This is followed by a low, infrasonic rumble that tapers off gradually. Garces and Buckingham duplicated the conduit of a volcano using a cylinder of rock filled with magma. The sharp boom of the initial blast comes from the sudden burp of gas that explodes near the surface. This creates acoustic waves that ripple out into the magma, bounce around inside the conduit and then die away producing the long, low rumble that subsides slowly.
By 1994, the two were ready to test their model against the real explosions. They chose one of the world's most studied volcanoes on Stromboli, an island among Italy's Lipari group. Because it was so well understood, there were real numbers to feed into the model. In May 1998, Graces spent some time recording the boom of infrasound on Sakurajima volcano in Kyushu, one of the Japan's southern islands. He found that he could pick out clear changes in the character of the infrasound spectrum before an eruption. "You could definitely see the change in the acoustic signal,' he said.
In the next few years, Garces predicts volcano models will emerge that can translate volcano sounds into information almost instantaneously. Such a volcanic stethoscope will convert the spectral fingerprint of tremor and explosion into the kind of measurement that would mean a lot to those living close to a volcano.