But infrasound also comes from the very heart of a volcano, produced by the turbulent fluid dynamics of magma flows deep beneath the surface. Here, 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 to its surroundings, the molten rock traps these sounds and acts like a waveguide, channelling 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, says Steve McNutt, a volcano seismologist from the Geophysical Institute at the University of Alaska in Fairbanks. This infrasound is Predicting volcanic activity by monitoring gas bubbles a complex mixture of low frequencies.
Geophysicists already eavesdrop on low-frequency vibrations using seismometers.
Many of the processes that generate infrasound inside a volcano also send out waves known as volcanic tremor that race through the ground. Measuring these waves can tell geophysicists much about a volcano, but so far, attempts to use changes in the amount of tremor as a warning of eruption have given mixed results. Sometimes you see none at all. Cracks and fissures No one knows why this is. But Garces believes that part of the problem lies with features in the ground beneath a volcano that scatter and distort the seismic waves.