Some volcanoes ‘scream’ at ever higher pitches until they blow their tops

Swarms of small earthquakes can precede a volcanic eruption, sometimes resulting in “harmonic tremor” resembling sound from some musical instruments. A new analysis shows tremor during a 2009 sequence at Alaska’s Redoubt Volcano glided to substantially higher frequencies, then stopped abruptly just before six of the eruptions.
Source: http://www.sciencedaily.com/releases/2013/07/130714160521.htm