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New research has discovered that volcanoes can sometimes 'scream' at ever-higher pitches just before they erupt. Scientists at the University of Washington analysed an eruption sequence at Alaska's Redoubt Volcano and found what they describe as audible 'screams' that followed seismic activity as the changed from steady pulses to rapid tremors. The researchers also believe that this research and the rise in the frequency of earthquakes and the tremors they cause in the ground could be used to predict future volcanic eruptions. The study said that it is not unusual for swarms of small earthquakes to precede a volcanic eruption. They can reach a point of such rapid succession that they create a signal called a 'harmonic tremor' that is inaudible to humans as it is such a low frequency. The researchers analysed an eruption sequence at Alaska�s Redoubt Volcano in March 2009. They found that the harmonic tremor rose to substantially higher frequencies and then stopped abruptly just before six of the eruptions. Some volcanoes emit sound when magma resonates as it pushes up through thin cracks in the Earth�s crust. But the scientists think that in this case the earthquakes and harmonic tremors happen as magma is forced through a narrow channel under great pressure into the heart of the mountain. They believe that the thick magma sticks to the rock surface inside the channel until the pressure is enough to move it higher, where it sticks until the pressure moves it again.