Volcano "screams" may predict when they explode, a new study says.

The screams come with shifts in seismic activity, then silence before eruption.

As seismic activity changes, the noises volcanoes make change from thumps or drum beats to quick successions of tremors. Then, right before volcanic explosion, the noise stops abruptly.

These trempors rise in pitch due to subterranean magma shifts.

The magma is feeding pressurized motlen rock toward the surface of the volcano before an eruption. As the magma flows through cracks in the core of the mountain, it generates small tremors and earthquakes, which create noises that rise in pitch slowly.

The new knowledge is part of a study of tremors before the 2009 eruption of Redoubt, a volcano in Alaska.

"The frequency of this tremor is unusually high for a volcano," explained Alicia Hotovec-Ellis, a University of Washington a doctoral student and researcher involved in the study.

"Because there's less time between each earthquake, there's not enough time to build up enough pressure for a bigger one. After the frequency glides up to a ridiculously high frequency, it pauses and then it explodes."

Earthquakes also preceded eruptions during earthquakes such as Mount St Helens in April 2005.

However, the new study analyzed the frequencies of the tremor, showing that they were higher and higher, and chronicled their abrupt stop less than a minute prior to the eruption of the volcano.

Dr Marie Edmonds, a University of Cambridge scientist who wasn't involved in the study, said that this work may help scientists predict volcanic eruption.  "This work is probably the most intensive treatment of this phenomenon. If you can get an idea of what is causing these types of patterns then you have a route to prediction of volcanic eruptions."

More research needs to be done. Edmonds added, "The question that arises is whether you can ever get these sorts of patterns without an eruption following?