Guttural screamer (Image: Game McGimsey/USGS)

Several volcanoes erupted in 2009 – but only one screamed in the process. Its unique guttural howls provide an unrivalled glimpse into the heart of a volcanic eruption, and may hold clues to the driving forces that cause volcanoes to blow their tops.

The Redoubt volcano near Anchorage in Alaska sprang to life in March 2009, exploding more than 20 times during a two-week-long eruption that spewed ash some 15 kilometres upwards into the stratosphere.

Before many of those explosions, local seismic stations detected a flurry of tiny tremors of magnitude 0.5 to 1.5. In the final minute before an explosion, the earthquake frequency peaked at about 30 events every second, which meant the seismic waves they generated merged together into a continuous stream. This then gave way to 30 seconds of eerie silence just before the explosion.


The seismologists studying Redoubt have dubbed the stream “the seismic scream“, because it built to a crescendo of increasing pitch, entering the lowest range of human hearing (listen to the scream here, sped up 60 times).

Surface hum

If you could somehow stand – or swim – inside the volcano’s magma chamber, you would have heard the “screams” as a loud continuous rumbling. By the time the sound reached the surface, however, it would have been dampened such that a person standing on the volcano’s flanks would probably only just be able to hear it – so more like a hum than a scream, says Eric Dunham at Stanford University in California, who is part of the team analysing the vibrations.

The phenomenon could tell us a lot about what was going on in the heart of the Redoubt volcano, says Dunham. Analysis of the seismic readings shows the tremors occurred some 2 kilometres beneath the crater, very near to where the magma chamber feeds into the base of the volcanic conduit, the pipe the magma spurts up to reach the vent at the volcano’s peak.

To work out why Redoubt screamed, Dunham and his colleagues generated a mathematical model of the seismic activity. Their work shows how a build-up of pressure within the conduit could have increased friction between the surfaces in the heart of the volcano, causing them to slip past each other in a quickening sequence of judders just before it exploded.

Exactly which surfaces scraped against each other is still unclear. “But it’s fun to speculate,” says Dunham.

It might be that the magma was thick enough to rasp against the walls of the conduit rather than flowing smoothly past them during its upward journey.

Magma howling

Alternatively, a sudden change in pressure in the run-up to eruption might have partially crystallised and stiffened some of the magma, forming a plug that blocked the conduit. The seismic noise may be a howl of protest as the rest of the magma scraped past the blockage on its journey upwards.

This scenario might also explain why the volcano fell silent before exploding, says Dunham. If the blockage eventually fractured and fragmented, the conduit would no longer generate earthquakes or screams – and at the same time, the magma would suddenly be free to race upwards and explode out of the vent.

Both scenarios make sense, says Helena Buurman at the University of Alaska Fairbanks, who has studied the 2009 Redoubt eruptions. She says that the explosions that followed the volcanic screams during the two-week eruption involved noticeably thicker magma than the entirely silent explosions during that time. “This suggests to me that the stick-slip mechanism between the magma and the conduit or even the brittle failure of the magma itself is the most likely explanation,” she says.

The 2009 Redoubt eruption is the only known example of a volcano screaming but establishing the exact cause of the screams – and the silence – will prove invaluable, because so little is known about what goes on at the heart of a volcano. “This region is often several kilometres beneath Earth’s surface, so it is quite challenging to study,” says Dunham. The screams may hold the key to working out how pressure changes in the heart of a volcano just before an eruption, and that could help establish why some events are particularly destructive.

Journal reference: Nature Geoscience, DOI: 10.1038/ngeo1879