Volcanologists have formed the best picture so far of the plumbing hidden beneath Mount St Helens, a volcano in Washington State that killed 57 people when it erupted in 1980.
Eric Kiser at the University of Arizona in Tucson and his colleagues set off small explosions around the volcano and traced how the resulting seismic energy raced through the ground. The team’s analysis showed that the seismic waves slowed as they passed through a particular patch between 3.5 kilometres and 14 kilometres below the ground’s surface. Seismic waves move more slowly when travelling through hot material, suggesting that the patch represents the volcano’s magma chamber.
The seismic waves slowed the most — representing the largest volume of molten material — at depths between 4 kilometres and 6 kilometres. That’s consistent with scientists’ best estimates of the source of the magma for most of the volcano’s last seven major eruptions.
Monitoring the upper levels of the magma chamber could help to improve forecasts of future eruptions.