An international team of scientists studying the Soufriere Hills volcano in the Caribbean's Leeward Islands has found deep magma controls eruption cycles.

While the volcano exhibits cycles of eruption and quiet, the scientists discovered magma is continuously supplied from deep in the Earth's crust, but a valve acts below a shallower magma chamber, releasing lava to the surface periodically.

"Continuous records of surface deformation are available for only a few volcanoes," said Professor Derek Elsworth of Pennsylvania State University. "The Soufriere Hills volcano has been erupting since 1995 and provides a peek into the processes occurring deep beneath this stratovolcano."

Stratovolcanoes, said Elsworth, are cone-shaped with steep sides created by episodic eruptions of magma that flow down from the cone a short way and create layer upon layer of volcanic material.

The researchers measured ground inflation and deflation as a window to the transfer of magma deep within the crust. "It is apparent," they wrote, "that the major changes in magma storage that have supplied the eruption are from depth, with the lower reservoir contributing only a third of the erupted volume,"

The study is reported in the journal Science.