A subterranean "highway from hell" enables some volcanoes to erupt at super-speed, a discovery that also offers options for predicting the danger, according to a study published in Nature.
Volcanoes disgorge molten rock generated within the mantle, the layer sandwiched between the earth's crust and fiery core. The magma gathers in a chamber beneath the volcano, progressively rising until the pressure, which can be detectable over time by rumblings at the surface, becomes too great and an eruption occurs.
Conventional wisdom has it that the mantle magma creeps upwards before it reaches the chamber, lingering for long periods several kilometres beneath the volcano.
But the research, published on Wednesday, suggests there are channels that run directly through the crust from the mantle to the magma chamber.
As a result, a volcano can be recharged and primed for action in a matter of months, and a clear danger to humans living nearby.
The evidence comes from traces of an eruption between 1963 and 1965 of Irazu, a Costa Rican strato-volcano located on the notorious Pacific "ring of fire".
A team led by vulcanologist Philipp Ruprecht, of Columbia University's Lamont-Doherty Earth Observatory in New York, analysed ash from an 2010 expedition to Irazu. Buried in crystals of the volcanic mineral olivine were spikes of nickel, which is a trace element in the mantle.
The spikes were a sign of an extraordinarily fast ascent, as a slower rise would have meant that the nickel would have melted and diffused through the crystals.
The magma charged 35 kilometres through the crust in just months, the team calculated.
"There has to be a conduit from the mantle to the magma chamber," geochemist Terry Plank said. "We like to call it the highway from hell."
The findings may explain why seismologists have sometimes detected mysterious earthquakes occurring at depths of 20 to 30 kilometres several months before great eruptions. These may herald mantle magma blasting its way through the hidden channels.
Examples include the 1991 eruption of Mount Pinatubo in the Philippines and Iceland's Eyjafjallajokull in 2010.