By Marta Serafinko
April 28 (Reuters) - For people living in communities in the shadow of a volcano, a long silence may give the impression that the geological beast is not merely sleeping but has gone extinct. But, as research detailing the life history of one in Greece shows, a volcano can slumber for more than 100,000 years before reawakening.
The new findings involving the Methana Volcano, located roughly 60 km (37 miles) southwest of Athens, contradict the notion that most volcanoes may be considered
"extinct" if they have not erupted for 10,000 years.
The scientists reconstructed 700,000 years of activity for Methana, marked by eruptions separated by long pauses. The findings show that its longest quiet period - lasting from about 280,000 to 168,000 years ago - was not a sign of extinction but rather a phase of substantial underground magma accumulation.
"This long period of quiescence at Methana happened during the prehistory, so we are piecing it together based on the chemical evidence preserved in the rocks and minerals. To understand what happened under Methana, we need to picture the volcano as the tip of an iceberg: at the surface we see only a little bit of it, while most of the igneous system is underground," said ETH Zürich volcanologist Răzvan-Gabriel Popa, lead author of the study published in the journal Science Advances.
Earth is made up of layers, including the outer crust we live on and the mantle immediately underneath, where magma forms. Volcanoes erupt when rising magma "floods" their underground chambers and spills excess material toward the surface, while an absence of magma supply causes them to fall quiet and gradually die.
"What we have now found is that in subduction zones, volcanoes can go quiet even when the mantle produces a lot of magma, but with a twist: this magma is superhydrous, and the volcano doesn't die, but it thrives, while taking a nap," Popa said.
Superhydrous magmas are extremely water-rich and are thought to drive this process.
"They ascend through the crust, they start bubbling like a fizzy drink," Popa said. "This gas bubbling triggers crystallisation, making the magma sticky and viscous, and it slows down a lot - by a factor of 100 to 1,000 - and becomes so lazy ... it can't continue towards the surface."
Instead, Popa said, the magma becomes trapped.
"Since the magma chamber can't evacuate all that excess material, no eruption happens, and the reservoir accumulates this crystalline, sticky magma that helps it grow," Popa said.
Over time, this could lead to more voluminous and potentially more energetic eruptions.
To reconstruct this history, the researchers dated more than 1,250 crystals of the mineral zircon in volcanic rocks and, where zircon was absent, used other minerals such as ilmenite to track magma chamber activity even during volcanically quiet periods.
Popa said such "silent" magma buildup can be detected with sensitive instrumentation.
"Magma accumulation at depth often triggers earthquakes that may be too small for us to perceive, but seismometers record them easily. The ground may also bulge by only a few centimetres per year, yet satellites and GPS can detect those changes," Popa said.
While Methana lies relatively close to the Greek capital, Popa said the volcano is not expected to pose significant threats at this stage, with any future eruptions likely to take the form of mere lava flows similar to those seen in the past.
However, Popa added, the process could be more widespread than previously thought, with many seemingly inactive volcanoes potentially continuing to build magma chambers underground, particularly in regions such as Greece, Italy, North and South America and Japan.
The scientists are now planning a research campaign on Ciomadul in Romania's Eastern Carpathians, a volcano that has been quiet for nearly 30,000 years but still may host an active magma chamber underground.
"It's important for our society to understand that for volcanoes, quiet doesn't always mean safe," Popa said.
(Reporting by Marta Serafinko in Gdansk, Poland; Editing by Will Dunham)
