Jan. 23, 2009 -- In May of 1960, the most powerful earthquake on record rocked southern Chile. At magnitude 9.5, the quake killed thousands of people and sent a massive tsunami rippling across the Pacific Ocean.
It also triggered a cluster of volcanic eruptions up and down the Andes mountains that persisted into the spring of 1961, according to new research.
Scholars have puzzled over whether earthquakes can cause eruptions for almost two millennia; in the 1st century, Pliny the Younger wrote about the relationship as a possible cause of the eruption of Mt. Vesuvius in 79 A.D.. Similarly, Charles Darwin reported that volcanoes in Chile awoke violently following an earthquake during his visit to Chile aboard the Beagle in 1835.
In recent years, scientists have worked out a solid correlation; volcanic eruptions in a given region are far more frequent for the few days following an earthquake than they are otherwise. Now Sebastian Watt of Oxford University and a team of researchers are adding a new twist -- some of the world's biggest tremors can make volcanoes blow their tops for up to a year afterwards.
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When the team measured earthquake activity against volcanic records southern in Chile for the last 150 years, they found two large clusters of eruptions occurred following quakes -- one after an 8.3 temblor in 1906, and again after the 1960 mega-quake.
In the southern half of the Andes, 25 historically active volcanoes produce a total of 1.3 eruptions per year, on average. But each time a huge quake rattled the mountains, it triggered between seven and eight eruptions. The team calculates the odds the events are just a coincidence are around one in 10,000.
"At no other time in the record do we see this level of activity," Watt said. "The next largest number of eruptions in a 12-month period is four."
The causes of the correlation remain almost as mysterious as they were in Pliny's day, though scientists have identified two likely culprits. Strong quakes centered close by volcanoes can open fractures in the fiery mountains, making it easier for pressurized, molten rock to explode toward the surface.
Further from a quake, deformation is less likely, but seismic waves still pack enough punch to shake the magma beneath a volcano. Liquid rock is full of dissolved gases, and agitating it is like shaking a bottle of seltzer water -- bubbles form, priming the system for an eruption.
But whether one process leads to the eruption, or both are important, is still unknown, Michael Manga of the University of California, Berkeley said. Mostly that's because volcanoes aren't well understood.
"This is deeply connected with improving our understanding of how volcanic systems work. In a sense it's a large-scale natural experiment," he said. "You take your volcano, you hit it a little with an earthquake, and you see how it responds."
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