Volcanoes: The prophetic silence before the violence

Tilca volcano in Nicaragua erupting(Credit: Diana Roman/Carnegie Institution of Science)

We often think of volcanoes as sleeping giants that could erupt in a rain of fire with little notice. But it turns out that restless volcanoes do, in fact, give us some notice about when they're going to blow – and it's not quite the kind of warning you'd expect. Scientists from around the world intensively monitored an active volcano in central America and say they've begun to crack the code.

The team set up a comprehensive sensor network around the active Telica volcano in Nicaragua to collect data from a sequence of eruptions. They discovered that the volcano went relatively quiet just before eruptions.

"It is the proverbial calm before the storm," explained Carnegie Institution for Science volcanologist Diana Roman. "The icing on the cake is that we could also use these quiet periods to forecast the amount of energy released."

Unlike dormant volcanoes that exhibit clear changes in seismic activity before most eruptions, discerning similar changes in an already active volcano is more difficult. That's because they tend to register much more seismic activity, making it harder to establish a baseline for relative calm.

Think of it as watching two people sleep and predicting when one will wake. It would be easier to predict that a sound, motionless sleeper is about to wake at the first sign of motion. This would be harder to do with a restless sleeper who tosses and turns frequently while still asleep.

The team tracked 50 explosions during the broader eruption period at Telica and found 35 were preceded by quiet periods of a half-hour or longer. Conversely, only two of the 50 came without any quiet "warning" period. They measured quiet periods ranging from six minutes to over 10 hours leading up to the largest explosion.

In other words, longer quiet periods at an active volcano could mean the silence will be broken by a particularly powerful eruption.

The researchers say that the calm periods are linked to vents being sealed off — perhaps by mineral precipitation or the settling of rock near the volcano's surface. This would trap gas underground but also lead to a build-up of pressure that would eventually blow. The longer the quiet period, the more pressure that builds up and the more violent the resulting eruption.

Like another recent method that uses atomic clocks and the theory of relativity, the measurement of time is key to the prediction process, although in a much more straightforward manner.

Roman worked with team members from Oxford, Penn State and national universities in Iceland and Nicaragua to publish the research in the most recent issue of the journal Earth and Planetary Science Letters.

Roman says their monitoring method could be used to create short-term eruption warnings.

"Our work has now quantified that quiet periods can be used for eruption forecasts and that longer quiet periods at recently active volcanoes could indicate a higher risk of energetic eruptions," she says.

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