Those of us who live in tornado-prone areas know the ‘calm before the storm’ adage all too well. Dark clouds warn of a stormy day and the short period of calm before just makes you feel uneasy. We can attach this same adage to another natural phenomenon. And it’s not one you would expect. Volcanoes.

Scientists generally know when a volcano can potentially erupt. They can’t predict exactly when, but they can look for the tell-tale signs. Increased seismic activity, changes in the surrounding ground and amounts of gas escaping.

Predicting a volcanic eruption is an end goal for many scientists. You would think any potential forecast would center around the increase in seismic activity. But, it’s actually the opposite.

New research shows lulls in activity right before an eruption. And how long this seismic silence lasts influences the amount of energy released when a volcano erupts.

Scientists from Carnegie, Penn State, Oxford University and the University of Iceland closely monitored 50 eruptions at the Telica Volcano in Nicaragua in 2011. Telica was chosen because it’s one of the more restless volcanoes out there. Near constant seismic activity and gas emissions make forecasting any potential eruption difficult.

During one month in 2011, scientists monitored 50 small to moderate ash eruptions. They didn’t notice and deep seismicity or major gas changes. That meant fresh magma wasn’t the driver of these eruptions. It was likely caused by an increase in gas pressure after vents were sealed off.

Out of the 50 explosions, scientists noted quiet periods in 48 of them. But they don’t usually last long. 35 of them had quiet periods lasting at least 30 minutes. The other 13 at least five minutes. The shortest quiet period came in at 6 minutes before an explosion. The longest? 10 hours.

Why do longer periods of calm equal stronger explosions? It’s actually quite simple. Remember, Telica’s eruptions were driven by increased gas pressure. The calm periods are caused as vents are blocked off. The longer these vents stay blocked, the more the pressure builds and the stronger the explosion.

“What is clear is that this method of careful monitoring of Telica or other similar volcanoes in real time could be used for short-term forecasts of eruptions,” Carnegie volcanologist Diana Roman said. “Similar observations of this phenomenon have been noted anecdotally elsewhere. 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.”

These predictions probably aren’t enough to be used for evacuations, but it does give scientists valuable insights into what’s going on deep inside volcanoes and more importantly, when.

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