How Often Do Volcanos Erupt? Calbuco In Chile Has People Wondering About Frequency

In the U.S., the natural disasters we worry about tend to be of a certain uniformity. If you live on the East Coast or near the Gulf of Mexico, you own storm windows to barricade your house down for hurricane season. In Tornado Alley — well, they call it Tornado Alley for a reason. And on the West Coast, all the latest buildings are earthquake-proof. But while volcanic eruptions are possible in the US, it's a form of disaster, unfortunately, that strikes South American countries pretty often. There are a number of active volcanoes in South America, and many of them are in Chile. This week, the Calbuco volcano erupted twice, spewing billowing ash into the sky and forcing 4,000 people to evacuate the area. Though there have not been any injuries or fatalities reported, the once-bustling city of Ensenada is now a ghost town, coated in volcanic ash.

Until this Wednesday, the Calbuco volanco had not erupted for 42 years, but in the space of two days, it went off twice. On Wednesday afternoon, it blasted a plume of ash more than six miles into the air. Then in the wee hours of the morning on Thursday, it went off again, forcing officials to close access to the areas surrounding the volcano and evacuate people. The city of Ensenada was hit the hardest, lying right under the volcano. Only about 30 people were left in the tiny town on Thursday, refusing to abandon their homes or animals.

MARTIN BERNETTI/AFP/Getty Images

Ash from the volcano, which is one of Chile's top three most dangerous, has spread as far as 56 miles north. The head of the sanitary planning division of Chile's health ministry said the possibility of infections and water contamination makes it imperative that people evacuate the area.

How often does this kind of eruption happen, and how dangerous are the consequences of ash inhalation? According to the United States Geological Survey, the effects of long-term ash inhalation are still unknown, but "short-term exposures to ash, however, are not known to pose a significant health hazard."

According to the International Volcanic Health Hazard Network, volcanic ash falls tend to elicit disproportionate anxiety for the relatively low level of hazard they pose.

In most eruptions, volcanic ash causes relatively few health problems, but generates much anxiety. People can be more fearful of the health hazards of volcanic ash and gases than of the risk of dying from more major hazards, such as pyroclastic flows. However, ashfalls can affect very wide areas around volcanoes and may cause major disruption to normal living.

Seismic activity from the volcano, which is a stratovolcano, was recorded at levels as high as 3.6. The explosion lasted about six hours, but this time lapse video shows it unfolding dramatically:

Rodrigo Barrera on YouTube

How common are events of this magnitude? The level of seismic activity is not at all uncommon: the scale of moment magnitude, a modified version of the Richter scale, goes as high as 10, and events under magnitude 6 aren't even listed on the Wikipedia page for earthquakes in 2015, which has to tell you something. And there were six volcanic events in the week from April 15 to April 21, none of which we heard about on the news because they did not emit the same large amounts of ash as Calbuco.

On another scale, this eruption is more of a big deal. The Volcanic Explosivity Index is the way geologists measure the size of eruptions, on a scale of 0 to 8. There is no definitive VEI rating for Calbuco's eruptions yet, but CNN includes the events on a list of notable blasts since 1902, most of which are rated 4 and above, so it is likely rated a 4. According to the USGS, in the 10,000 years leading up to 1994, there have been 278 eruptions rated VEI 4, but only 84 rated VEI 5. The eruption of Mount Vesuvius was likely a 5, about 10 times more powerful than a 4, and it left the city of Pompeii covered in volcanic ash and lava flows that had dire consequences for its residents.

Thankfully for the citizens of Ensenada, this ash fall — though dramatic — will hopefully be relatively benign.