Mexican authorities on Sunday raised the warning level for the Popocatepetl volcano to one step below red alert as smoke, ash and molten rock spewed into the sky posing risks to aviation and far-flung communities below.
The huge volcano that towers above Mexico City is considered one of the most dangerous in the world because some 25 million people live within a 60-mile radius.
Sunday’s increased alert level – to ‘yellow phase three’ – comes a day after two Mexico City airports temporarily halted operations due to falling ash, which also forced 11 villages to cancel school sessions.
The yellow warning means ‘remain alert and prepare for a possible evacuation,’ according to the warning system developed by Mexico’s National Disaster Prevention Center.
The yellow phase three activated Sunday means ‘intermediate to high activity’ and is triggered when the volcano shows ‘significant explosions of increasing intensity that shoot fragments (of rock) over considerable distances.’
The next step, a red alert, triggers mandatory evacuations, and dozens of shelters have already been opened in areas surrounding the crater as a precaution.
The residents of Mexico City and the villages close to the foot of Popocatepetl aren’t the only ones keeping a close eye on the towering peak.
Every time there is a sigh, tic or heave in the volcano, there are dozens of scientists, a network of sensors and cameras, and a roomful of powerful equipment watching its every move.
The 17,797-foot volcano, known affectionately as ‘El Popo,’ has been spewing toxic fumes, ash and lumps of incandescent rock persistently for almost 30 years, since it awakened from a long slumber in 1994.
The volcano is 45 miles southeast of Mexico City, but looms much closer to the eastern fringes of the metropolitan area of 22 million people.
The city also faces threats from earthquakes and sinking soil, but the volcano is the most visible potential danger – and the most closely watched.
A severe eruption could cut off air traffic, or smother the city in clouds of choking ash.
Ringed around its summit are six cameras, a thermal imaging device and 12 seismological monitoring stations that operate 24 hours a day, all reporting back to an equipment-filled command center in Mexico City.
A total of 13 scientists from a multidisciplinary team take turns staffing the command center around the clock.
Being able to warn of an impending ash cloud is key, because people can take precautions.
Unlike earthquakes, warning times can be longer for the volcano and in general the peak is more predictable.
Monitoring gases in nearby springs and at the peak – and wind patterns that help determine where the ash could be blown – also play a role.
The forces inside are so great that they can temporarily deform the peak, so cameras and sensors must monitor the very shape of the volcano.
Authorities came up with the simple idea of a volcano ‘stoplight’ with three colors – green for safety, yellow for alert and red for danger – to explain the situation to the millions of people who have grown so used to living near the dormant beast.
For most of the years since the stoplight was introduced, it has been stuck at some stage of ‘yellow.’
The mountain sometimes quiets down, but not for long.
It seldom shoots up molten lava – instead it’s more the ‘explosive’ type, showering out hot rocks that tumble down its flanks and emitting bursts of gas and ash.
The center also has monitors in other states; Mexico is a country all too familiar with natural disasters.
Because the city’s soil is so soft – it was built on a former lake bed – a quake hundreds of miles away on the Pacific coast can cause huge destruction in the capital, as happened in 1985 and 2017.
A system of seismic monitors along the coast sends messages that race faster than the quake’s shock waves.
Once the sirens start blaring, it can give Mexico City residents up to half a minute to get to safety, usually on the streets outside.