“Popocatépetl provides an excellent example of how the perception of risk is strongly influenced by the recent eruptive history and not by a full geological record, particularly among the people dwelling in the endangered areas and the media.”
I think the above is probably a good quote that describes one of the biggest issues that geologists face when trying to properly educate the public about volcanoes, and more specifically Popocatepetl. In 1994 when Popocatépetl reawakened from it’s 60+ year slumber, it was only natural for the media to be in an uproar thinking only of the worst case scenario. Almost 20 years later, the opposite is more likely to be true where it’s hard to get locals excited or worried about the gigantic volcano lurking right next door to Mexico’s two largest cities.
Popocatepetl’s Recent History
While some locals near Popo may (improperly) believe the giant volcano to be a sheep in wolf’s clothing, it’s not hard to see why they might think this. Since reawakening, Popocatépetl has been in a state of almost constant eruptive activity, mostly consisting of small explosions, daily exhalations of ash, and the occasional vulcanian eruption that occurs when a lava dome collapses. Activity of this nature may occasionally appear like a larger eruption during a dome collapse, but the largest eruption since 1994 has been a small VEI-3 eruption that forced evacuation of nearby towns, yet resulted in very little actual disruption.
Since the initial evacuation, there have been subsequent evacuations that were largely unnecessary due to weak explosions causing very little damage. This causes problems for local authorities as they enter into a “boy who cried wolf” situation, in which locals may be reluctant to leave in the event of an actual larger eruption. The problem here, is the evacuations are important since it’s not really possible to predict whether a small explosion will occur, or a much larger eruption, which Popocatepetl certainly is capable of.
Popocatepetl’s Rather Violent Past
While Popocatepetl has been frequently active with smaller eruptions throughout it’s entire history, the smaller eruptions are likely a cyclical part of an eruption process that leads towards much larger plinian eruptions. In the last 23,000 years, there have been at least 7 Plinian eruptions from Popocatépetl that have been determined by finding ash flow deposits and pumice falls in the region around the volcano (Siebe et al., 1996; Siebe & Macías, 2004). Prehistoric and historic eruptions repeatedly formed large volumes of hot pyroclastic flows and air falls that extended 20 km southeastwards and 10 to 15 km northeastwards (Source).
For some volcanoes, sending pyroclastic flows 20km away would simply be an awesome and exciting event to watch from afar. For Popocatepetl, this is a very different scenario as 30 million people live within view of the volcano. Puebla (2 million population) is situated approximately 45km from the volcano’s eastern flank, and Mexico City (8+million population) sits approximately 55km Northwest of the volcano.
How Lateral Blasts and Landslides Change Everything
So if the pyroclastic flows only reach on average 15-20 km from the volcano during a large eruption, the major cities should be okay, right?
This is largely true with a few very relevant exceptions. First, there is still a large population living much closer to the volcano, including towns directly on the slopes of the edifice. But more importantly, Popocatepetl likes to aim it’s eruptions like cannons, instead of shooting straight out from the top. In other words – the massive peak likes to lose it’s side, forming lateral blast eruptions similar to Mt. St. Helens in 1980.
Lateral blast eruptions are interesting since much more of the eruptive power is directed in a single direction radiating sideways from the edifice. Whereas a normal eruption goes straight up, then disperses around the edifice, a flank eruption can send debris flows, pyroclastic flows, and landslides an incredibly far distance from the point of eruption.
The first large Plinian eruption that occurred roughly 23,000 years back created a debris avalanche that is simply massive. To form a comparison, Mt. St. Helens’ 1980 plinian flank eruption traveled approximately 23km from the summit, with the total volume approximating 2.5 cubic kilometers of material. Popocatepetl’s debris avalanche traveled as far as 70 km southeast of the summit, and totaled a volume of roughly 30 cubic kilometers of material, more than 10 times the size of St. Helen’s 1980 landslide eruption. The flow covered an area over 300 square kilometers, and the ensuing flank collapse produced a caldera structure roughly 6.5 x 11 kilometers in size. Source. Since the first episode, there have been at least two more flank collapse eruptions of the edifice, likely on a smaller scale than the original.
So while it’s unlikely that any normal pyroclastic flows from an eruption would reach Mexico City or Puebla, if a flank collapse occurs pointing to either the northwest or the east, all bets are off in terms of how far the eruption and debris flow would reach. Also, given the current height of the edifice, it would be pretty safe to assume that any collapse of a mountain as large as Popocatepetl would be nothing short of enormous.
What Causes the Frequent Flank Collapses?
As with most things relating to volcanoes, there are many factors that influence events of this nature. With that said, Popocatepetl has the age old issue of being too big to carry it’s own weight. Unlike skyscrapers, volcanoes aren’t engineered to stay standing upright. They’re built by somewhat random outpouring of lava (lava flows), intrusions within the edifice, dome building and collapse, and other similar events. Over the years, all that material piles on top until it can’t hold it’s own weight any more.
For some volcanoes, they’ll never collapse since the slopes are at such a shallow angle, the edifice is rather stable – think of Icelandic or Hawaiian shield volcanoes here. Some volcanoes simply aren’t active enough to ever collapse. Other volcanoes have magma that is too sticky and brittle to ever form a large edifice before blowing itself apart – think of Pinatubo or Taupo here. For Volcanoes like Popocatepetl, you get the perfect storm of edifice building, where the magma is runny enough that it can flow out and build a large edifice, but sticky enough that it forms a tall, steep, and brittle edifice.
Popocatepetl currently is the second highest mountain in Mexico, and one of the most prominent volcanoes in the world, standing 3,020 meters from it’s base with a pretty steep slope. When you add in earthquakes, erosion, and magmatic intrusions, you get a very instable mountain that likes to topple over when a decent sized blob of magma rises from depth. This results in the lateral blasts that have been much more common at Popocatepetl than most volcanoes.
Other Risks to Surrounding Cities and Villages
In the grand scheme of things, a large flank eruption is fairly unlikely to happen any time soon, and in the event that it does, it probably won’t be aimed directly at one of the major cities in the area. But that doesn’t mean there aren’t major risks from a standard large or mid-sized eruption of Popo. Ancient archaeological sites have been discovered beneath lahar deposits caused by the large eruption that occurred around approximately 800 a.d.
More than just lahars and ash flows, ash-fall is often an overlooked problem from large eruptions. If significant ash were to fall in a city such as Mexico City, how would they accomodate for it? Most buildings are not designed to carry an additional 2 foot layer of rock on their roofs, which results in the collapse of many small or mid-sized buildings. Would clean drinking water be available for the population of 8+ million residents in the municipality alone? These are some of the questions that become much more relevant when thinking of eruptions near large urban populations.
Geological Environment and Ruminations on Future Activity
Popocatepetl has been studied extensively since it’s reawakening in 1994. While there have been many detailed studies performed, there hasn’t been a lot of conclusions reached on how large the magma chamber is, and exactly how deep the chamber is. What is known however, is that the magma chamber is deeper than normal, sitting more than 6km below the edifice. Above the magma chamber, most geologists have concluded that there is a complex system of sills and dikes above the primary magma chamber.
Seismicity occurs in two primary areas below the volcano. The first area is predictably in a pocket below the summit vent. Interestingly, there is another pocket of seismicity that occurs to the southeast of the main cone, close to the base of the volcano. This seismicity sits near a northeast trending fault. While the fault is thought to be tectonic in nature, it’s formation and activation relates directly to rising magma within the edifice, or relaxation of the mountain as a whole (source). It’s not yet known whether this fault is a sign of growing instability or not, but it leaves some room for speculation at the bare minimum.
While the volcano has reawakened since 1994, there has interestingly been very little inflation despite the renewed volcanic activity. The lack of inflation is a bit of a mystery, but some speculate it may be a result of the extra-deep magma chamber, and the lack of a shallow reservoir for magma to accumulate.
One interesting note is that most volcanologists consider the current activity of the volcano as a continuation of a cycle that started after the last plinian eruption occurred roughly 1100-1300 years ago. Eruptions have seemed to be cyclical in this regard with Plinian eruptions occurring around 800 AD, 400-800 BC, and 3100 BC. This leaves an approximate interval period of 1200-1800 years between large eruptions, which is somewhat similar to Krakatoa and Pinatubo, although this time period is subject to change at any given time.
Another interesting thing I noticed which is 100% speculation is that there appears to be a vent of some sort opening at the base of the volcano. This may just be fire or fumaroles degassing more vigorously during an eruptive sequence, or it may be the initial start of a ring fracture. It may be nothing, but it’s at least worthy of discussion. Look carefully at the base of the edifice during the eruption, and you can see an area where gas seems to be vigorously outpouring through cracks in the mountain. The area this occurs in is slightly north on the eastern flank of the edifice. See the video below for reference.
Someone was kind enough to plot out some faults as they would roughly appear on the summit. It definitely seems possible that the degassing could be coming from the middle fault on the mountain in the image below, although the likely cause is debris and volcanic bombs that landed in the region of heavy smoke.