Some days ago, a video called my attention on Facebook, a video of a beautiful explosion of Popocatepetl volcano in Mexico. I ended up in a YouTube channel called Volcano Time-Lapse. This youtuber had dubbed the explosion as the perfect explosion, and for a good reason. At night-time, with not a cloud in the sky, the magnificent white snow-clad Popocatepetl stood in full view of the Tlamacas webcam 4.5 kilometres away, when a brilliant explosion scattered countless particles of light high up into the air, some beyond the webcam’s view, over a kilometre above the crater rim, which then came down like a meteor shower over the upper flanks of the volcano, lighting them up to a distance of up to 2 kilometres from the active crater. The perfect explosion was on May 15. There had been several explosions that day, although not all as photogenic. After learning this, I tried to find more information on Popocatepetl on the MOUNTS Project page, which shows multiple satellite-based data streams, like thermal anomalies or SO2. A rapid increase in sulphur dioxide emissions was apparent since the start of 2023, with a particularly steep rise during February-March, and a peak from 5 May to 8 May, at 4500-5500 tonnes of SO2 per day. How much is this? Not particularly high for Popocatepetl, who is an extremely proficient long-term, sulphur dioxide degasser. But much higher than the 200-500 tonnes per day that were typical in January.
I was curious about this rise, knowing it could mean that something was building up. However, we have had an enormous amount of volcanic unrest towards the end of 2022, and the start of 2023, but very little actual volcanic activity. Cotopaxi, Villarrica, Laguna del Maule, Nevado del Ruiz, Aniakchak, Tanaga, Trident, and Chiles-Cerro Negro have all rumbled but so far not erupted in a substantial manner. This comes to show that volcanic unrest does not often culminate in eruptions, or at least not immediately. But finally, one volcano has provided.
There are four different webcams run by the CENAPRED pointing at Popocatepetl, but that doesn’t help much if the sky is cloudy. Right now, as I write this article, May 22, that happens to be the case. But I believe the volcano is probably erupting behind the clouds. It has been nearly two days of eruption, with a continuous plume of ash pouring into the sky, and lava fountains frequently playing inside the crater. The upper wind has blown eastwards/northeastwards all the time, carrying the ash into Puebla, and protecting Mexico City of this huge, gray inconvenience. People are referring to Puebla as Silent Puebla and discussing the possibilities of selling the ash. There have been some impressive eruption moments, bordering dangerous for the local inhabitants, where lava fountains were continuously showering the uppermost slopes of the volcano in lava bombs. There is a risk for pyroclastic flows if this goes on for an extended period. But for the most part, the eruption has been of a very low intensity. I have watched the webcams several times, but haven’t noticed any instances of the lava bombs flying as far as they did during the perfect explosion of May 15. My guess is that lava is rising up in the crater, and filling it with a lava dome, and in the process is ejecting some ash and lava bombs. But I’m not even sure of this, and much less do I know where will it lead, which makes me all the more curious about it. Will the crater overflow? Will a series of large vulcanian explosions destroy the dome? Could it go full plinian? Or will nothing happen?
The Trans-Mexican Volcanic Belt
I wouldn’t miss an opportunity to talk about the volcanic arc that I personally find one of the most interesting in the world. The Trans-Mexican Volcanic Belt. And it is interesting to consider Popocatepetl in the context of this fascinating volcanic arc.
Like other volcanic arcs, the Trans-Mexican Volcanic Belt is located over a subduction zone. However, unlike most other volcanic arcs, which are more or less parallel to the trench where oceanic crust is subducting, the TMVB is oriented obliquely with respect to the trench. This is probably due to subduction becoming shallow to the southeast, making a small flat slab. Even so, this is a pretty unique shape that does not show up in other flat slab areas of the world. The chain of stratovolcanoes is not particularly long, at 670 km length from Fuego de Colima to Pico de Orizaba, cutting roughly E-W across Mexico, although a few other atypical volcanoes extend the belt a little further both to the east and to the west.
To the east we have the San Martín Tuxtla volcano, which is a shield volcano, constructed of numerous scoria cones, and aa-dominated lava flows, with a composition of alkali basalt and basanite, primitive and alkaline types of magma. It is not unusual for volcanic arcs to have back-arc provinces of basaltic alkaline lavas, and the San Martin Tuxtla volcano is not particularly large either. Although it is a young vigorous shield volcano that has erupted twice historically, in 1664 and 1793-96. In Mexico, the basaltic volcanoes are located outside of the main orogenic area of folding and thrusting of the crust. 3 other major Cenozoic alkaline provinces occur along the eastern coast of Mexico, all outside the main orogen. At least one of them, the Sierra de Tamaulipas province, is still just barely active in the form of the Aldama Volcanic Field. These three provinces are outside the TMVB, to the north.
The western end of the TMVB turns into a graben, the Tepic-Zacoalco Rift. The deepest portion of this rift is 200 km long and 70 km wide, but it continues eastward, slowly fading away into the central section of the TMVB. This rift is part of a wider area of extensional tectonics. It is located at the exact southern end of the Basin and Range Province, and has the mid-ocean ridge of the Gulf of California only 400 km away. The Basin and Range is a vast area of horsts and grabens that extends all the way from Tepic-Zacoalco in the south, to Idaho and Oregon in the north. Two grabens radiate southward from the Tepic-Zacoalco rift towards the Middle America Trench and isolate the area known as the Jalisco Block. Colima volcano fills one of these two grabens.
The Tepic Zacoalco Rift was the site of a rhyolite ignimbrite flare-up 5-3 million years ago. Now it is home to a characteristic style of volcanism. A family of several closely packed, small, central volcanoes occupy the rift. Some of them have alkaline compositions, other subalkaline, and some have both. They are often evolved, most of them have erupted rhyolite and trachyte lavas. The volcanoes are topped with small stratovolcanoes or calderas, while viscous lava flows and domes erupt along the flanks from fissure swarms oriented NW-SE, parallel to the rift. In the map, I refer to them as the rift volcanoes. Ceboruco is one of the rift volcanoes, which made a 3-4 km³ rhyodacite plinian eruption around 990-1020 AD, followed by voluminous andesite-dacite lava flows from fissures across the summit.
South of the Tepic-Zacoalco Rift, we can find a few volcanic fields, consisting of cinder cones, lava domes, and monogenetic shield volcanoes. These fields are minor, but it is not their size that is interesting but their chemistry. Some of the lavas in this area have a strongly potassic composition, occasionally phonotephrite composition according to the TAS diagram. According to the minerals they contain, some of these lavas can be classified as minettes, and one of the fields in the ultrapotassic belt, the Mascota Volcanic Field, which is active, is thought to have erupted the youngest minettes in the world. I don’t know what the exact definition of a minette is. Alkaline magmas can have rare mineral compositions, so a plethora of terms has been created to describe all these types, minette being one of them. Potassic alkaline magmas often happen in volcanic arcs or related areas, like, for example, in Italy, usually in places where the setting is unconventional, as opposed to sodic alkaline magmas that are more common in back-arcs or intraplate areas. The ultrapotassic volcanic fields of México are at the very western end of the Middle America Trench, and comprise an area of minor volcanism ahead of the more intense volcanism in the Tepic-Zacoalco Rift. This unusual setting is probably the source of its rare chemistry.
I have described the volcanoes near the E and W ends of the TMVB. But the proper arc is the 670-km section that extends from Colima to Orizaba. In this area, there are 3 types of volcanoes.
First, the rhyolite volcanoes, which are found in slightly back-arc positions. These volcanoes erupt rhyolite in the form of extensive lava flows and plinian eruptions from circumferential fissures, a bit like Laguna del Maule does as I have talked about in some of my articles. Two of the rhyolite volcanoes, Acoculco and Los Azufres, have massive uplift domes cut by normal faults, probably overlying shallow granitic plutons. Another, Los Humeros, collapsed 160,000 years ago producing a 230 km³ ignimbrite. A second collapse of Los Humeros generated a 37 km³ ignimbrite 70,000 years ago. A trapdoor resurgent dome pushed upwards the floor of the second caldera to the level of the rim. During the Holocene, Los Humeros has been extremely active and turned to more mafic compositions. Circumferential fissure eruptions from every side of the caldera have issued flows of trachyte, trachyandesite, and basaltic trachyandesite, inundating vast tracts of land with rugged black lava.
Second, we have the Mexican shield fields. A Mexican shield is a type of monogenetic volcano, typically several hundred meters tall, and several kilometres wide. A Mexican shield is enormous. They mostly consist of many piled up lava flows erupted from a cinder cone during a very long-lasting eruption, maybe a decades-long eruption, of mainly basaltic-andesite or andesite composition, although more rarely dacite or basalt. Mexican shields happen alongside smaller cinder cones and andesitic-dacitic lava domes making up volcanic fields. Most Mexican shields are found in México, as the name correctly implies, most Mexican shields elsewhere are probably found in the Cascades. The largest concentration of such volcanoes is in the Michoacan-Guanajuato Volcanic Field, which has nearly 400 monogenetic shields, plus a much greater number of cinder cones, lava domes, and maars, including the historical Paricutín and Jorullo, entirely covering an area of 220 by 160 kilometres in volcanic material, the largest volcanic system of its kind. The Chichinautzin volcanic field immediately south of Mexico City is not so large, but is much younger, and the shields are more concentrated.
And third, we have the stratovolcanoes. Popocatepetl, Pico de Orizaba, Fuego de Colima, and a few others, are classical stratovolcanoes that make tall edifices, with steep upper cones around a central crater, formed in a combination of lava flows, lava domes, pyroclastic flows, tephra layers, debris avalanches, lahars, and reworked volcaniclastic material. Most Mexican stratovolcanoes are on the large side of stratovolcanoes in terms of volume. The stratovolcanoes erupt mainly andesite and low-silica dacite. They are arranged in seven groups or complexes, most of them elongated north-south. Activity started in the north and gradually moved southward. The presently active stratovolcanoes are the southern end of these north-south complexes.
Arc volcanism in Mexico has been on and off over time. An earlier episode happened around 22-11 Ma, and formed an arc of very similar shape and location to the one that exists today, only a little smaller. The volcanoes extended E-W from the east side of Michoacán Guanajuato volcanic field to the area of Pico de Orizaba. Lavas erupted were predominantly andesites and low-silica dacites. This was followed by a hiatus in andesitic volcanism, where basaltic volcanic fields formed lava flows, or calderas produced rhyolitic ignimbrites. Arc volcanism only resumed very recently. The Michoacan Guanajuato volcanic field started to form 3 million years ago, but about two-thirds of the volume have erupted within the last 1 million years. The complexes of stratovolcanoes seem to have become active at some point between 3 to 1 million years ago. Activity in both the stratovolcanoes and Mexican shield fields migrated southward, presumably due to the steepening of the subduction, and rollback of the flat-slab, while rhyolite systems developed in the back-arc.
The Valley of Mexico
Intense volcanic activity during the Miocene and the Quaternary has almost completely concealed any basement rocks across an enormous area of the Trans-Mexican Belt. The volcanoes have altered the topography, obstructed former valleys, and led to the formation of a series of basins, filled with volcaniclastic material. Two important endorrheic basins, ringed by volcanoes, are the Oriental Basin, surrounded by Los-Humeros, Pico de Orizaba, and other volcanoes, and the Valley of Mexico Basin.
The Valley of Mexico is surrounded by volcanoes on all sides. North lie old eroded Miocene volcanic edifices, and younger Plio-Quaternary Mexican shields and cinder cones. To the west, you find a chain of several eroded stratovolcanoes, known as Sierra de las Cruces, formed over a steepening subduction, with ages ranging from 2.87 at the north to 0.39 Ma at the south. In the south stands the active Chichinautzin Volcanic Field. The cinder cones and monogenetic shield volcanoes of Chichinautzin stand taller than any skyscraper, over 1000 meters above Mexico City, and probably much higher from the bottom of volcaniclastic-filled basin, whose existence we probably owe to the very Chichinautzin Volcanic Field who forms a mighty lava dam, 20 kilometres wide and 1-2 kilometres tall, blocking drainage to the south. Some of the cones and craters of this volcanic field extend into present-day Mexico City.
A lava flow in the Chichinautzin volcanic field, around 20 AD, destroyed the city state of Cuicuilco, that at the time dominated the basin. At the time, the basin was occupied by a large lake known as Lake Texcoco. The volcanoes of Chichinautzin formed islands and peninsulas in the lake, and Cuicuilco was on the southern shore. After Cuicuilco, other city states emerged in this important cradle of civilization. This includes Teotihuacan, the largest populated center in the pre-Columbian Americas. The Aztec-capital of Tenochtitlan was also constructed here, on artificial islands within Lake Texcoco. And presently, the Valley of Mexico is home to over 20 million people.
Along the east side of the Mexico Basin lies Sierra Nevada, a series of north-south stratovolcanoes, with the main volcanoes being Tlaloc, Iztaccihuatl and Popocatepetl. Sierra Nevada means snow-clad range, which alludes to the volcanic lovers Iztaccihuatl and Popocatepetl, the second and third highest mountains in México, that are often covered in ice and snow due to their enormous height. The oldest volcano in the Sierra Nevada range is Tlaloc, which started to grow 1.8 million years ago, and was still in full activity by 0.9 Ma. Tlaloc is an edifice with shallow slopes and modified by erosion. The youngest eruptions from this volcano were rhyolitic plinian eruptions that happened only 44,000-21,000 years ago. Iztaccihuatl started to grow at 1.1 Ma. The northern lower portion formed first. The name Iztaccihuatl means white woman in Nahuatl, because it resembles a woman lying on its back, and is often covered in snow. Various parts of the woman, head, chest, knees, feet, are different volcanic peaks formed of viscous lava flows and domes. The highest peak still preserves the summit crater. The feet formed around 440,000 years ago, and the rest of the peaks are probably younger since they seem better preserved. One of the latest eruptions happened 80,000 years ago from the northern flank and formed a massive dacite coulee, up to a few hundred meters in thickness.
Popocatepetl, or El Popo as the Mexicans often refer to it, is the southernmost and youngest volcano of Sierra Nevada. Unlike Itzaccihuatl’s stratovolcano chain, Popo comprises a relatively simple cone crowned by a singular crater. Both mountains reach an enormous size and are partly joined together. Popocatepetl stands at 5426 meters above sea level, and Itzaccihuatl at 5230. The height above the pre-eruptive basement of both volcanoes is probably around 3500-4000 meters, which is very large considering that only a handful of subaerial volcanoes in the world are over 4000 meters tall. This is more than the height above the present-day surrounding basins, because they are filled with several hundred meters or so of ash, debris avalanches, reworked volcanic material, and lava flows.
The northern flank of Popocatepetl is dissected by deep valleys to a substantial elevation of more than 4000 meters above sea level. So the volcano is not particularly young. The northern flank is similarly eroded as the feet of Iztaccihuatl. So I think the main construction of Popo and the southern part of Itza, the head, chest, knees, and feet, was probably simultaneous. But I haven’t seen any ages of the oldest lavas of Popo’s cone. It probably reached an elevation similar to that of today early on, but was later destroyed and reconstructed cyclically during a series of landslides.
In the last 30,000 years, Popocatepetl has produced five VEI 5 plinian eruptions. These eruptions are the best studied of the volcano. Rare plinian eruptions are more impactful and are preserved over a wide area as layers of tephra, so they are more easy to study that the more minor, more typical eruptions.
The first plinian event happened 27,800 cal BP (calibrated years before the present, with present meaning 1950). This event is very interesting. Part of the volcano collapsed in a debris avalanche of at least 10 km3 which reached as far as 72 km downslope from the volcano, destroying everything in its path, something that would have catastrophic consequences if it were to repeat. Following the avalanche, a massive plinian eruption ensued, presumably triggered by decompression, which erupted 1.9 km³ DRE (dense rock equivalent). This is the most evolved plinian eruption of Popo, with 64.8 wt% SiO2 whole rock composition, while the other plinian events range in between 60.6-62.7 wt% SiO2. Popocatepetl does not change much in composition, so this difference is significant. It is probably some of the most evolved magma erupted by the volcano overall.
After this lateral collapse, Popo grew back. Now there is little sign of the collapse scarp and the top forms a symmetrical cone. The upper 1 kilometre of the volcano is very steep, like a huge cinder cone some 4-5 kilometres wide, and it has probably grown from proximal pyroclastic ejecta, from vulcanian explosions and lava fountains. The most typical eruption style probably consists of lava domes/ponds that fill up the summit crater, and blow up in vulcanian explosions showering the upper cone in lava bombs. Or at times, activity probably grows into continuous fountains that feed small clastogenic flows. In fact, much of the upper cone seems to be made of steeply dipping layers of welded spatter, which may have been small fountain-fed flows. The present eruption of Popocatepetl briefly behaved this way, producing continuous fountains over the rim.
The second most typical activity of Popocatepetl probably consists of lava flows. The next 1-2 km of elevation below the upper cone are covered in voluminous lava flows that reach almost 20 km away from the crater. Many of these lava flows probably issued from the summit of Popocatepetl when lava rose in the crater and overflowed. But close to the crater they are all buried in thick ejecta, so this may not have happened in a while. A few of the lava flows have clearly issued from flank vents along a NE-SW rift system.
Edit: After publishing the article, I have estimated the volumes of the two major flank eruptions: the Ecatzingo and Nealticán lava flows. I used elevation contours to get an estimate of each lava flow’s volume, which turned out to be identical at 3.6 km³. Both eruptions were long lived, particularly the Ecatzingo lava flow, piling up numerous small tongues of lava, and each fissure erupted 3.6 cubic kilometres. Nealticán is the youngest-looking lava flow of Popocatepetl, younger than Ecatzingo and the flows from the summit.
The rarest eruptions are the five full-blown plinian eruptions, even though most of the research is dedicated to them. Following the 27,800 BP eruption, the next plinian eruption did not happen until 17,000 cal BP. Authors have referred to this later eruption as the Tutti-Frutti Pumice, or as the Pumice With Andesite. This eruption is the largest and most complex of Popocatepetl. It erupted 2.9 km³ DRE. Eruption started with many thin layers distributed all around Popo, probably resulting from vulcanian to subplinian explosions. It then evolved into a fully plinian eruption where the plume was carried northwest and dropped 10-15 cm of ash across present-day Mexico City. This eruption is the most primitive composition of the plinian events, ranging in 60.5-61.4 wt% SiO2, the magma in the plinian phase was more evolved and came from a shallower reservoir than in the preceding explosions. The primitive character is best seen in the glass composition. Magma is a mix of solid, liquid, and gas. The whole rock composition refers to the solid+liquid composition, while the glass in a volcanic rock represents the part that was molten upon eruption and solidified quickly without forming crystals. So the glass composition is the composition of the molten portion of the magma upon eruption. The glass of the Tutti-Frutti Pumice has 62-63 wt% SiO2, while the other eruptions have 65.5-69.1 wt% SiO2 glasses. So the magma should have been more fluid.
The three remainder plinian eruptions happened at 3700 BC, 300 BC, and 800 AD, and had DRE volumes of 2 km³, 1.2 km³, and 0.5 km³ respectively. There is a trend of the plinian eruptions becoming increasingly frequent and increasingly small. This could be seen as a sign of escalating activity at Popocatepetl during the past 30,000 years. The last plinian eruption inundated some human settlements of the time in lahars, which are probably the greatest hazard of Popocatepetl’s plinian eruptions, since lahars could potentially reach into major cities like Puebla, or even Mexico City itself.
Probably shortly after the 300 BC plinian eruption, the Nealticán lava flow erupted from the NE flank. This lava flow is enormous. With 4.2 km³, the Nealticán lava flow involved a larger volume of magma than any of the plinian events. The eruption was long-lived and probably lasted a few or several years. One interesting thing about the flow is that the early lavas have 61-63 wt% SiO2 while the later lavas have 63-64 wt% SiO2, so that the eruption changed from andesite to dacite. This is a little counter-intuitive, since the dacite, more evolved, should have come out first. Dacite would pond in the upper part of the storage under Popocatepetl and be the first to erupt. My speculation is that the magma came from a storage somewhere else, possibly under the immediately adjacent Iztaccihuatl volcano, that erupts viscous dacitic lavas from many vents over a broad area. Popocatepetl may have drawn magma from a shallower storage than its own, under Iztaccihuatl, from the bottom up, erupting the least evolved magma first, and gradually changing towards more evolved dacites. I’m thinking of a similar relation as that of Hualca Hualca-Sabancaya, in Peru. Hualca Hualca has the magma, but Sabancaya does the erupting. Here, Iztaccihuatl would be playing a similar role as Hualca Hualca, and Popo would be playing Sabancaya.
Additionally, Popocatepetl has erupted several times historically. Not much is known about the older eruptions. Popo was dormant from 1804 to 1919. There was some minor activity in 1919-1927, including explosions and the growth of a small lava dome. From there the volcano was dormant until 1994, when it awoke with intense fumarolic activity and explosions. Since 1994, the smoking mountain has been in near continuous eruption. Lava fills up the crater, vulcanian explosions blow up the lava and deepen the crater. This repeats again and again.
The biggest development in the eruption might well be the present episode. As of now, the volcano has had sustained plumes of ash and lava fountains for 3 days in a row. I suspect the crater might be filling up with lava. A rising magma column. As far as I know, this kind of activity has not been described in this volcano before, so it is hard to know what to expect. I can see the possibility of almost anything happening, as well as nothing in particular happening. Even a plinian eruption is possible if my Iztaccihuatl speculation is correct, since vast amounts of dacitic-andesitic magma could very well start flowing into Popocatepetl. Although the most likely option based on what we know about the volcano is that some powerful vulcanian explosions will blow the lava away, or maybe that bigger lava fountains will form and shower the upper cone in spatter, with a small risk of pyroclastic flows from collapsing spatter and ejecta. Only time will tell.
To follow the eruption, the following links are most useful, which include webcams and seismograms:
I will watch with curiosity how this volcanic situation evolves, which I think could teach a lot about how this volcano works. A volcano that we don’t know much about other than the big five plinian eruptions.
Alvarez, G. M., C, M., Fucugauchi, J. U., & Uchiumi, S. (1991). Southward migration of volcanic activity in the Sierra de Las Cruces, basin of Mexico? – A preliminary K- Ar dating and palaeomagnetic study. Geofisica Internacional, 30(2), 61–70. https://doi.org/10.22201/igeof.00167169p.19188.8.131.524
Hasenaka, T. (1994). Size, distribution, and magma output rate for shield volcanoes of the Michoacán-Guanajuato volcanic field, Central Mexico. Journal of Volcanology and Geothermal Research, 63(1–2), 13–31. https://doi.org/10.1016/0377-0273(94)90016-7
Israel Ramírez-Uribehttps://doi.org/10.1130/B36173.1Claus Siebe Magdalena Oryaëlle Chevrel Dolors Ferres Sergio Salinas; The late Holocene Nealtican lava-flow field, Popocatépetl volcano, central Mexico: Emplacement dynamics and future hazards. GSA Bulletin 2022;; 134 (11-12): 2745–2766. doi:
Macías, J. L., Arce, J., García-Tenorio, F., Layer, P. G., Rueda, H. N. R., Reyes-Agustín, G., López-Pizaña, F., & Avellán, D. R. (2012). Geology and geochronology of Tlaloc, Telapón, Iztaccíhuatl, and Popocatépetl volcanoes, Sierra Nevada, central Mexico. In Geological Society of America eBooks (pp. 163–193). https://doi.org/10.1130/2012.0025(08
Siebe, C., Salinas, S., Arana-Salinas, L., Macías, J. L., Gardner, J. V., & Bonasia, R. (2017). The ~ 23,500 y 14 C BP White Pumice Plinian eruption and associated debris avalanche and Tochimilco lava flow of Popocatépetl volcano, México. Journal of Volcanology and Geothermal Research, 333–334, 66–95. https://doi.org/10.1016/j.jvolgeores.2017.01.011
Sosa-Ceballos, G., Gardner, J. V., & Lassiter, J. C. (2014). Intermittent mixing processes occurring before Plinian eruptions of Popocatepetl volcano, Mexico: insights from textural–compositional variations in plagioclase and Sr–Nd–Pb isotopes. Contributions to Mineralogy and Petrology, 167(2). https://doi.org/10.1007/s00410-014-0966-x
Sosa-Ceballos, G., Gardner, J. V., Siebe, C., & Macías, J. L. (2012). A caldera-forming eruption ~14,10014Cyr BP at Popocatépetl volcano, México: Insights from eruption dynamics and magma mixing. Journal of Volcanology and Geothermal Research, 213–214, 27–40. https://doi.org/10.1016/j.jvolgeores.2011.11.001