Popocatepetl is the taller of two volcanoes some 70 km SE of Mexico City. Its neighbor Iztaccihuatl 17 km to the north is connected by a high ridge between the two volcanoes.
This region is densely populated, with Mexico City hosting a metropolitan population of 21.3 million and Puebla City to the east of the volcanoes with a metro population of 3.25 million. While both cities have much smaller listed populations, 8.9 and 2.5 million respectively, it is the larger total that must be considered by any disaster planners dealing with the impact of potential eruptions. Over a million live within 35 km of the volcanoes.
The pair of volcanoes are two of only three volcanoes in Mexico to host glaciers. They are also two of the three highest peaks. Popocatepetl’s coverage has most disappeared due to a combination of changing weather and the ongoing series of eruptions. By 2001, its glaciers were gone, though snow and ice remained. Popocatepetl’s total height is over 5,400 m, 3,020 m above the surrounding valleys.
Its neighbor Iztaccihuatl is called the Woman in White because its four peaks draped with glaciers look like a women in repose from a distance. It is the third highest peak in Mexico topping out at 5,230 m. It is a massive volcano of 450 km3, built of four overlapping cones constructed along a NNW – SSE – line to south of the ancestral Llano Grande caldera.
Eruptive products from both volcanoes are primarily andesites and dacites.
An Aztec legend connects the two volcanoes as lovers. Iztaccihuati was a princess who fell in love with one of her father’s warriors (Popocatepetl). The father sent the warrior away to war, promising his daughter’s hand when he returned. Once he was gone, the father told the daughter the warrior died. She died of grief. Popocatepetl returned to find his love dead and took her body to a spot outside Tenochtitlan and kneeled by the grave. The gods covered them with snow and turned them into mountains. Popocatepetl became an active volcano, raging at the loss of his beloved.
The name Popocatepetl comes from Nahuati words translated as “it smokes.” Alternate names include El Popo and Don Goyo. Don Goyo comes from the mountain’s association with local St Gregorio lore.
There is a National Park centered on both volcanoes. Both host significant hiking activity, though much of that has ended on Popo since the start of its recent activity.
Volcanic activity from both volcanoes, Popocatepetl in particular, had significant historical impact on natives that lived in the valleys below them, nicely bracketing what is called the Classic Period of Mesoamerican archeology a period 1038 – 1622 years long ending around 822 AD when the most recent massive eruption covered the surrounding valleys with tephras, pumice, and most importantly significant lahar debris that destroyed local irrigation and in turn agriculture. It appears there was sufficient warning to get out alive, though not sufficient to remove larger objects from the buried villages. It was during this period that the natives built the massive Great Pyramid of Cholula, the largest pyramid in the world.
Popocatepetl is a mature, highly active stratovolcano built on the remains of its earlier cone. There have been at least three previous cones destroyed by cone collapses producing massive debris avalanche deposits mostly to the south. There have also been at least three Plinian eruptions separated by 1,000 – 3,000 years that destroyed human settlements living in the valleys below with lahars, pyroclastic flows, and massive tephra falls. Following each eruption, locals repopulated the valleys below the volcano and the flanks of the volcano itself.
Major eruptive products are andesites, basaltic andesites, dacites, and basalts. The typical crater on top is about half a kilometer in diameter.
Initial construction of the volcano began some 730,000 years ago. The ancestral volcano named Nexpayantla started its construction about the same time as the Pies volcano of the Iztaccihuatl volcano complex. It was built mainly of andesitic and dacite lava flows. Flank activity was dacitic to rhyolitic building large dacitic lava domes. The volcano also had multiple dacite and rhyolitic dikes. It finished with a flank collapse to the south, the Lower Tlayecac avalanche deposit.
The new cone, Ventorrillo built though the remains of the first volcano. It was primarily andesitic lava flows with no observed flank activity. This cone was destroyed with a large Bezymianny-type eruption to the SW producing the Upper Tlayecac avalanche and Tochimilco pumice. There is a debris fan from the north called the Calpan that came from the collapse and eruptions of the Pies volcano. The most recent flank collapse debris avalanche and Bezymianny-class eruption is dated somewhere 22,900 – 23,500 years old. There are three other flank collapses identified at Popocatepetl, with one of them from neighboring Iztaccihuatl Pies volcano component to the north.
As an aside, it appears that in this part of the world, each vent is named as a separate volcano, not unlike what we saw with Nevados de Chilian. https://volcanohotspot.wordpress.com/2016/02/11/phreatic-eruptions-continue-at-nevados-de-chillan/
This eruption produced the White Pumice, one of the thickest and most voluminous Plinian deposits. The eruptions appears to be triggered by a catastrophic failure of the SW flank of the volcano. This highly mobile avalanche reached as far as 72 km from the cone. It covers 1,200 km3 and has a volume of 10.4 km3. Dispersal axis of the tephras has nearly 2,500 km2 covered by over 10 cm of ash. Estimated pumice fallout is just under 2 km3 DRE of dacites. As the eruption waned, the eruption column fluctuated, it collapsed periodically, emplacing pyroclastic flows around the volcanoes. Subsequent years erupted 4.4 km3 of degassed andesitic – dacitic lavas. One of these flows covered 68 km2, reaching 22 km from the crater. This VEI 5 eruption ended up producing 6.3 km3 of juvenile magmas and another 10 km3 of debris avalanche.
The present day cone is constructed by multiple andesitic to dacite lava flows. They have been glaciated multiple times. The current cone has flank vents along two well-defined lineaments. There are another 12 Plinian pumice deposits layered with the lava flows. Four large volcanoclastic fans and five valley fill deposits (lahars) sit below the volcano, mostly to the south, SE and SW. These were emplaced by multiple pyroclastic flows, lahars and fluvial deposits. Glacier melting coincident with several of the Plinian eruptions may have caused the lahar deposits.
Multiple Plinian eruptions are dated starting 12,900 – 14,700 years ago. Two more emplaced pyroclastic flow deposits north and south of the edifice in 9640 and 4980 BP. There have been at least three major relatively recent Plinian eruptions, between 3195 – 2830 BC, 800 – 215 BC, and 822 – 823 AD. A total of 10 Plinian eruptions have been identified with 6 of them dated.
The most recent Plinian eruptions bracketed what is called the rise and fall of the Classic Period of Mesoamerican archeology a period 1038 – 1622 years long.
Plume heights during these eruptions is estimated at 25 km, meaning that they could have had global impact. Each major eruption would impact over 3,000 km2.
Lahars are a significant hazard from this volcano due to the variable coverage of ice and glaciers. Lahars reached the base of the great pyramid of Cholula. The sequence of mudflows is several meters thick and contains pumice, pottery shards, obsidian artifacts, and carbon. Carbon dating of the sequence at Cholula puts its age at 790 AD with a 350 year bracket around the estimate which brackets the well-defined 822 – 823 AD date for the second Plinian eruption. The available geologic information suggests that the city and surrounding agricultural areas were destroyed by lahars, leaving the pyramids sticking out of a muddy wasteland. Much of this disruption was of the destruction of the irrigation network constructed to support farming that fed the city.
Popocatepetl was dormant for 50 years before the current eruption sequence started in 1994. Prior to that, there were five VEI1 eruptions in the 20th Century, with a VEI2 in 1925. There were four eruptions, in the 19th Century, the most recent ones with uncertain dating. All of them VEI 1s. A single eruption took place in 1720, a confirmed VEI1. This appears to be a straggler from a period of increased activity from 1488 – 1720, with 20 mostly VEI 2 eruptions. A single VEI 3 took place in 1519.
Recent activity has been in the VEI 2 range, with a VEI 3 during the period March 1996 – Nov 2003. The most recent continuing activity started in Jan 2005.
The current sequence of eruptions typically sends ash plumes 1 – 4 km above the summit. Multiple emissions of gas and steam occur daily. Some include ash. Dome growth and destruction takes place continuously, with dome fragments being ejected from the crater and rolling down the upper flanks of the volcano. The red glow from the ongoing explosions is normally visible during the night.
Explosions in July 2016 put ash plumes to 8.5 km. Ashfall was observed 30 km WSW of the volcano. Satellite instruments recorded SO2 plumes and thermal bright spots from the crater. Total coverage by SO2 emissions was 100,000 km2. There were also small landslides recorded by the seismic network monitoring the volcano. Domes typically fill the crater before being destroyed by the next round of explosions.
Iztaccihuati, the Woman in White, is a volcanic complex that sits to the north of Popocatepetl. It is geologically older than Popocatepetl, with volcanic activity migrating south over time. Its most recent eruption was sometime more recent than 11,000 years ago. The youngest vents are located at the summit and a summit ridge at the 5,100 m height. It is located 60 km SE Mexico City and 45 km NW Puebla. It is the third highest peak in Mexico.
The volcano was constructed from two main formations, the oldest is the Llano Grande which is in turn built from three main volcanic units, the Pulpito del Diablo, Caldera Llano Grande, and Los Pies Ancestrales volcano. This includes associated domes and lava flows. Oldest rocks date back perhaps 1.09 Ma.
The second unit is the Iztaccihuatl Formation that includes the most recent activity and what are called five aligned volcanoes (likely volcanic vents migrating over time to the south), La Cabeza, El Pecho, Las Rodillas, Los Pies, and associated lava flows. This unit dates 80,000 – 580,000 years ago, with the most recent activity being a dacite lava flow 9,000 years ago.
Construction of Iztaccihuatl began around 900,000 years ago. Eruptions progressed in two phases. The initial volcano was a large shield with a summit caldera constructed between 900,000 – 600,000. Pyroclastic cones and lava flows erupted from the flanks of the shield. Younger activity took place more recently than 600,000 years, mainly lava flows and pyroclastic materials erupted from the summit and flanks. Most volcanic activity ended 80,000 years ago.
The initial cone built the Llano Grande, a 220 km2 shield topped with a 4.5 km diameter caldera. The initial phase of eruptions produced andesitic to rhyolitic lavas over 9 km3 in volume.
The second phase produce younger andesites and dacites that are differentiated into 125 km3 of pyroclastic breccias from vents around the summit and similar rock types that buried most of the ancestral flank vent Pies. Two plug domes El Solitario and Los Yautepemes, were built NW of the summit. By far the most voluminous lavas during this phase were 26 km3 of dacites.
The Los Pies cone was constructed 410,000 – 340,000 years ago. Its summit has a 1.3 km horseshoe open to the SE. The amphitheater collapsed to the SE with debris avalanche and associated pyroclastic deposits. It covers an area of 550 km2, extends up to 45 km from the peak, and contains perhaps 1.5 km3 of Los Pies cone. Date of the collapse is not precisely dated with a range between 250,000 – 630,000 years ago.
Later stages of volcanic activity reflect the inability of dense basaltic influxes of new, hot magmas to successfully penetrate a relatively large chamber of low density hornblende dacite magma.
The Trans Mexican Volcanic Belt is a subduction driven region, with the Cocos Plate subducting eastward under the North American Plate. The Cocos Plate sits a bit over 100 km below the surface near Mexico City / Puebla. The North American Plate of the region is a bit thicker than 50 km. The flat slab detached some 7 Ma, and subduction of the Cocos Plate drove the subducted plate sharply downward, only a few tens of degrees from vertical under the Mexico City region, which in turn provided decompression / dehydration melts and a depression around Mexico City.
The massive outbreak of volcanic activity across the Trans Mexican Volcanic Belt generally worked its way west to east between 11 – 3.5 Ma. The activity is thought to be related to flat plate subduction beneath the North American Plate. The activity migrated trenchward over the last several million years due to the flat plate rollback, detachment of the eastern portion of the flat, subducted plate, and rips and tears in the subducted plate under Mexico. Current volcanic activity is driven by flux and slab melting, with the rear portion of the belt characterized by more differentiated magmas, mafic lavas with little subduction fluids and higher asthenosphere fingerprint (decompression melts).
The entire region is quite tectonically active, with multiple earthquakes, primarily subduction driven, offshore and multiple earthquakes in the interior. This is particularly bad, as Mexico City is built on a lake bottom which tends to amplify the effects of shaking ground. The most recent large earthquake 120 km SSE Mexico City was a M 7.1, less than two weeks after a M 8.1 some 650 km to the south.
So how does the melt make its way to the surface? There are some faults in the region, generally trending NNW – SSE which appear to provide a zone of weakness. Their deeper structures are not readily apparent, but appear to be positioned by allowing (encouraging?) activity to migrate south over time as it has with Iztauccihuatl – Popocatepetl. Even current activity in Popo appears to be migrating southward, with a second location of earthquake swarms SSE of the main crater.
Popocatepetl is an active, growing volcano with an ample and vigorous magma supply. The most recent flank collapse / directed blast eruption was some 23,000 years ago. Recent Plinian eruptions have been 1,000 – 1,500 years apart, though these numbers ought not to be used for planning purposes.
Overall volcanic activity under the region appears to be slowly migrating south over time. Due to its proximity to nearly 25 million people in the municipal areas of Mexico City and Puebla, this is a highly dangerous and potentially very deadly volcano.
While lahars are no longer a significant threat, mostly due to the disruption of the glacier field topping the volcano, an extended period of heavy rains will be sufficient to mobilize loose eruption debris on its flanks downslope into the inhabited valleys. Alternately, should there be another extended hiatus in activity, glaciers and an ice cap will quickly return, and along with that cap, the millennia-old lahar threat resurrected.