Popocatépetl, in southern Mexico, is North America’s 2nd-highest volcano. This volcano has been actively erupting for millennia—the name comes from Aztec (Nahuatl) words translated as “Smoking Mountain”. The classic cone-shaped peak with a central crater rises only about 44 miles (70 km) to the southeast of Mexico City, probably the most populous city on the continent, and is close to many smaller towns. And the recent activity of the volcano is bringing additional concern about the possibility of a significant eruption.
During late May 2023, researchers recorded small volcano-tectonic earthquakes, minor to moderate explosions ejecting ash and incandescent material upwards, and nearly constant emissions of steam and ash from the volcano. This activity resulted in air quality alerts and caused airports to shut down temporarily in Mexico City and the nearby town of Puebla, and school cancelations in at least 11 towns surrounding the volcano.
In the past 3,000 years, Popocatépetl has produced three Plinian eruptions—explosive volcanic types similar to those of Krakatoa in Indonesia in 1883, Mount St. Helens in Washington in 1980, Mount Pinatubo in the Philippines in 1991, and Hunga Tonga in 2022. The most recent eruption occurred approximately 1,100 years ago (likely 823 CE). The Aztecs recorded frequent eruptions in the centuries before the Spanish arrived in 1519. Since 1519, there have also been over 15 major eruptions. After a quiet period in the mid-1900s, smoke and gas began emanating from the crater in 1993 and have continued more or less constantly ever since.
The 17,887 ft (5,452 m) tall Popocatépetl is within the 620-mile (1000 km) long Trans-Mexican Volcanic Belt. This impressive chain of active and dormant volcanoes came to the attention of the world in 1943 when Paricutín broke through the fertile soil and began erupting into the cornfield of an astonished farmer. Approximately 200 miles (322 km) west of Mexico City, Paricutín erupted for 9 years. In the first year, the cone rose about 1,475 ft (450 m) from the base, and eventually reached an elevation of 1,706 ft (520 m)—a dramatic expansion of real estate!
Over the past tens of millions of years, volcanic eruptions and large earthquakes have produced mountain chains along the Pacific coast of the Americas. These include the 5,530-mile (8,900 km) Andes Mountains in South America, the 400-mile (640km) Sierra Nevada extending through California, and the Cascade Range from Northern California through Oregon, Washington, and into Canada, a distance of about 700 mi (1,100 km). Each of these roughly north-south aligned ranges parallel a location where enormous oceanic and continental tectonic plates are converging. These are subduction zones, where dense and metal-rich oceanic crust plunges beneath less dense continental crust, in an interaction that melts rocks that rise to feed volcanoes. The Trans Mexican Volcanic Belt is also next to a subduction zone and is active because of this tectonic feature. But it has a highly unusual orientation: oblique, or roughly perpendicular, to the subduction zone. Why? What makes the geologic setting of southern Mexico so different from the great expanses of mountains bordering North and South America?
Researchers think they know the answer. And it is a specific type of subduction–specifically, flat slab, or shallow-dipping subduction—a phenomenon that has fascinated me for many years.
A Few Things We Know About Subduction Zones
Like many people, I am drawn to mountains. As a California native, I grew up roaming around the tall peaks of the Sierra Nevada. From a young age, I was curious about how such impressive landscape features formed. When I first visited the Andes Mountains in 2006, the enormous size and majesty of these mountains astounded me. As a geologist by this time, I understood the basics of how the Andes rose, but I also had lots to learn. Some of this involved delving into the emerging research on flat slab subduction.
The Andes Mountains provide a textbook example of a mountain range built where continental and oceanic plates are converging. These mountains are relatively young, and frequent earthquakes and volcanic eruptions are clear evidence that active subduction is underway. The diagram below shows the major features of the interaction between the continental South American and oceanic Nazca plates.
Off the coast of South America, the oceanic crust riding along on the Nazca plate begins a downward plunge toward the mantle in a deep offshore trench known as the Peru-Chile trench. This enormous feature stretches for approximately 3,660 miles (5,890 km) and reaches a maximum depth of about 26,500 feet (8,070 m) below sea level. The trench parallels the South American coast offshore about 100 miles (160 km), where it defines the Nazca/South American plate boundary. When subducting oceanic crust plunges beneath continental crust at an offshore trench, the cold oceanic lithosphere, known as the slab, sinks into the mantle with a low slope. Earthquakes originate at the trench and increase in frequency eastward towards the continent, following the slope of the cold slab as it sinks downward. The earthquakes closest to the trench and coastline are shallow and then become progressively deeper towards the east. Instrumentally recorded earthquakes outline the descending plate along an inclined plane called a Wadati-Benioff zone (named for the two seismologists who independently discovered them).
The plate interface where the South America and Nazca plates intersect has distinctive features. Alignments of earthquakes show there are a series of distinct segments. These interfaces angle down into the Earth with initially steep angles around 30 degrees. On some segments, the steep angle continues downward, but there also are shallow sloping, or flat slab, segments. On these, at depths of approximately 95 miles (150 km), the plate interface flattens out to less than 5 degrees and extends for hundreds of miles beneath the continent.
Today, in the Central Andes, the steeply dipping segments and the flat, or nearly horizontal, segments show important differences. Flat slab segments coincide with volcanic gaps, with volcanic eruptions currently occurring above steeply dipping sections of the subducting plate. Also, the frequency of earthquakes is higher above flat-slab regions than the adjacent steep slab segments. Along the Andes Mountains, scientists estimate the amount of energy released in earthquakes is 3 to 5 times higher on average on shallow-dipping segments of the plate, compared to the more steeply dipping segments.
Flat slab subduction can extend for hundreds of miles beneath a continent. The Rocky Mountains, in an unusual position in the center of the North American continent, are believed to have formed beginning around 70 million years ago because of the convergence between the North American and Pacific tectonic plates. At the time the Rockies were rising, they were located almost 1,000 miles (1,600 km) from the plate boundary. Flat slab subduction caused these mountains to form and uplifted the Colorado Plateau.
Unraveling the Unusual History of the Trans-Mexican Volcanic Belt
Popocatépetl and Paricutín are part of the volcanic chain that extends approximately west to east, from the Pacific Ocean to the Gulf of Mexico. Along the northwest-southeast oriented subduction zone known as the Middle American Trench, the edge of the North American plate is overriding the northern part of the Cocos plate and a microplate that broke away from it, the Rivera plate. The volcanoes in the Trans-Mexican Volcanic belt are aligned obliquely to the Middle American Trench, an unusual orientation. Also, the volcanic products produced in the belt are strangely variable, from the andesites that are typical of a subduction zone setting to some rocks that show a geochemical signature more typical of a continental plate interior. There are relatively few earthquakes, and these are concentrated between the offshore trench and the coast.
Scientists have found that segments of the downward sloping slab of the Cocos and Rivera plates exhibit typical steeply sloping angles, while the central segment that extends beneath Mexico City has a shallow dip. The model below shows how slab angles may have changed over time.
Since interpreting this diagram might be difficult, here is a short explanation: changes in the angle of the subducting slab over tens of millions of years, including development of a shallowly dipping segment, have caused volcanic activity over a broad, east-west oriented region. Go any farther and this topic will rapidly become even more complex, so I’ll wrap up this post!
The earthquake and volcanism situation in southern Mexico differs significantly from that in other parts of the Americas. Geology is fascinating!
For more information about the volcanic activity of Popocatépetl, check out the interesting and informative blog posts on Volcano Café: https://www.volcanocafe.org/popocatepetl-and-the-trans-mexican-volcanic-belt/ and also on Volcano Hotspot: https://volcanohotspot.wordpress.com/2018/08/31/popocatepetl-mexico-citys-active-volcano/. The sources listed below (available on Google Scholar) have great info on tectonics. And, as you may have guessed, the book I have written about the Andes Mountains and the ancient cultures that flourished there includes discussions of the tectonic setting. An academic press will publish my book and it should be available early in 2024 — stay tuned!
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