Popocatépetl

The Popocatépetl volcano is located in central Mexico, 40 kilometers east of the outskirts of Mexico City. It is in sight of 30 million people, most living on the debris from previous eruptions. The volcano has erupted explosively in the past, and it became active again during the 1990s, raising major concerns about the safety of people in this heavily populated area.

Volcanism in Mexico

The Popocatépetl volcano in central Mexico rises to 5,465 kilometers above sea level. Although snow is usually on its summit, oranges, mangoes, bananas, and palms grow at its base.

The volcano results from tectonic activity in the region, where the Cocos oceanic plate is underthrusted or subducted below the continental plates of the Caribbean or Americas. The subduction process produces melting along or above the subducted plate, creating fairly viscous melted rocks such as andesites. These stiff or viscous melts do not readily flow up through the volcano, often resulting in a plug of partially solidified lava that causes a buildup of gases. The gas pressure may eventually become so great that it explodes out of the volcano at its weakest point.

Such an explosion may take the form of a Plinian eruption, in which copious amounts of volcanic ash, gas, and larger particles are blown upward out of the central cone. This type of eruption has occurred at Popocatépetl several times throughout history. As the particles settle out of the atmosphere, they produce ashfalls. Sometimes, the ash and gas may be blown out laterally and flow down the volcano's sides as ash flows. Heavy rains produced by hot updrafts over the hot, central eruptions may mix with the abundant volcanic ash, producing lahars that move rapidly and for many kilometers downstream and river channels.

Eruption History

Scientists believe that Popocatépetl has experienced major Plinian eruptions every one thousand to three thousand years since 4000 BCE. Some of the most recent of these events have destroyed human settlements. Large eruptions occurred sometime between 3195 and 2830 BCE, between 800 and 215 BCE, and between 675 and 1095 CE. These eruptive cycles began with ashfalls and ash flows in small volumes, producing several 2- to 10-centimeter-thick volcanic layers. The subsequent phase of Plinian eruptions produced a thick sequence of coarser grained pumice fragments. It is estimated that the eruption column of fragments and gas would have reached heights of more than 25 kilometers. This would have significantly impacted the world climate as the particles spread high into the atmosphere. These significant eruptions also produced lahars and hot ash flows that moved rapidly down valleys and were deposited on the plains.

The area around Popocatépetl has long supported fairly large populations because of the fertile soils, the presence of water, and the climate. For example, agricultural and ceremonial towns were established between 1000 and 100 BCE. Pottery shards, building foundations, and agricultural furrows dating to this era have been found in pumice deposits, suggesting that the settlements suffered widespread destruction due to volcanic activity. Subsequently, areas near the eruptive center declined in population between 100 BCE and 100 CE, while areas to the north became more populated.

By 750 to 800 CE, several large cities containing as many as 150,000 people each had been established in the region surrounding Popocatépetl. Two of these, Cholula and Teotihuacán, were large religious centers. Archaeologists have determined that Cholula was abandoned around 800 CE, a phenomenon accompanied by a significant decrease in population in the general area. Various reasons were initially proposed for this decline, including drought, invasion, food shortages, and the exhaustion of soil nutrients. However, this period also correlates with a major Plinian eruption in which deposits from the eruption covered more than 3,000 square kilometers of the area. The remains of lahars at the base of the pyramid in Cholula contained pieces of pottery and volcanic glass artifacts. Several researchers who have studied these eruptions have suggested a scenario in which the area was covered by a muddy plain of lahars, with only the great pyramids projecting above the mud.

Modern Activity

After a long period of dormancy, Popocatépetl became active again during the early 1920s. The first stages of activity began with steam and ash emissions during the summer of 1920. A lava dome formed on the crater's floor in the latter part of the same year and then stopped growing. The dome eventually exploded during a minor eruption in January 1922.

Relatively little activity occurred at Popocatépetl between 1922 and December 1994. During that time, researchers noted a gradual increase in earthquake activity and gas emissions enriched in sulfur dioxide. On December 21, 1994, ash emissions also began, which continued into the middle of 1995. The activity then quieted until emissions resumed on March 5, 1996. Ash was observed covering snow and glaciers on the summit, and ash and gas were thrown as high as 800 meters above the crater. A viscous dome of lava about 400 square meters in area was observed on the central crater floor on March 29, 1996, indicating that the lava phase of the eruption had begun. Lava continued to flow until most of the inner crater was filled.

On April 30, a large explosion blew a hole in the surface of the lava dome. This event resulted in the deaths of five climbers who had defied warnings to stay away from Popocatépetl. A rain of warm volcanic debris was deposited in nearby towns. Particles 0.5 centimeters in diameter were observed 12 kilometers away from the volcano, while sand-sized material was observed at a distance of 60 kilometers.

Volcanic activity continued sporadically through late 1997 as lava domes periodically built up on the crater floor, only to be destroyed by often spectacular explosions. An explosion on October 28, 1996, at 9:05 am, emitted large amounts of ash and gas. This was followed by a lesser explosion the following day at 10:11 pm. Minor explosions also occurred on December 21, 1996, and between February 17 and June 1997. An eruption on April 24, 1997, produced an ash and gas plume 13 kilometers high and 8 kilometers wide. It eventually spread out to a plume that was 65 kilometers long. Meanwhile, the lava body first observed within the crater in March 1996 remained unchanged.

The largest ash emission since the beginning of the eruption cycle in 1994 occurred on June 30, 1997. It began with several earthquakes related to the volcanic activity at 4:56 pm. The first ash eruptions began at 5:11 pm. and lasted 135 minutes. The second ash eruption began at 7:26 pm and lasted for 90 minutes. The ash plume rose to more than 13 kilometers within a few minutes, and ash began falling in towns around the volcano two to three hours later. The ashfall shut down the airport in Mexico City for twelve hours, leading to numerous press reports. A red alert was posted, but no evacuations were ordered. No casualties were reported, but minor lahars were reported in an area with heavy rain, causing the flooding of one house. Several lava tongues up to 2 kilometers long flowed south to southeast down the volcano on July 3 and 4.

More small, sporadic eruptions were reported through the end of 1997, with the most significant eruptions occurring on August 12, in late September, early October, and early to late December. The biggest eruption occurred on December 24, during which incandescent material was thrown from the volcano, and grass fires were started to the east. Intermittent activity continued through 1998 and 1999, with several significant eruptions. A thirty-second explosion sent ash 5 kilometers into the atmosphere on January 1, 1998. Larger explosions began in late November 1998, producing ash plumes 3 kilometers high. Ashfalls were reported on the southwest side of Mexico City. Explosions on November 25 broke windows and walls in several villages 60 kilometers away, and large volcanic blocks landed 5 kilometers away. People were warned to stay at least 7 kilometers away from the volcano. Large explosions in December 1998 shattered windows, fractured walls, opened doors, and knocked tiles off roofs. Popocatépetl continued to be active through 1999. Significant eruptions occurred on March 17, March 23, June 16, and June 22. The eruption on March 17 was large enough to result in evacuation centers being put on alert.

Popcatépetl remained in an active pattern throughout the first decades of the twenty-first century, often emitting ash, producing steam and gas emissions, and causing tremors. In 2023, the Mexican government warned people to stay away from the area to avoid the violent eruptions occurring from the volcano. In February 2024, flights were diverted from nearby airports due to ash and smoke from the volcano. Mexican authorities ordered residents to stay at least 12 kilometers away from Popocatépetl. Ashfalls, lahars, and pyroclastic density currents from Popocatépetl continued to threaten residents of Mexico City and the surrounding area throughout the first half of 2024.

Significance

Much has been learned about the long-term eruptive cycles of volcanoes by studying ancient volcanic deposits. If authorities can understand the potential danger of Popocatépetl in terms of its historic eruption cycle, they can set up better evacuation procedures in case of a catastrophic eruption. The authorities have such evacuation and emergency procedures in place should a large eruption cycle begin.

Scientists are also interested in the effects of volcanic eruptions on the global climate. Several of Popocatépetl's major eruptions appear to have been powerful enough to send volcanic ash into the stratosphere, allowing it to spread worldwide. For example, analysis of ice cores from the Greenland ice sheet has suggested that a major eruption occurred somewhere in the world around 822 or 823 CE, corresponding to the approximate date of a major eruption of Popocatépetl. Such large eruptions appear to affect the world's climate by lowering the penetration of sunlight, thus causing a cooling of the earth's surface.

Principal Terms

andesite: a volcanic rock with intermediate silica content that contains the minerals plagioclase, biotite, and hornblende; because the molten equivalent of andesite is often viscous, gases can build up in it and produce explosive eruptions

ashfall: fine-grained volcanic material that settles out of the atmosphere

ash flow: a moving mass of hot ash and gas that flows from a volcano; the hot material is deposited as it spreads out on flatter areas

lahar: a volcanic mudflow formed when volcanic ash and minor coarse volcanic particles mix with the abundant rainfall that often accompanies eruptions

Plinian eruption: a rapid ejection of large volumes of volcanic ash that is often accompanied by the collapse of the upper part of the volcano

pumice: volcanic glass, usually light in color, that contains so many bubbles that it may float in water

subduction: a tectonic process in which one lithospheric plate is underthrust below another plate into the mantle

Bibliography

Kornei, Katherine. “How Dangerous Is Mexico's Popocatépetl? It Depends on Who You Ask.” Eos.org, 29 Jan. 2024, eos.org/features/how-dangerous-is-mexicos-popocatepetl-it-depends-on-who-you-ask. Accessed 19 July 2024.

Lockwood, John P., and Richard W. Hazlett. Volcanoes: Global Perspectives. Wiley, 2010.

Oppenheimer, Clive. Eruptions That Shook the World. Cambridge University Press, 2011.

Roberge, Julie. “Global Volcanism Program—Popocatépetl.” Global Volcanism Program, volcano.si.edu/volcano.cfm?vn=341090. Accessed 19 July 2024.

Yablonski, Steven. “Video Shows Violent Eruption of Mexico's Popocatepetl Volcano.” FOX Weather, 27 Feb. 2023, www.foxweather.com/extreme-weather/popocatepetl-volcano-eruption-video. Accessed 14 Apr. 2023.

Yeung, Jessie. “Mexican Volcano Spews Massive Columns of Ash and Smoke, Forcing Flight Cancelations.” CNN, 27 Feb. 2024, www.cnn.com/2024/02/28/travel/popocatepetl-volcano-mexico-smoke-ash-intl-hnk/index.html. Accessed 19 July 2024.