Each century its own. The 19th century was owned by Krakatoa and Tambora. (The mystery volcano of 1808/09 should probably be added to make it a list of three, but it is hard to credit the unknown.) The 20th century was the century of Pinatubo (1991) and Mount St Helens (1980). There were other eruptions, of course. Novarupta (1912) rivalled Pinatubo in size but in the publicity stakes it did not get anywhere near. The deadliest eruptions were Pelee (1902) and Nevado del Ruiz (1985), both smaller eruptions which happened to badly affect nearby population centres. As an aside, compared to previous centuries, Indonesia played only a minor role in the 20th century. But there is one event that is among the most significant but most overlooked eruptions of the century, a Pinatubo look-alike. El Chichon is the one that got away.
The obscurity was already there even before the beginning. To the locals, the mountain peak was hardly notable. The region was heavily forested, and in the series of peaks there seemed little that set El Chichon apart. At 800 meters above the surrounding country side, the 1260-meter high peak was less tall than some other peaks in the area. And in any case, there was little evidence for any volcanic activity. Mexico’s volcanoes are mostly further north, in its central zone, while to the south the Guatemalan volcanoes cluster together near the Pacific. El Chichon is mid-way between, an isolated, small volcano in an area that does not seem to be particularly volcanic, neither close to the Pacific nor to the central zone. Its unassuming nature it had in common with Pinatubo, a decade later.
El Chichon is certainly remote. It lies mid-way between Tuxtla Gutierrez, the capital of the state of Chiapas, and Villahermosa, capital of Tabasco, each about 70 kilometer away. There are towns in the area but the remoteness meant that it was never monitored. Neither was there much suggestion that it should be: this volcano seemed safely dormant or extinct, and Mexico has many much more urgent volcanoes. In view of the importance of the unexpected eruption, it is a pity that it wasn’t seen as more of a priority. Much of the run-in of the eruption was now missed, and even the eruption itself was poorly observed, not helped by the unerring scheduling which made each of the major explosions happen at night.
The mountain consisted of an outer crater rim, 2 kilometer in diameter, with a main central lava dome, several small domes to the northwest and southeast of the crater, and one crater structure inside the outer crater. Two rivers collect water from El Chichon: the Platanar river from the NE flank of the volcano and the Susnubac– Magdalena river from the SW flank. Both are tributaries of the Grijalva river which flows towards the Gulf of Mexico.
In hindsight, the activity had started in 1930 when locals reported hearing sounds from the mountain. Local stories told of a possible eruption a century earlier. A geologist, Frederich Mülleried, visited and he found that in the moat between the dome and the crater rim there was strong fumarole activity. After his 1932 report, the mountain became classified as a volcano. Shocks were felt in 1964, but nothing more happened until minor earthquake activity started in 1980, continuing into 1981. in December 1980 and January 1981, two geologists, Canul and Rocha, conducted fieldwork in the area for the purpose of hydrothermal energy surveys, and they heard and felt ‘strong noises and small earthquakes’ coming from the crater. They stated in their report that there was ‘a high volcanic risk’. But this report was not released by the Comisión Federal de Electricidad for whom it was written. This was an important missed opportunity.
On 26 February 1982 intense seismic activity began, starting with an M2.9 and reaching up to M4 in mid-March. There were 10 or more earthquakes per day, at a depth of some 5 kilometer. This was detected on a nearby seismometer network, installed in 1979 to monitor a dam, but the data was not inspected until after the eruption. This was another missed opportunity. Scientists only became aware of the local reports of the seismic activity on March 20. On March 26 they attempted to set up monitoring stations on the mountain but their helicopter flight was cancelled and rescheduled for March 29. This probably saved the lives of the scientists involved. On March 27 the earthquakes became much shallower, around 2 kilometer in depth, and increased in frequency to 60 events per day – again, this was only detected in hindsight.
March 28, 1982, was the fateful date. After 9pm, the earthquakes ceased. This probably indicated that the rising magma had fully filled the conduit. For 90 minutes the earth remained quiet, until, just before midnight, at 11:32pm local time (5:32 GMT), the mountain exploded in what has been called a phraetoplinian event, i.e. driven by vaporising ground water. The eruption lasted a few hours. This being night in the pre-webcam era, it was not directly eye-witnessed, at least not by survivors. Weather satellites picked up the eruption 40 minutes later. By that time, the cloud had spread north-eastward, over 100 kilometer in diameter, and had reached well into the stratosphere, 27 km high. By morning, it covered the Caribbean from the Yucatan to Haiti.
The local area suffered ash fall, mainly to the northeast where the ash was 5 centimeters thick at a distance of 70 kilometer, and 15 centimeter at 20 kilometers. Worst affected was the northeast flank of El Chichon. In the village of Nicapa, 7 kilometers from the summit, a church sheltering refugees from the eruption collapsed, brought down by an M3.5 earthquake. 10 people were killed and many more injured. Further fatalities occurred to the north, caused by the fires which had been ignited by the hot ash. The numbers are not well known but it is estimated that some 100 people died overall on that night. Photos taken a few days later showed that a quarter of the lava dome had disappeared: the eruption had come through a narrow vent in the dome and partially destroyed it.
Confusion reigned among the local population. Villahermosa remained dark for several days, with limited visibility even at mid-day. The green jungle of eastern Mexico was coloured white-gray by the ash and even ships in the Gulf of Mexico reported ash fall on their decks. Farmers were seen walking from one village to the next trying to find out what had happened.
On March 30, two more explosions followed, a smaller one at 9am and a bit larger event at 3pm. The southerly wind spread the clouds and haze to the north, reaching Mexico City. Even at this distance, 700 kilometer, the visibility was reduced to 3-8 kilometer. There was a small explosion the next day, but otherwise the volcano seemed to be calming down. Over these four days, there were six explosions in total. These were again phreatic: ground water was interacting with the hot rock. There was little ash and tephra from these smaller eruptions.
In the mean time, the Mexico emergency plans had sprung into action but, being untested, had ran into some problems. The plans were not designed for a major eruption. They relied on bringing in personnel and equipment by helicopter and plane, but this was not possible: the air space over south-eastern Mexico was closed because of the ash. It also required a presidential order which took a day to obtain. The plans came into action on April 1. In the mean time, the limited management showed. Roads became clogged with people fleeing the eruption, hindered by a counter flow of people coming to rescue their relatives. And this on poor roads covered in thick white ash.
Still, the army started evacuating a large area around El Chichon, out to 15 kilometer from the mountain and covering over twenty thousand people. This commendable action may have avoided massive casualties in the events to follow, but it also met problems. The impassable roads made it impossible to reach the communities nearest the summit, and people there had to decide for themselves whether to evacuate or not, without reliable information. Scientists were not sure whether the eruption had finished, and an unnamed chief scientist advised the army that the danger had passed. The seismologists who knew better had not been invited to these meetings. This uncertainty led to the evacuation order being rescinded on April 2nd. The relative calm also encouraged people to try to return to their homes. They walked into the fire.
On the 3rd of April around 02:40 in the morning, as later determined from the seismic record and from atmospheric waves recorded in Texas, the eruption resumed. The story becomes more complex at this point, because after this the seismic records begin to diverge from local reports and papers. The 02:40 eruption was large but less than the first one. However, it was followed by a very major event at 03:00. These two events became conflated in the reports. The second one was probably the explosion that destroyed the dome. The force of this blast has been reported as almost 100 kilo-ton of TNT, and the atmospheric pressure signals as similar to an atmospheric nuclear explosion. Being the middle of the night, eye-witness reports are scarce. The deposits contain evidence for water: this was a major phreatic event, perhaps when rising magma had reached the groundwater table. Near the summit, Volcan village was buried under 80 cm of ash, but further away, Nicapa only had 7 centimeters. This suggests a ground blast, where the ash column did not reach any great altitude. After this, there is less evidence of water in the ejecta, and it seems the ground water was now exhausted. Water-loaded flows will have temperatures up to 100 C, while dry flows can be much hotter: charcoaled trees after a pyroclastic flow are evidence that there was less water. Both types of flows occured at El Chichon. The explosion set the scene for the next, and final, events.
That evening, at 19:35, a major eruption occurred, which is reported by some as the largest event of the El Chichon eruption. The column reached a height close to 30 kilometer. And it was certainly very large, with a total energy was similar or larger than that of the preceding event. During the next few hours, the eruption fluctuated, with explosions every few minutes. A mushroom shaped cloud was seen. Again Nicapa was in the line of fire, with ash up to 40 centimeters thick and many roofs damaged from the falling stones, up to half a meter in size. But even with this devastation, the village was found to have had a lucky escape. Two pumice flows coming down the mountain had reached to about 2 kilometers, containing rocks up to 1 meter in size, and a pyroclastic flow had covered the same path, strong enough to topple trees and leave a thin layer of boiling hot ash. Scientists at Ostuacan, 11 km from the mountain saw the first explosion, being warned by their seismograph. Later they saw the glowing pyroclastic avalanche coming towards them. Once the ash fall started, visibility went down to meters. The pyroclastic flows and surges coming down the mountain had in fact reached 7 kilometer from the summit in all directions. This devastation only became clear later.
The time of these devastating pyroclastic flows is unclear. Early papers attribute them to this final, huge explosion. But other evidence suggest that they happened during the previous eruption, in the early morning of April 3rd, rather than the subsequent evening. This discrepancy shows how difficult it can be to piece together a complex eruption, especially when it happens at night amidst tremendous confusion. People close enough to know what happened did not survive.
The deposits show that the pyroclastic flows had come before the associated tephra falls. This order is unusual for plinian eruptions although not unique – there are examples at other volcanoes. To get an early pyroclastic flow, either the eruption column needs to collapse very fast, or the flow must come directly from the crater. What perhaps happened is that the explosions had fully removed the old dome, and there was suddenly much less weight pushing on the magma chamber. The release of this pressure caused the volatiles to come out of the magma, including the penetrating ground water, causing the entire magma chamber to explode as well. The cold debris flow was probably crater rock fragmented by the explosion. The hot pyroclastic flow came because this cold fragmented rock made the eruption column very dense. The hot gas was unable to carry the weight and a mix of cold debris and hot pyroclastics came racing down the mountain. The deposits left by these flows and surges are up to 15 meters thick. Some were stopped by the outer crater wall, but the gaps in this wall let the flow enter the steep valleys. The flows raced down, until the gradient shallowed and the cold flows slowed down and settled.
Nicapa had come off lightly, being just far enough away. Much worse had happened elsewhere. Nine small villages closer to the summit than Nicapa had been destroyed: Volcan, Tanchichal, Francisco Leon, Guayabal, Trinidad, Naranjo, Yaspac, Volcan Chich6nal and the western part of Guadalupe Victoria. Some of these had already been abandoned, apart from a few people looking after the crops, but Francisco León, 5 kilometer from the summit, had a population of about 1000, of whom half were still there. About 300 to 400 people died there, including a geologist, Salvador Soto Pineda, who was part of the scientific team studying the eruption. Nearby, a river was seen to boil but the heat made it impossible to approach.
On April 4, at 5:22 in the morning, the fourth of the large eruptions came. There were two separate explosions, 8 minutes apart, followed by several hours of activity. Weather satellites showed that the ejecta had again penetrated the stratosphere, and a huge sulphur-bearing cloud began to spread. This eruption caused extensive tephra falls and a variety of pyroclastic flows. Again the north-eastern region bore the brunt of tephra fall. At this time, visibility in Ostuacan became reduced to about 20 meters.
The eruption was now largely over, apart from a minor event on April 5, but the impact continued. The pyroclastic debris had blocked some of the rivers. The worst case was on the Rio Magdalena, to the southwest, which was now crossed by a 50-meter thick debris pile. The river continued to bring in water, and a 5-kilometer long lake developed, covering the hot pyroclastics. On May 26, the dam gave way and within an hour, the lake emptied. People living in the danger area had already been evacuated. The most remarkable thing was the heat of the lahar. Ten kilometre downstream, a water temperature of 82 C was measured. A hydroelectric plant further downstream still had people working in it: three people suffered bad burns from the water and one person died.
In hindsight, there were local Mayan legends about a boiling flood in the area, which may have referred to a similar event in the long distant past. But before the eruption, such stories would not have been understood.
El Chichon is listed as a VEI-5, both because of the tephra volume and the height of the eruption columns. The tephra amounted to 2.2 km3. The dense-rock-equivalent is of course rather less than the tephra, but even that is estimated at 1.1km3, with a minimum of 0.5 km3.
The original lava dome had a volume of 0.16km3; the new, 300-meter deep crater may add another 0.1km3. This fits the minimum DRE, but for the larger amount that is commonly listed there is a ‘missing rock’ problem: the ejecta seem to need more volume than can fit into the hole the eruption generated. The remainder must have come from excavation of the surface beyond the new crater, and from subsidence caused by the emptying of the magma chamber. It is not immediately obvious that these two suffice. Perhaps there had been considerable inflation before the eruption, or the amount of DRE ejecta is somewhat overestimated.
The total number of fatalities is not well known. This was a remote jungle location with few records of who lived where. The most complete records were from the local churches, and these indicate that some 2000 people disappeared, presumed dead. Most of the casualties probably occurred in the early morning of April 3rd. There was widespread economic impact of course, in loss of agriculture, but this was manageable for Mexico.
The climate effect is often mentioned. El Chichon produced an unusually large amount of sulphur, compared to the size of its eruption. Sulphur is a bit of a common thread in the area: 60 million years earlier, the impact that killed the dinosaurs has been so devastating in part because it hit the sulphur-rich rocks of the Yucatan. El Chichon was similar in size to the eruption of Mount St Helens, but it produced 7 times as much SO2. The ash contained up to 2 weight percent sulphate. Anhydrite (made of CaSO4) crystals were also in the deposits. Anhydrite is rare in volcanic rocks. The large amount of sulphate entering the atmosphere could have given a significant cooling.
Heating in the upper troposphere was indeed seen. This is the first indication of cooling (because the sun’s rays are absorbed at high altitude, these high altitudes warm up whilst the ground cools down. The CO2 greenhouse has exactly the opposite effect: the ground layer warms but the shielding of the ground radiation cools the upper atmosphere.) But in fact, rather little effect was seen in global temperatures, with at most a small short-lived dip. By coincidence, a strong El Nino had just started and this counteracted the cooling effect. It kept the temperatures too cool for a strong El Nino and too warm for a volcanic winter – but just about right for the time. Of course this was just before the strong global warming started to dominate the climate – it was considerably cooler than the current climate.
After the eruption, El Chichon had lost its lava dome and in its place there was now a 300 meter deep crater, 1 kilometer wide. The outer rim was still there. The forest was -and remains- gone. The height is less than before because of the loss of the central dome: it now peaks at 1100 meter.
El Chichon is unique in location and in the composition of its ejecta. But it does not stand alone. It is part of a minor volcanic belt called the Chiapanecan Volcanic Arc. It is the youngest, and the only non-extinct volcano in this belt. The belt runs from El Chichon to southwest, parallel to the Pacific coast. It ends where the Guatemalan volcanoes begin, following a line much closer to the Pacific.
Out in the Pacific, the ocean floor shows a linear feature pointing straight at El Chichon. This is the Tehuantepec Ridge, and it is part of the Cocos plate. It seems hard to believe that it would not be connected to this unusual volcano! But the precise relation is not clear. One suggestion is that this ridge is a fault which allows water to penetrate down. As the Cocos plate and its ridge subduct, the water causes a serpentine layer to form in the mantle wedge above the subducted plate. The serpentinization allows melt to form, and this feeds the volcanic activity.
The model is illustrated here. However, El Chichon may be a little far downstream of the subduction, for this model. More research may still be required before we can understand why El Chichon lives in isolation.
El Chichon is in fact part of an ancient complex. The oldest structure is a crater situated 4 kilometer to the northwest, which is dated to 1.64 million years ago. A dike extended 14 kilometer southwest, following a fault line: this happened 1.1 million years ago. El Chichon itself began to develop at its current location some 370 thousand years ago. The outer crater, the Somma, is dated to 250 thousand years ago. At the time, the full volume of the volcano would have been about 18 km3 – this was never a large volcano. Various smaller peaks on the slopes around El Chichon are now seen as related volcanic ‘lateral’ domes, emplaced since El Chichon began to develop. One of the domes caused the collapse of the southwestern rim of the Somma crater, and thus formed after the Somma. The lateral domes tended to explosively self-destruct. The latest of these was the Guayabel cone, 3 kilometer southwest, which left a horseshoe collapse structure on the side of El Chichon: this may have happened as recent as 10 thousand years ago.
In addition to these secondary domes, the central crater has re-activated a number of times even during the Holocene, the latest such event happening in 1982. The earliest dated event was at 8500 BP. It was followed by 4000 years of quietness or minor eruptions only. It is worth pointing out that the rainfall in this area can be phenomenal (perhaps explaining the hydro-magmatic nature of this volcano), and erosion rates are therefore also high. Quite a bit of history may have been lost.
The deposits suggest that a major eruption occurred 4000 BP, followed by 600 years of dome building, which subsequently blew up. Now the crater remained open for another 600 years until another explosion occurred. Activity resumed 2000 BP, with several explosive events within a few hundred years. A dome developed, and blew up around 1500 BP. 250 years later, a plinian eruption occurred from an open conduit, and 900 BP this repeated itself. A large eruption happened around 550 BP. After that, the new dome developed, until it was destroyed in 1982.
There is evidence for at least 11 eruptions during the past 8000 years. The recent recurrence time is about 300 years. The 1982 event was fairly typical in size: this volcano does VEI 4’s and 5, but would have difficulty with larger eruptions. The eruptions of 1400 AD (B in the table below), 700 AD (D) and 1900 BC (K) may have been a little larger than the 1982 event. Some of the events were similar in that a central dome was blown up. Others appear to have occurred from a vent, taking place in an environment similar to the current situation of a crater with a lake.
This history shows that El Chichon has always been a dangerous volcano. It wasn’t large enough to devastate huge areas, and never has done eruptions that are global in impact. But locally, it could have wiped out communities living along the rivers, and over much of the east of Mexico and the Yucatan it could have given a troublesome covering of ash. However, claims that this volcano could have played a role in the collapse of the Mayan empire seem overstated. There are bigger beasts further south more likely to have done this.
There is an important point to make here, though. El Chichon was not recognized as a volcanic danger, because it had had no historical eruptions. The recurrence time was too long for either human memory, or for written history. This is actually quite common. A significant fraction of large eruptions come from volcanoes without a living history of such. Pinatubo and Krakatoa come to mind, but so does Vesuvius in Roman times. (And in this context, the recent developments in Öræfajökull may sound a warning bell.) Add to this that a volcano that self-destructs regularly never builds up a traditional volcanic edifice – so it may not stand out in the landscape. Its danger is hiding in the shadows. And that is the message, loud and clear, of El Chichon.
Albert, February 2018
J.L. Macías et al. / Journal of Volcanology and Geothermal Research 175 (2008) 444–458: Hazard map of El Chichón volcano, Chiapas, México: Constraints posed by eruptive history and computer simulations
H. Sigurdsson, S.N. Carey and J.M. Espindola, Journal of Volcanology and Geothermal Research, 23 (1984) 11—37: The 1982 eruptions of El Chichon volcano, Mexico: stratigraphy of pyroclastic deposits
S. De la Cruz-Reyna and A. L. Martin Del Pozzo, Geofísica Internacional 48 (1), 21-31 (2009): The 1982 eruption of El Chichón volcano, Mexico: Eyewitness of the disaster
R. I. Tilling, Geofísica internacional 48 (1) (2009): El Chichón’s “surprise” eruption in 1982: Lessons for reducing volcano risk