A brief history of Cotopaxi eruptions: a geologist looks to the past for clues of what to expect from the volcano and describes the worst case scenario

By George Lassiter

The editor of this website asked me to provide some historic background on the Cotopaxi volcano, which is currently in an eruptive stage. He also asked what we could expect if the volcano does in fact erupt.

In the first case, I can provide fairly reliable information. I made three trips to Ecuador between 1991 and 1995 to study volcanoes and learned much about their history, including Cotopaxi’s, in the process.

In the second case, I can only offer an old geologist’s best guess.

Cotopaxi is a classic example of a stratovolcano, which means it has a conical shape that has been built up by a series of eruptions. Like some other stratovolcanoes, it has grown very rapidly following a massive eruption about 4,400 years ago which destroyed the old volcano. More about that eruption in a minute.

The Cotopxi volcano.

Cotopaxi is one of the most active volcanoes in the world. During the 1700s and 1800s, which was an especially active period, the volcano had eight sizable eruptions. Looking back over the past 2,000 to 3,000 years, the volcano has had, on average, one large eruption per century. It has been relatively quiet in the last 130 years, with several minor eruptions between 1902 and 1907, and even smaller eruptions in the early 1940s.

The last large eruptions occurred in 1877 and 1880. From a historic perspective, the 1877 eruption was the most devastating. Pyroclatic flows and lahars covered more than 200 square miles around the eruption, wiping out dozens of small towns and killing an estimated 10,000 people. Although the 1880 eruption was about the same size, its impact was much less for the simple reason that all the towns and farms in its path had been destroyed three years earlier.

The 1877 and 1880 produced massive lahars from the melted ice of the glacier, some of them flowing down river channels 100 miles from the crater.

An aerial view of the Cotopaxi crater.

Given the volcano’s history, it is easy to say that Cotopaxi is overdue for a major eruption. However, estimating when and how strong an eruption will be is only an educated guess.

Most of the current indications are that Cotopaxi will erupt during the current active cycle. The director of the Ecuador Geophysical Institute, Mario Ruiz, believes an eruption is imminent and can come at almost any time (click here, for an interview with Mario Ruiz). Like any other prudent geologist, Ruiz also says that it is possible that volcanic activity will subside and the threat will be reduced.

Even though history tells us we are due for a large eruption, it also tells us that the range of intensity can vary enormously, from being relatively minor and only affecting people and property close to the mountain, to being catastrophic, killing thousands.

Unfortunately, because of the recent experience with the Tungurahua volcano near Ambato, I believe Ecuadorians have been lulled into a dangerous complacency about volcanic eruptions. Since 1999, when Tungurahua entered its current cycle of activity, people have been warned many times that a big eruption could occur but none has. To many, it has been a case of hearing “wolf” cried too many times.

But Tungurahua is no Cotopaxi. In worst case scenarios for both volcanoes, Cotopaxi is a thousand times more powerful. Comparing the two is a fool’s game.

For the worst case scenario for Cotopaxi, we can look at the eruption 4,400 years ago that I mentioned above. It was the largest eruption in South America in the last 11,000 or 12,000 years. It dwarfed more recent benchmark volcanoes such as Tambora and Krakatoa. The ash that it blasted into the atmosphere cooled the planet by five or six degrees Fahrenheit for several years and was probably responsible for mass extinctions of plants and animals.

In local terms, it sent flows of debris 250 miles to the northwest, reaching the Pacific Ocean in several places. It covered the Los Chillos Valley, which today is a densely populated area southeast of Quito, with a hundred feet of volcanic debris in some places. It did the same to the west and east of the mountain.

It is extremely unlikely that Cotopaxi will have another event like that one, even though the geologic forces are still in place that produced it. I mention it only as a metric by which to measure the power of Cotopaxi. Its lesson is to take any potential eruption very seriously and for the population close to the volcano to pay close attention to the government’s evacuation plans.

From everything I hear, the government of Ecuador is moving quickly and intelligently to develop emergency plans in case of an eruption. It is certainly getting very good information from Mario Ruiz and his staff at the Geophysical Institute.

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George Lassiter is a retired geology professor and researcher who conducted field studies of Ecuador’s volcanoes in the 1990s. He commented in an earlier article about Cuenca’s exposure to the effects of a Cotopaxi eruption (click here to read the article). He lives part-time in Cuenca.