nevado del ruiz colombia 1985 eruption case study

Benchmarks: November 13, 1985: Nevado del Ruiz eruption triggers deadly lahars

by Bethany Augliere Thursday, October 20, 2016

The 1985 eruption of Nevado del Ruiz in Colombia unleashed deadly lahars that swept through Armero, killing 20,000 people in that town alone. Credit: U.S. Geological Survey.

By Bethany Augliere

On Nov. 13, 1985, at a little after 9 p.m. local time, Nevado del Ruiz, a volcano about 130 kilometers from Colombia’s capital city of Bogotá, erupted, spewing a violent mix of hot ash and lava into the atmosphere. Less than three hours later, the earth rumbled as mudflows towering nearly 30 meters high swept through the countryside, several villages and eventually the town of Armero, where it killed 70 percent of the town’s residents. All-told, these mudflows, called lahars, killed more than 23,000 people.

Although it wasn’t considered a large eruption, heat melted snow and ice from a glacier capping the volcano and unleashed three lahars — a mixture of rock, volcanic debris, mud and water. The lahars roared down the volcano’s flanks at 30 kilometers per hour, picking up everything in their path, including trees and vehicles, as well as sediment and water from the rivers whose paths they followed. Ultimately, the lahars pulverized and entombed the towns in their wake. The next day, several more eruptions occurred.

The 1985 eruption of Nevado del Ruiz was Colombia’s worst natural disaster, the second-deadliest volcanic disaster of the 20th century (behind the 1902 eruption of Mount Pelée in Martinque) and the fourth-deadliest in recorded history.

In many ways, the eruption did not come as a surprise, wrote Barry Voight, then a geologist at Penn State University, in an analysis of the event. But, despite that, emergency management failed, causing unnecessary loss of life.

In the wake of the tragedy, two agencies — the U.S. Geological Survey (USGS) and the U.S. Agency for International Development’s Office of U.S. Foreign Disaster Assistance — came together to launch the international Volcano Disaster Assistance Program ( VDAP ) to track and monitor the world’s 1,550 potentially active volcanoes. Since its inception 30 years ago, VDAP has aided in more than 30 international crises, including the 2007 eruption of Nevado del Huila in Colombia, during which 4,000 people were safely evacuated.

“Ruiz is an absolutely seminal event in modern volcanology by virtue of demonstrating the hazards of long-reaching lahars from snow- and ice-clad volcanoes,” says Jeffrey Marso , a geologist with USGS and VDAP team member who went to Colombia in the days after the eruption. “We now know,” he says, “that a relatively small eruption on a high snow- and ice-clad volcano can produce lahars that threaten populations many tens of kilometers away.”

Path to Eruption

The summit crater of Nevado del Ruiz in late November 1985. Credit: U.S. Geological Survey.

Nevado del Ruiz is a large stratovolcano, similar to Washington’s Mount St. Helens and Mount Rainier, reaching more than 5,300 meters above sea level and topped by glaciers. “Nevado” means “snow-capped.” It’s one of the northernmost volcanoes in the Andean Volcanic Belt, a volcano chain running along the western coast of South America. Ruiz is the second-highest volcano in Colombia and sits in the Los Nevados National Park, which is home to seven other volcanoes.

In recorded history, Ruiz had two notable eruptive events. In 1595, three Plinian eruptions killed 636 people when a lahar swept down the nearby river valleys. In 1845, 250 years later, mudflows again flooded the upper valley of the Lagunillas River, killing more than 1,000 people.

Ruiz began stirring again nearly a year prior to the 1985 eruption, with early activity marked by earthquakes and fumarolic activity. In November 1984, climbers on the mountain reported gas emerging from the summit crater. In December, three earthquakes were felt within 20 to 30 kilometers of Ruiz. This activity continued into the next year.

So, in early 1985, emergency planning began. Geothermal researchers for the Central Hidroeléctrica de Caldas (Chec) visited the summit in January and noticed a new pit in the bottom of the crater had developed. In response, a civic committee formed in the closest large city, Manizales, with the support of Chec and local government, to monitor Ruiz. But, on Feb. 23, the region’s only seismograph broke.

In March, three geologists, including John Tomblin, a seismologist from the United Nations Office of Disaster Relief, visited the area at the request of Colombia’s civil defense agency. “The abnormal activity at Volcán Ruiz corresponds to typical precursory events for an eruption of magnitude,” Tomblin concluded, after witnessing vapor columns hundreds of meters high. He recommended the immediate installation of a seismograph and the preparation of a hazard map.

In the meantime, the Colombian Institute of Geology and Mining (INGEOMINAS) requested seismographs, other equipment and technical expertise from local companies as well as several countries, including the U.S., Costa Rica, Ecuador and Mexico. A Colombian electric company and Costa Rica sent seismographs and some other equipment, and USGS sent additional equipment to help operate the seismographs. The USGS Deputy Chief for Latin America wrote in a note to the United Nations Disaster Relief Organization: “The opportunity is clear, and it is unfortunate that we can spare no one from the Hawaii or Cascades Observatories. … If the volcano is to blow, let us hope that both we and the Colombians are prepared.”

On Aug. 8, the Swiss Disaster Relief Corps and Swiss Seismological Service sent seismologist Bruno Martinelli to provide technical support. The four seismographs sent by Costa Rica and the Colombian electric company had been poorly located, he noted in written communication. Once they were relocated, he wrote, “the seismic system could be considered to be suitable for monitoring the volcano.”

On Sept. 11, Ruiz blew gas and steam in a phreatic eruption for seven hours, but no magmatic eruption followed. It caught the attention of the government, however, which began to develop a response plan. Scientists began working to develop a draft of a volcanic hazard map. The dangers of lahars were obvious. Martinelli viewed the map before leaving and wrote: “The danger of lahars, especially along the Rio Azufrado and Lagunillas, was the guiding theme of this work.”

On the night of the deadly eruption, everything should have been in place to evacuate. Before leaving Manizales in September, Martinelli commented that he “was convinced that everything would be done to limit the possible damage.”

The Devastation of Armero

Lahars devastated the region, destroying everything in their paths: homes, agricultural fields, schools and roads. Credit: U.S. Geological Survey.

At 3 p.m. on Nov. 13, 1985, Ruiz sent up steam and gas in another phreatic eruption. Then it settled, and the rain started. This had happened before without a major eruption and “there was no clear instrumental indication in the following hours that a bigger eruption was coming,” Marso says. “In general, the Colombians knew an eruption was possible, but didn’t have hours and hours of warning that it was coming,” he says. Local authorities met, but with no way to forecast the next eruption, no clear, authoritative evacuation was ordered. Six hours later, the situation turned serious when a magmatic eruption started. Once an eruption starts, Marso says, “there’s a fixed time frame for evacuation — at that point, the clock was ticking.”

In Manizales, scientists and officials detected the eruption as soon as it began at 9:09 p.m. However, Marso says, “it turns out there was just too little time.” After a 20-minute magmatic eruption, it took just an hour for the first lahar to reach the closest town of Chinchiná. There, a thousand people lost their lives, and 200 houses and three bridges were destroyed. The first lahar reached Armero about 11:30 p.m., with more lahars following; there, they killed more than 20,000 of the town’s 29,000 residents and left another 5,000 injured.

The lahars destroyed everything in their paths: roads, bridges, farm fields, aqueducts and telephone lines. They wiped out 50 schools, two hospitals and more than 5,000 homes. The region lost 60 percent of its livestock, 30 percent of grain and rice crops, and half a million bags of coffee. About 7,500 people were left homeless.

Lessons Learned

The lahars descended through the steep, narrow drainages and river canyons on Nevado del Ruiz at speeds reaching 50 kilometers per hour. Credit: U.S. Geological Survey.

Marso, who arrived a couple of days after the eruption with two other scientists, describes it as a frenetic and chaotic time. “There was a lot of ash and mud on the landscape,” he says. The U.S. military came to aid in search and rescue operations. “Once we got there, our primary focus was to quickly establish monitoring, because at that point, we didn’t really know what had happened — or what could happen,” Marso says. Even after the eruption, he says, some 90 percent of the ice was still at the summit. If more, or larger, eruptions were to occur, even more people — including those involved in search and rescue — would be endangered. “We spent quite a lot of time flying, putting instruments in the field.” Eventually, other scientists across disciplines showed up to study the volcano.

Volcano emergency management is not easy, Marso says. It takes communication and trust among scientists, local residents and government officials. “In the case of Ruiz, it almost worked perfectly,” says Marso, until it didn’t. In the aftermath, researchers realized that one of the biggest problems was that Armero and the towns nearest the volcano were without power so communication broke down; the scientists had no ability to alert the townspeople to evacuate. With two hours and 21 minutes between the eruption and the first lahar reaching Armero, the town could have been evacuated if communication systems had worked.

“A good partnership between science and civil authorities is essential to protecting people,” Marso says. The Ruiz eruption was a lesson for the world, and resulted in the formation of VDAP. Before Ruiz, a few scientists had been pushing to make a response team happen, but the question was how to fund it, Marso says. “Ruiz made it easier.” Now, VDAP maintains a response team and cache of equipment to respond quickly.

When Mount Pinatubo in the Philippines began to awaken in April 1991, VDAP responded. At the time, most officials thought that Pinatubo was not active and that an eruption would not be dangerous. However, by the third week of April, VDAP sent in a three-person team with monitoring gear. In collaboration with local scientists from the Philippine Institute of Volcanology and Seismology (PHIVOLCS), the team set up a network of seismometers and various instruments around Pinatubo to determine if an eruption was imminent. At the same time, scientists worked to understand the eruptive history of Pinatubo.

As activity increased, and an eruption became more likely, the teams developed response plans and hazard maps. When Pinatubo erupted on June 15, the surrounding area had already been evacuated. In total, about 300 people were killed, but 5,000 could have been. Pinatubo was the second-largest eruption of the 20th century (behind the 1912 eruption of Alaska’s Novarupta), but the combined PHIVOLCS-USGS team had forewarned emergency managers, who had moved most people out of harm’s way.

Colombia, in particular, did not want this kind of tragedy to repeat itself and made serious efforts to prevent it from happening again, Marso says. In 2007, Nevado del Huila — the highest volcano in Colombia — reactivated. VDAP assisted the local INGEOMINAS volcano observatory (now called the Servicio Geológico Colombiano) to improve the monitoring network. In 2008, when the volcano erupted at 9:45 p.m., more than 4,000 people were evacuated from the nearby town of Belalcázar. When a lahar swept through the town less than an hour later, no one was killed. “It was an amazing success, and a testament to the work done in Colombia, with assistance from the U.S., VDAP and other organizations, to see that crises do not become disasters,” Marso says. In 2008, “the Colombians got it absolutely right.”

Nevado del Ruiz hasn’t produced a catastrophic eruption since 1985, though it remains active. Early in 2016, a magnitude-2.9 earthquake occurred under the volcano. And, throughout this year, Ruiz has ejected occasional ash emissions, some reaching up to 1 kilometer high. Today, Marso says, “it’s probably one of the best-monitored volcanoes in the world.”

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Nevado del Ruiz Eruption of 1985 and Its Impact on the Community

Introduction, armero community in 1985, geological profile of the catastrophe, impact on the community, lessons for city planners.

Natural disasters tend to have a profoundly destructive impact on the communities that happen to be situated close enough to the epicenter. Should the unfavorable natural conditions coincide with the cumulative human error, the toll on resources and human lives alike may become very high. Economy, culture, and education in the affected communities may suffer grave consequences if the natural disaster hits them. However, such cases also allow drawing conclusions from the negative experiences and considering them for the urban planners and early warning systems alike. This was exactly the case with the catastrophic eruption of the Nevado del Ruiz volcano in Colombia that is now studied as a seminal event in modern volcanology (Augliere, 2016). The eruption provoked pyroclastic flows and lahars that destroyed a prospering community of Armero nearby and caused more than 20,000 casualties. Although the town has never been rebuilt, its tragic fate is a lesson in the necessity of learning geological history and improving early warning systems to avoid similar outcomes in the future.

As of 1985, when the Nevado del Ruiz erupted, Armero was a prospering community in Tolima, one of the 32 departments of Colombia located in the central part of the country. While not a large town by any account, it was still a seat of the eponymous administrative unit and had as many as 29,000 residents (Augliere, 2016). In economic terms, it was a place of “considerable… economic value” and a center of agricultural production responsible for a significant share of Columbian rice, sorghum, coffee, and sugar (Voight, 1990, p. 380). One of the reasons for this agricultural prosperity was Nevado del Ruiz volcano situated 45 km from the town, as the repeated volcanic eruptions had created a highly fertile mineralized soil (US Geological Survey [USGS], n.d.). Culturally, the town was predominantly populated by the Spanish-speaking white and mestizo Colombians. In terms of education, the community also fared reasonably well, as many of the 50 schools destroyed by the eruption were situated in Armero (Augliere, 2016). To summarize, Armero of 1985 was, by all accounts, a thriving agricultural community with fairly bright economic, cultural, and educational prospects.

Before delving into details of the eruption of 1985, it is necessary to briefly describe the sequence of geological events that led to the catastrophe. Nevado del Ruiz is a cone-shaped stratovolcano “built from successive layers of lava, ash, and pyroclastic-flow deposits” (“Nevado del Ruiz Volcano,” 2010). It is fed by the magma generated on the boundary between the overriding South American tectonic plate and subducting Nazca plate (“Nevado del Ruiz Volcano,” 2010).

Nevado del Ruiz is a glaciated volcano, meaning that the high temperatures of the eruption provoke the expansive melting of the nearby ice (“Global Volcanism Program,” 2020). As a result, whenever Nevado del Ruiz erupts, it produces lahars – large flows of liquid mud and pyroclasts, such as solidified magma from previous eruptions and fragmented lithics (Gómez-Arango et al., 2018). Significant earthquakes in 1984 and early 1985 led to the formation of a new crater in Nevado del Ruiz (Voight, 1990). Since that moment, the volcanic activity has been steadily rising, and a phreatic eruption occurred on September 11, 1985 (Voight, 1990). The volcano remained active, and the next major eruption came two months later.

Following almost a year of precursory activity, Nevado del Ruiz erupted on November 13, 1985. Lahars resulting from the 20-minute magmatic eruption affected several communities, such as the nearby town of Chinchiná, where approximately 1,000 people lost their lives, and 200 houses were destroyed (Augliere, 2016). However, it was Armero that bore the brunt of the catastrophe and suffered the vast majority of the casualties associated with it. Out of the 23,000 victims of the eruption and following lahars, more than 20,000 lived in Armero, meaning that the once vibrant town has lost more than two-thirds of its population (“How volcanoes work,” n.d.).

Additionally, more than 7,000 people became homeless in the wake of the catastrophe (Augliere, 2016). The immediate economic impact was also severe: the regions had “lost 60 percent of its livestock, 30 percent of grain and rice crops, and half a million bags of coffee” (Augliere, 2016). Overall, the losses associated with the eruption amounted to $1,000,000,000, or approximately one-fifth of Colombia’s GNP at the time (“How volcanoes work,” n.d.). Thus, the immediate adverse impact on the Armero community was significant in economic, cultural, and educational terms alike.

The long-term impact proved to be just as catastrophic, as the community never recovered from the lahars that engulfed it. The town was inundated completely, and no attempt has been made to restore or rebuild it (“Nevado del Ruiz Volcano,” 2010). Most of the survivors evacuated to the neighboring towns and villages, but the community of Armero was utterly destroyed. The seat of the Armero administrative unit within the Tolima department was relocated, and the former town of Armero remains unpopulated ever since. Hence, the eruption of Nevado del Ruiz is an example of a natural catastrophe that did not merely affect a given community’s economic, cultural, and educational conditions but wiped the entire community off the map.

One lesson the city planners can learn from the events of 1985 in Armero is the importance of considering geological history when planning a settlement. Nevado del Ruiz has a long history of volcanic activity and successive eruptions that lead to highly destructive lahars. There are historical records of the eruption in 1595, with the mudflows sweeping across the valleys of the Guali River and the Lagunilla River, resulting in a death toll of 636 (“How volcanoes work,” n.d.).

Another eruption occurred in 1845, once again causing catastrophic lahars and resulting in hundreds of people dead and significant property damage (Voight, 1990). As a 19 th -century observer noted, “It is astonishing that none of the inhabitants of these villages, built on the solidified mud of old mass movements, has even suspected [its] origin” (Voight, 1990, p. 350). This astonishment applies fully to Armero of 1985, as the town was built directly above the mudflows of the previous eruptions (Voight, 1990). An assessment of the geological history of the region could have alerted urban planners to the fact that the town’s position was highly vulnerable in the case of an eruption.

Another lesson to be learned from the events of 1895 is the importance of stable infrastructure as a prerequisite for effective early warning systems. The national government of Columbia, as well as the regional administrations, have been well aware of the possibility of the eruption for the better part of 1985 (Voight, 1990). After the eruption of November 13 was indicated, two hours had passed before the first lahar reached the outskirts of Armero, but this window of opportunity was squandered. It was, in no small part, the result of the faulty infrastructure: the towns nearest to the volcano lost their power soon after the eruption and, therefore, could not receive the warning in time (Augliere, 2016).

The local radio station also went off the air son after the eruption, thus leaving the people of Armero unwarned and unprepared for the catastrophe. One may agree with Voight (1990) that the main reason behind the tragic outcome was “cumulative human error” (p. 383). One of the facets of this error that has to be considered by urban planners is the importance of infrastructure, allowing swift and effective information exchange.

As one can see, the eruption of Nevado del Ruiz in 1985 was a natural catastrophe that had a profound negative impact on the community of Armero in Tolima, Columbia. Before the eruption, the town of Armero was a thriving agricultural community that fared well in economic, cultural, and educational terms. However, after a series of earthquakes in 1984-1985 provoked increased volcanic activity and led to the formation of a new crater, Nevado del Ruiz erupted in November after a prolonged period of precursory activity. The immediate results were catastrophic for Armero: the town lost more than 20,000 of its 29,000 residents, and the total costs of the disaster amounted to $1,000,000,000. The long-term effects proved to be severe a well: the community was wiped off the map entirely and never recovered. The 1985 eruption of Nevado del Ruiz provides valuable lessons for city planners, such as the utmost importance of knowing the regional geological and the necessity for stable infrastructure for providing effective communication.

Augliere, B. (2016). Benchmarks: November 13, 1985: Nevado del Ruiz eruption triggers deadly lahars . Earth Magazine. Web.

Global Volcanism Program | Nevado del Ruiz (2020). National Museum of Natural History. Web.

Gómez-Arango, J. A., Murcia, H., & Borrero, C. (2018). Finding eruptive mechanisms through pyroclasts from the current eruption (1984–present) at Nevado del Ruiz volcanic complex, Colombia. Journal of Volcanology and Geothermal Research, 364 , 48-58.

How volcanoes work . (N.d.). San Diego State University. Web.

Nevado del Ruiz Volcano, Colombia . (2010). Earth Observatory. Web.

USGS. (N.d). Lessons Learned from the Armero, Colombia Tragedy . Web.

Voight, B. (1990). The 1985 Nevado del Ruiz volcano catastrophe: Anatomy and retrospection. Journal of Volcanology and Geothermal Research, 44 (3-4), 349-386.

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Case Study on the Nevado Del Ruiz Volcanic Crisis

Environmental geology (205-bwt-03), dawson college.

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Andrew Singer Catherine Pappas-Maenz Earthquakes and Volcanoes 14 th of April 2019 Case Study on the Nevado Del Ruiz Volcanic Crisis

[The Nature of the Volcanic Hazard] In February 1985 the National Institute of Geology and Mines (INGEOMINAS) went on a visit to Nevado Del Ruiz to observe the volcano after the volcano started to see a rise in general activity. [6] Eight months later on November 13th, 1985, Ruiz erupted which led to destruction and more importantly, led to over 23,000 people losing their lives. While the eruption was relatively small, many wondered why this volcano led to so much destruction and why more precautions were not taken. However, through research it is evident that there were certain measures in place to ensure the volcano would not do too much damage. In fact, scientists warned the government long before the actual explosion that this volcano would eventually erupt, they just could not pinpoint a date. Although it is a controversial statement, the blame of disaster lies within the government of Colombia. They were the ones who failed to take the necessary measures in order to protect their citizens. They had received warnings from multiple organizations that they needed to evac- uate Armero and some other surrounding cities. The vulcanological organizations even warned them that Armero would be hit the hardest. There was even a banner at the mass funeral which read, "The volcano didn't kill 22,000 people. The government killed them." [1] Through the evid- ence presented it is clear to observe that no, the volcanic eruption was not preventable, but the deaths were. The government of Columbia should be held responsible for their neglect towards their people. [The Rescue Attempt]

The volcanic eruption sent out pyroclastic flows which eventually led to massive lahars to sweep through Armero. The lahars covered the city in mud leaving many people trapped. [1] Mudflows more than a mile-wide destroyed roads and bridges blocking access to the city. [1] The consistency of the mud would not allow for rescue equipment and heavy machinery to reach the city. The only real method of rescue was by airlifting people out. [1] The disaster occurred at night which also delayed the rescue attempts. It was impossible to reach all the potential survivors because there were not enough helicopters. When other helicopters did arrive, it was too late to be rescued. The government of Colombia should not be blamed for their rescue efforts. They did not have the money, the means, or the methods to save some of the potential survivors. [6] However, if the government would have listened to the warnings in the first place, it would have costed them much less to evacuate these people. It also would have saved the lives of many people. This tragedy could have been averted if the government would have taken the appropriate action in order to preserve the safety of their population. [3] [Lessons for the Future] After the Nevado del Ruiz tragedy, two agencies, the U. Geological Survey (USGS) and the U. Agency for International Development’s Office of U. Foreign Disaster Assistance united to make the International Volcano Disaster Assistance Program (VDAP). [6] The goal of this movement is to monitor the world’s 1550 active volcanoes. The goal of this unit is to make sure the mistakes made in Armero are not make again in the future. Volcanic eruptions are not and will not be preventable. Sooner or later, no matter how dormant it has been, an active volcano will explode. It is up to the scientific community and governments alike to work together and stop the death and destruction that volcanoes bring with

how many lives could have been saved on that day if everybody had been evacuated prior to the eruption. All that potential and all those good people were lost for the sole reason of neglect and discernment from their own government. Now that it is been a long time since this disaster occurred, society should look back at this disaster as a lesson as to what can happen if we are not prepared for the worst. We should also look back and remember those 23 000 people who died and work towards bringing them justice and reducing volcanic destruction.

Works Cited

• Watch this video to give you a better understanding of difficulties during the rescue efforts. Warning: some disturbing footage (approx minutes long). youtube/watch?v=3XMS-quxdGg2.
• Studies Find Warnings of Volcano Were in Vain nytimes/1986/06/29/us/studies-find-warnings-of-volcano-were-in- vain?pagewanted=all3.
• Columbia’s Rescue Operation Draws Divided Assessments nytimes/1985/11/24/world/colombia-s-rescue-operation-draws- divided-assessments?pagewanted=all4.
• USGS Lahar Hazards volcanoes.usgs/hazards/lahar/ruiz.php5.
• Preventing Volcanic Disasters: The Critical Nature of Communication london-nerc-dtp/2016/05/09/preventing-volcanic-disasters-the-critical- nature-of-communication/6.
• Nevado Del Ruiz – Summary Paper- Eruption, Outcome and Consequences Document prepared by: Catherine Pappas Maenz, Dawson College
• Multiple Choice

Course : Environmental Geology (205-BWT-03)

University : dawson college.

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Report on Nevado del Ruiz (Colombia) — November 1985

Scientific Event Alert Network Bulletin, vol. 10, no. 11 (November 1985) Managing Editor: Lindsay McClelland. Nevado del Ruiz (Colombia) Seismic swarms, latest with inflation; more on 13 November activity and products

Please cite this report as: Global Volcanism Program, 1985. Report on Nevado del Ruiz (Colombia) (McClelland, L., ed.). Scientific Event Alert Network Bulletin , 10:11. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198511-351020

Nevado del Ruiz

4.892°n, 75.324°w; summit elev. 5279 m, all times are local (unless otherwise noted).

Since the 13 November eruption, activity at Ruiz has been limited to emission of a vapor plume and a few seismic swarms, one accompanied by measureable inflation. Work by numerous geologists has yielded new information on the 13 November eruption, its products, and pre-eruption activity.

Pre-13 November activity. The most vigorous seismic energy release at Ruiz occurred in the days preceding the 11 September ash emission. The rate of energy release increased prior to the 13 November eruption, but more gradually than before the 11 September activity. Hypocenters were concentrated N and NE of the summit with best-located events concentrated at depths 0-1 km below sea level (figure 3).

The quoted material below is from a report from Rodolfo Van der Laat, Eduardo Parra, and Heyley Vergara.

"After 11 September, when there was a significant ash emission, activity at Ruiz had decreased notably through 10 November. The activity caused concern in Manizales (30 km NW of Arenas Crater), but the presentation by INGEOMINAS of a preliminary volcanic risk map (figure 4) calmed the population.

"Seismic activity reached a maximum of 60 events per day 19-21 October, declining by 3 November to 3-5 daily locatable events. An increase in temperature of the thermal vent 'La Hedionda' (on the NE flank) may have been related to the increase in seismic activity.

"The height of the plume during this period decreased from about 3 km at the end of September and the beginning of October to about 800 m, with an occasional nucleus of ash 200-300 m high. There were two main fumarolic vents: one yellowish (sulfur), the smaller one gray/coffee-colored (ash derived from mud).

"A tilt network was established, detecting a general deflation 26 October-3 November, with small pulses of inflation of the order of 5-10 µrad per day. At the beginning of November, the first measurement was made of a geodesic net to monitor horizontal deformation by the Instituto Geográfico Agustín Codazzi."

13 November eruption and products. Details of the 13 November eruption sequence remain uncertain and field investigations were still in progress at press time. An initial explosion at 1530 deposited a very thin, fine-grained layer of ash around the summit and NNE of the volcano. The main explosion started at 2108 or 2109 and continued for 20-30 minutes. Five kilometers from the crater, tephra from the main explosion was 7 cm thick and included 30-cm pumice fragments, but the deposit thinned rapidly and was only 1-2 mm thick at Armero with similar amounts at Mariquita and Honda (75 km NE). Preliminary estimates by Haraldur Sigurdsson and Steven Carey place the volume of tephra at roughly 39 x 10 6 m 3 . Cloudy weather and lack of nearby wind data on 13 November impeded determination of the height of the Ruiz eruption column. Based on the position of tephra diameter isopleths, Sigurdsson and Carey inferred that the top of the eruption cloud reached [31] km altitude, but emphasized that most of the tephra probably remained in the upper troposphere [Naranjo and others, 1986].

Mudflows that moved E down the valleys of the Lagunillas and Azufrado rivers and inundated Armero were overlain by airfall tephra within 5-10 km of the volcano. However, the mudflow that moved W down the Río Claro valley to Chinchiná contained fresh pumice, and the fluid mudflow that traveled down the Gualí river washed tephra off vegetation, suggesting that both were generated after tephra ejection. The Armero mudflows emerged from both the Lagunillas and Azufrado valleys, which join upstream from the city. The first wave of mud, probably from the Lagunillas, was apparently colder, lighter colored, more water-rich, and formed a more extensive deposit than the second wave, probably from the Azufrado, which was hotter, coarser, and darker-colored. Donald Lowe estimated that outflow from the mouth of the Río Lagunillas reached about 47,500 m 3 /s. Preliminary calculations by Sigurdsson and Carey yield a volume of about 30-60 x 10 6 m 3 for the deposits of the Armero, and Gualí and Chinchiná valley mudflows, plus about 30-90 x 10 6 m 3 of water, roughly 6-18% of the pre-eruption volume of the summit ice cap. The Lagunillas mudflow probably included water from a lake that had been trapped behind a debris dam in that valley's headwaters for at least several months. Other estimates suggested that about 5% of the summit ice was removed during the 13 November eruption.

Preliminary chemical analyses of a few samples of the 13 November pumice suggest that it is a hypersthene andesite, very similar to an earlier pumice that was probably from Ruiz's last large eruption, in 1595. Little systematic variation was found in different-colored samples that had suggested mixed magma in hand specimen (table 1).

Table 1. Preliminary analyses of bulk compositions of Ruiz pumice (1-4) and glass septa (5-6). Numbers 1, 2, 5, and 6 are from electron microprobe analyses by William Melson and Deborah Reid Jerez; 3 and 4 are X-ray fluorescence analyses by Joseph Taggart.

Post-13 November activity. No significant eruptive activity occurred in the succeeding weeks. The vapor column varied in height from 200-300 m to 1-1.4 km. Rates of SO 2 emission measured by COSPEC were 200 t/d on 18 November, 50 t/d on the 19th, and several thousand tons per day on 22 November. Possible new fissures have been observed near the summit along with possible development of a depression SW of the summit. However, the fissures may have been pre-existing features exposed by clearer weather and seasonal snowmelt. Slight advances of some of the summit glaciers have been noted, but no large-scale ice movements were apparent and there was no evidence of significant melting from below.

Six telemetering seismometers have been installed, ringing the summit at elevations of 4,000-4,500 m, supplementing the four-station seismic net that was in place before 13 November. Telemetering tiltmeters were emplaced at 4,200 m elevation on the NW flank, 4,600 m elevation on the NNW flank, and on the NE flank, and 8-10 EDM lines have been established, in addition to the dry-tilt network installed on the N flank in October.

Seismic energy release was at relatively low levels shortly after the 13 November eruption, but the slope of the energy release curve steepened in the succeeding weeks. Earthquake swarms that were small but of increasing energy occurred 19-20 and 27 November, and 6-7 December. Maximum magnitudes were 2.5-3 in the November swarms; the 6-7 December activity included two magnitude 3-3.5 shocks. Locations were available for only a few events, which were centered along a generally N-S trend, usually somewhat N of the crater. The swarms were not accompanied by measurable tilt episodes or obvious changes to the plume. The rate of seismic energy release doubled during the first day of a stronger swarm 12-13 December and Civil Defense personnel were put on alert. The same day, the NW flank tiltmeter recorded a 5 µrad tilt event, the first change recorded in the weeks since it was installed, and a NW flank EDM line shortened 14 cm between measurements 11 and 13 December. However, seismicity declined 13 December, and the seismic energy release curve was nearly flat 14-17 December.

Geological Summary. Nevado del Ruiz is a broad, glacier-covered volcano in central Colombia that covers more than 200 km2. Three major edifices, composed of andesitic and dacitic lavas and andesitic pyroclastics, have been constructed since the beginning of the Pleistocene. The modern cone consists of a broad cluster of lava domes built within the caldera of an older edifice. The 1-km-wide, 240-m-deep Arenas crater occupies the summit. The prominent La Olleta pyroclastic cone located on the SW flank may also have been active in historical time. Steep headwalls of massive landslides cut the flanks. Melting of its summit icecap during historical eruptions, which date back to the 16th century, has resulted in devastating lahars, including one in 1985 that was South America's deadliest eruption.

Information Contacts: P. Medina , Comité de Estudios Vulcanológicos, Manizales; A. López R. , INGEOMINAS, Bogotá; R. Van der Laat , Univ. Nacional, Heredia; E. Parra , INGEOMINAS, Medellín; H. Vergara , INGEOMINAS, Tolima; H. Sigurdsson and S. Carey , Univ. of Rhode Island; S. Williams and D. Lowe , Louisiana State Univ.; A. Londoño C. , Univ. Nacional, Manizales; Néstor Garcia P. , Industria Licorera de Caldas, Manizales; R. Stoiber and B. Gemmell , Dartmouth College; D. Harlow , USGS, Menlo Park, CA; C. Hearn , D. Klick , D. Herd , and R. Tilling , USGS, Reston, VA; J. Taggart, Jr. , USGS, Denver, CO; W. Melson and D. Jerez , SI; P. Clemente-Colón , NOAA/NESDIS.