When the concept of volcanic catastrophe is brought to mind, most people envision large-scale plinian eruptions such as Mt. St. Helen’s famous lateral blast, or Pinatubo’s majestic VEI-6 eruption in 1991.
Large eruptions in theory do pose a greater risk to local inhabitants than small eruptions would, but that doesn’t necessarily mean you need a large eruption to cause a catastrophe. In 1985, this concept was proven when Nevado Del Ruiz woke up with a VEI-3 eruption that is responsible for one of the greatest natural disasters in history.
Background on Ruiz’s Recent History
Around the time of 1984, Nevado Del Ruiz had started to see an incline in overall activity, including increased seismicity, increased sulphur emissions, and bursts of small phreataic eruptions. By November 1985, Ruiz reached a tipping point and finally erupted. Geologists noticed a cluster of long-period earthquakes prior to it’s signature eruption, but unfortunately failed to identify it as a tell-tale precursor of an eruption.
The 1985 eruption was not particularly large on a geologic scale. VEI-3 events happen almost every year, and sometimes may occur multiple times within a single year. The problem with Ruiz is that it doesn’t require large eruptions to devastate the surrounding landscape. Much like Mt. Rainier, the most notable facet to the Ruiz summit is the massive glacier that sits on top.
When Ruiz erupted in 1985, there was just enough heat to melt a large portion of the glacier perched on top of the summit, which created an unimaginably large lahar. This Lahar swept down the mountain’s slopes and wiped the community of Armero from the face of the earth. The event was so devastating, that only 1/4 of the community’s residents survived, leaving approximately 23,000 people dead within a very short period from the initial eruption.
Current Risk Mitigation
After the 1985 disaster, Colombian authorities knew what they were dealing with, and made it a point to actively prevent risk associated with another eruption. While residents still have the 1985 event in mind, it is estimated that around 500,000 people are still at risk if Ruiz were to suffer another eruption similar to the event in 1985. The primary issue authorities face is that Lahars can travel incredibly fast, and in an extremely short period of time, the Ruiz Lahars can reach a distance of over 75km from the summit of the volcano.
Thankfully, Ingeominas, the Colombian authority is much more prepared now for a potential eruption, and they have taken steps to reduce any possible tragedy that could occur.
Is another Eruption Imminent?
As of 2010, Ruiz once again started to experience activity similar to what started occurring in 1984 prior to the 1985 eruption. Earthquake swarms, increased sulphur emissions, and eventual phreatic eruptions caused authorities to evacuate residents in 2012, and eventually the volcano did experience a minor ash eruption which was likely caused by a phreatic explosion. After this event, the volcano briefly died down in activity before experiencing more vigorous activity as of April 2013.
In April 2013, a large earthquake swarm started in a zone approximately 6-7km below the Arenas crater. The swarm has yet to stop since early April, and has since risen from 6-7km deep to 3-5km in depth. Since the commencement of the swarm, there have been over 17 events that have registered larger than 2.5m in magnitude, including several events that registered 4.0.
Additional to earthquakes, deformation, increased sulphur deposition, and increased fumarolic activity has been noticed since the advent of the April earthquake swarm.
All these events point to the likelihood of a magmatic eruption, which hasn’t been seen at Ruiz since the 1985 event. Keep in mind, volcanoes are not predictable, and Ruiz could easily cool down and hold off on erupting at any time. Ingeominas currently has the volcano on yellow alert, although if the trend continues, it would seem logical for the volcano to be upgraded to an orange alert status if the swarm continues to rise.
One thing to note is that if a magmatic eruption were to occur in the coming months, that would represent an uncharacteristically short repose time between eruptions based off the brief eruptive history we’ve seen in the last 6000 years or so.
What To Expect if Ruiz Erupts?
Ruiz’s magma like many stratovolcanoes in Colombia is dacitic in nature. Dacitic magma is quite viscous, which traditionally leads to more explosive eruptions. For comparison, Mt. St. Helens and Pinatubo also are dacitic volcanoes, which partially explains their violent eruptive histories.
While Ruiz does have the magma for more explosive eruptions, it has been relatively benign in terms of eruptive power in the last 11,000 years. In the last 11,000 years, most of the eruptions of Ruiz have been similar to the 1985 eruption, with numerous VEI 2, 3, and an occasional VEI 4 eruption occurring. As was previously mentioned, most of the damage and destruction that has occurred in this period has been a result of landslides and glacial melting resulting in large-scale debris avalanches and lahars.
While it’s most likely an eruption would be relatively similar to the 1985 eruption, Ruiz has had a very long past, and hasn’t always been so benign. Ruiz has gone through three separate periods in it’s 1.7 million year history, and the current period is relatively young in a geological time scale. The prior two eruptive periods were characterized by massive caldera forming eruptions, with multiple calderas (some as large as 10km across) that would indicate eruptions that would dwarf Mt. St Helens’ famous eruption. It would probably be a safe assumption to believe that Ruiz also experienced many smaller events during those time periods, but such smaller eruptions are difficult to spot and stratify, whereas massive eruptions are much easier to spot over long time periods.
One of the greatest risks that has a decent chance to occur would be some form of slope failure or edifice collapse. The Ruiz summit has been greatly destabilized by glaciation coupled with the greater hydrothermal system within the edifice, and there have been multiple instances of slope failures and landslides in the last 11,000 years.
While the most likely risk for a magmatic eruption would be a mid-sized event that creates large lahars, Ruiz really has the potential to do anything she pleases, and can’t be discounted by the local residents and government.
Update: Hazard map added so one can see how far from the volcano Armero really is.