Though I walk through the valley of the shadow of death, I shall fear no evil
It is said that within great beauty lies grave danger and few places are more beautiful than Lake Atitlán. Poets have filled cheap notebooks with particularly flowery hyperbole in their attempts to catch the beauty of the caldera lake and the encircling volcanoes roaring out of the lake mist, catching fire as the morning sun hits them turning the green into imaginary gold and fiery red. It is truly a place of dreams, and as with every dream there is a spot of darkness hinting at a lack of a sign: Beware! Here be Dragons.
I rarely write about supervolcanoes. To date I have only covered two real supervolcanoes. The first was the perfectly balanced volcanic system of Tondano and the second was the rather comatose Yellowstone. I have though on occasion mentioned that some volcanoes have better PR agents than others. Where Yellowstone in its dying shroud sprouts a scientific volcanic paper a month, the rest of them will be happy to merit one per year for all of the known supervolcanoes.
It is sad that things are like this, but it is in the end hard cold political facts running science. Yellowstone will get you money to write a pointless paper and it is all about the number of papers, not the quality or merit of research that runs academia in the twenty-first century.
In the end it might not matter that much, the main reason I do not write about supervolcanoes is that they are such a boring bunch. After all, they very rarely do anything interesting, and when they do they either suffer a side vent eruption like Tondano, calmly inflate like Uturunku, or do a bradyseism and goes back to sleep. Or they do nothing and get papers written.
So let us get back to the matter at hand, the rather interesting Lake Atitlán.
A brief background
Lake Atitlán as we see it today is the result of activity after the 300 cubic kilometer Atitlán III eruption that happened 84 000 years ago. In the wreckage of the caldera collapse, the volcano started a new cycle towards yet another major explosive event, by the rapid serial construction of 3 major volcanic edifices, one after another and each of them built more rapidly than the previous.
The oldest of the 3 volcanoes born after the Los Chocoyos caldera event is the 3020 meter high Volcán San Pedro. It was built during 44 000 years of eruptive activity ending 40 000 years ago. After that volcanic activity started at the Volcán Tolimán which in the next 30 000 years grow into a 3 158 meter high edifice.
On the flank of Tolimán is the failed volcano of Cerro de Oro that might have erupted 5 000 years ago. Instead the new main volcano was born on the rim of the old caldera instead of within it as Volcán Atitlán started erupting 10 000 years ago. This volcano grew very rapidly and matured into a classic 3 535 meter strato volcano.
In historic times Volcán Atitlán is the only erupting volcano in or near the Atitlán Caldera. The volcano suffers mostly from small to medium sized explosive eruptions. In the 384 years between 1469 and 1853 the volcano suffered 12 eruptions ranging from VEI-2 to VEI-3, but after 1853 the volcano has gone into a dormant state without any obvious signs of a pending eruption.
The area is highly seismic and in 1976 the area was struck by a M7.5 earthquake that killed 26 000 people. The earthquake most likely fractured the bottom of the lake and within a month the lake water level dropped two meters. It though rapidly filled up, most likely due to later seismicity closing the fractures.
Lake Atitlán is more than 300 meters deep and is covered with a 300 meter deep layer of sediments. The sedimental layer is filled with prodigious amounts of methane that effectively hinder analysis of the actual caldera bottom 600 meters below the surface.
Sedimentation occurs with a normal rate of 0.5 centimeters per year, and that accounts for half of the 20 meter uplift of the water level in the last 2 000 years. We know the rate of uplift from C14-dating of artifacts found in a sunken Mayan village.
The lake is suffering from extensive deep hydrothermal activity releasing large amounts of energy (290mW/h inside the lake and 230mW/h outside), this creates an ideal environment for the silt producing cyanobacteria inhabiting the lower two thirds of the lake (the source of the methane in the sediment).
To compound the problem the lake is endorheic, lacking any outlet of surface water, instead release happens through seepage into two river systems through the caldera rock. This traps the water at depth and makes it very stagnant and a perfect breeding ground for highly toxic cyanobacteria.
The Caldera today
By now the caldera has filled in with about one third due to the construction of the three volcanoes, and another third has been filled in by silting and resurgent pistoning of the caldera floor due to magmatic intrusions.
Under the caldera floor there is believed to be a large reservoir of magma with an unknown amount of eruptible material. The amount of hydrothermal energy seems to signal that the amount of hot material is either very large, or quite close to the surface. The lack of doming though seems to indicate that the main reservoir is at depth since the resurgence is uniform over the area.
During the last 2 000 years there is evidence of episodes with highly increased silting due to hydrothermal energy increase. This most likely are signals of magma moving upwards into more shallow repositories.
During the last 15 months the surface of the lake has risen uniformly more than 200 centimeters, something that has caused great problems for the local Mayan residents.
The dangers of Atitlán Caldera
There are four principal dangers from the current volcanic system. I will briefly expound upon them in the order of possible risk for anyone nearby.
The largest danger is obviously seismic. Another large earthquake may happen here at any time. The second largest risk is of course that Volcán Atitlán suffers a renewal of volcanic activity and erupts. The eruption would not be that large, but evacuation of nearby population might be problematic.
The second least likely risk is due to the unique sedimental setup and the large amount of deep water cyanobacteria. With time the amount of methane trapped in the sediment will be higher than is possible to contain within the confines of the sediment. As the lake gets closer to the threshold of containment the likelihood of an earthquake setting off a chain reaction increases and as that happens the bottom sediment will explode out of the lake bringing with it a deadly mixture of cyanobacteria, SO2 and methane poisoning everyone within the caldera. For those who are unfamiliar with these phenomena I recommend googling the Lake Nyos disaster.
The least likely risk is that a large fast explosive eruption (VEI-6) will empty out the magma reservoir under the caldera floor at such a speed that the roof of the reservoir collapses and a supereruption occurs.
There are many possible ways to explain what is currently happening with the water level in Atitlán Caldera. There are though three that are more likely than the others.
Seismic activity could have pushed together the natural pathways for the seepage out of the lake into the rivers. There is though no evidence that the rivers fed by the seepage have suffered a decrease in water reception.
Then there could be increased upwelling of hydrothermal water into the lake. This would lift the level of the cyanobacteria boundary upwards and there would be cyanobacterial (algae) blooming. We know for a fact that there have been large problems with such blooms during the last 15 months.
The third cause could also be piston-like inflation of the caldera floor due to magmatic intrusion either at depth, or in a shallower reservoir under the lake itself. The most likely thing in the end is a combination of a shallow emplacement causing a dramatic increase in hydrothermal activity.
Locally the belief is that the increase in surface cyanobacteria is due to release of waste water and other nutrients into the lake. This is highly unlikely to be the main cause; the waste water flow into the river has been roughly the same for decades. And in the end this would have caused normal algae bloom of garden variety green algae, not cyanobacteria. Why now? Well, for starters waste water contain too little sulphur to feed the bacteria, also you need something that cooks off the oxygen, and explaining that with sunlight warming the water does just not cut it. It is much more likely that an increase in hydrothermal activity has elevated the temperature of the lake and deposited the necessary sulphur to feed the cyanobacteria.
Most likely this is all caused by a deep intrusion of magma at depth, of which a small part has intruded in a shallow reservoir under the lake. This in turn caused increased turbulence of the water at depth as the levels of hydrothermal activity sky-rocketed. In the end there is an increased risk of eruptions occurring in Volcán Atitlán or from a new vent. I do though not believe that this will cause any large eruption within the foreseeable future. After all supervolcanoes are well known to inflate at prodigious rates and very rapidly without erupting, or erupting through one of the side vents that functions as safety valves.
Sorry all, no supereruption seems to be around the corner even though there is a bit of pause for thought when something on this scale shows signs of unrest.
CARL & CARMEN MORATAYA