As you may have already read, one of the reasons I chose to follow a career on Limnology was lake Atitlán (Guatemala). As if my emotional attachment to the lake wasn’t enough, it’s geological and natural history is fascinating (and several aspects are still a mystery!). However, some features of this beautiful ecosystem are still misunderstood on popular media, and many websites that talk about Atitlán still refer to it as “a lake on the crater of an old volcano”. I understand this confusion (and I kind of blame geologists for it — just kidding!). I will try to clear up this misunderstanding here! So let’s start at the very beginning: how do lakes originate?
Lake Types
Lakes around the world can be classified in different ways: based on their thermal stratification, if they are temporal or permanent, or by origin, to mention a few. One of the most widely accepted classifications of lakes by origin within the scientific community up to this day is the Hutchinson (1957) classification.
According to Hutchinson, throughout the world, we can find glacial lakes, tectonic lakes, fluvial lakes, solution lakes, aeolian lakes, landslide lakes, shoreline lakes, meteorite lakes, man-made lakes, and volcanic lakes. Within each of these categories, further sub-types can be found! However, you only have to worry about one type for today: caldera lakes (a sub-type from volcanic lakes).
What is a Caldera?
Imagine a volcano. Inside a volcano, we can find its magma chamber. The fluids and gases in the magma chamber are supporting the weight of the land above it (the volcano we can see!). Every now and then, some types of volcanoes release most of their chambers’ content during explosive eruptions. With the magma chamber empty, there is no more physical support and all the now-fractured land collapses. This new, bowl-shaped structure is what we call caldera.
So when a volcano is originating, outward explosions of rocks and other materials eventually form a volcano with its crater on top. A caldera, on the other hand, is the inward collapse of a volcano after an explosive eruption or magma retreat.
So, repeat after me: a caldera is not a crater.
Who would call it a crater?
Well, here’s the part where I understand the confusion. Calderas are further sub-divided into three types: resurgent calderas, shield volcano calderas, and crater-lake calderas. As you can find anywhere online, a great example is Crater Lake in the U.S. (Oregon). The lakes’ name is Crater Lake. It is a crater-lake caldera, but NOT a crater! Thank you very much, geologists.
Wouldn’t there still be some fiery stuff down there?
Well yes, whether near subducting plates or hotspots, there would still be a big chance that there is more trapped magma and gases in the lithosphere below. Some crater lakes around the world have small volcanic cones within the lake, fumaroles and hydrothermal vents.
And…where does all the water come from?
After a massive caldera-forming-explosion, part of the caldera will be filled with the land collapsing on it. A caldera can further get filled with water from precipitation (be it rain, snow, etc) as long as this surpasses the evaporation and infiltration rate. The water table level will also influence this and can contribute to the lake formation.
However, lake Atitlán is not the collapse of just one volcano!
Here’s where things get more interesting! Lake Atitlán (most of Central America, actually) is located in a very tectonic/volcanic zone. All the volcanoes we see today have not been there for a long (geologic) time and before them other volcanoes existed in the region.
Between 14-11 Ma, a big series of five eruptions took place, known as Maria Tecún. This collapsed into the first Atitlán caldera, located a bit north from today’s caldera. Some million years later (9-8 Ma), the San Jorge eruption “created” the Atitlán II caldera. Finally, there was a third cycle around 1 Ma. Back then, four volcanoes are thought to have existed in this area: volcano Paquisis, Tecolote, San Marcos and Xejolon (you can see this better in the map below). The explosion of these ones, known as “Los Chocoyos”, was followed by the collapse that formed the present basin (estimated 84,000 years ago). Shortly after this, the 3 post-caldera stratovolcanoes that we know today formed on the southern parts of the caldera (including the “Cerro de Oro” – what geologists call a “parasitic andesite dome” or lava dome).
We can find ashes from Los Chocoyos eruption all the way up to Florida!
Los Chocoyos eruption was really massive. It has been estimated that approximately 270 cubic km of rock and ash were expulsed. Based on geological studies, ash from Los Chocoyos eruption have been found all the way up to the Gulf of Mexico and the Florida peninsula, and all the way down to Costa Rica and on ocean cores west of Ecuador!
It has been estimated that the total depth of the III caldera was of 900 m! Initially, the first 300 m were filled with the collapse of the fractured rock after the eruption. Another 300 m were eventually filled with water (today, the deepest points on the lake are about 318 m deep). The other 300 m are where we can go hiking today!
Now, I don’t want to discourage your hiking or diving trips to Atitlán, but Newhall et al. (1987) suggest that the three volcanoes we can enjoy today in the Atitlán basin (and the super nice thermal waters near Santa Catarina Town) might represent the beginning of an Atitlán IV cycle. Any day now!
Just kidding. The dormancy periods may last several million years, and the last records of volcanic activity from Volcano Atitlán were around 1892, so we might be safe for now.
I hope I cleared things up!
Repeat after me: Lake Atitlán is not a crater!
Fun Fact!
As a native-Spanish-speaker, the term “caldera” was always pretty normal for me. However, whenever I talked about lake Atitlán in English, I always used the English word for caldera: cauldron. Yes, those big witchy pots. Through time I realized that even on scientific papers, textbooks and English-speaking professors they would also use the Spanish term when referring to these types of lakes.
I don’t know if there are other reasons why we are still ignoring the fact that the word caldera CAN be translated into many other languages, but we do owe the introduction of this Spanish term into the scientific vocabulary to a German geologist: Christian Leopold von Buch. After visiting the Canary Islands, where some caldera formations have the term Caldera on their name as called by the locals, he introduced the term in Spanish and it seems that that was the end of it (lets be grateful there was no Google Translate back then, otherwise we might still be calling them Hexenkessel. We dodged a bullet there!).
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References
- Drexler, J.W., W.I. Rose, R.S.J. Sparks, & M.T. Ledbetter. 1980. The Los Chocoyos Ash, Guatemala: A Major Stratigraphic Marker in Middle America and in Three Ocean Basins. Quaternary Research. 13:327-345
- Newhall, C.G. 1987. Geology of the Lake Atitlán Region, Western Guatemala. Journal of Volcanology and Geothermal Research. 33(1987):23-55.
- Newhall, C.G., C.K. Paull, J.P. Bradbury, A. Higuera-Gundy, L.J. Poppe, S. Self, N. Bonar Sharpless, & J. Ziagos. 1987. Recent Geologic History of Lake Lake Atitlán, a Caldera Lake in Western Guatemala. Journal of Volcanology and Geothermal Research. 33(1987):81-107.