Geological history
of the Atitlan Basin

Geological history of the Atitlan Basin

Central America, as part of the Pacific Ring of Fire, is geologically very active. Proof of this are the thirty-seven volcanoes in Guatemala (although there are about 288 volcanoes and structures that appear to have originated from volcanoes (INSIVUMEH, 2000). Volcanologists believe that the three giant calderas that can be detected today in the Atitlán area are related to a hot spot.

The oldest, Atitlán I, identified with the help of satellite images, had a cycle 14 to 11 million years ago. It was followed by Atitlán II, with a cycle 10 to 8 million years ago, and the most recent, Atitlán III, is about 1.5 million years old and has had four huge eruptions.

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The gigantic eruption left a large hollow and eventually collapsed the volcanic buildings, filling in some 300 metres of the caldera floor. Eventually the waters filled more than 300 metres of the old caldera. From the level of the lake to the top of the caldera is usually another 300 metres.
 
The eruption that formed the caldera is known as the Los Chocoyos eruption and ejected up to 300 km3 of tephra. The huge eruption vaporised life in what is now Guatemala and dispersed ash over an area of about 6,000,000 square kilometres. This ash has been detected from Florida to Ecuador, and can be used as a stratigraphic marker in both the Pacific and Atlantic oceans (known as Y-8 ash in marine deposits)[8]. A chocoyo is a type of bird often found nesting in the relatively soft ash layer of the area's bluffs.

Three distinct volcanoes, one true caldera

After the eruption of Los Chocoyos, continued volcanic activity has built up three volcanoes in the caldera. Volcán Atitlán lies on the southern edge of the caldera, while volcán San Pedro and volcán Tolimán lie within the caldera. San Pedro is the oldest of the three and appears to have stopped erupting about 40,000 years ago. Tolimán began to grow after San Pedro stopped erupting and is probably still active, although it has not erupted in historical times. Atitlán has developed almost entirely in the last 10,000 years, it is the only active volcano in the area with eruptions in 1469 and between 1717 and 1721. It was also intermittently active between 1826 and 1856. Of these last eruptions, the only strong one occurred on 3 May 1853, when ash darkened the skies around the lake. Cerro de Oro is a young lava dome less than 5,000 years old.