Geology of Lake Atitlan Volcanoes

Matt Purvis

Lake Atitlan fills the Atitlan caldera, a huge hole in the ground caused by a massive volcanic eruption 85,000 years ago. Every volcano has a magma chamber, an area underground where lava collects before erupting from the volcano. The magma chamber under Atitlan was so big that when the lava was ejected, the land above the magma chamber collapsed into the space underground.

The eruption and subsequent collapse formed the Atitlan caldera. 
The cliffs surrounding the lake mark the edges of this caldera, showing where the land collapsed and the size of the magma chamber, roughly 18-20 kilometers in diameter. The eruption has been named the Los Chocoyos event after the birds that now burrow nest holes into the extensive deposits.

To bring the size of Los Chocoyos into perspective, it helps to compare it with the eruption of some modern day volcanoes. When Mt. St. Helens in the U.S.A. blew up in 1980, 1km3 (one cubic kilometer) of rock, lava and ash shot from the volcano. This seems small compared with the 1991 eruption of Mt. Pinatubo in the Philippines, which ejected around 10 km3 of magma, the largest in living memory.

The Big One

Both of these modern eruptions are dwarfed by Los Chocoyos 85,000 years ago. A total of 270km3 of volcanic debris was thrown out of the caldera, blanketing Guatemala with ash and rock, 200 meters deep in some places.

Ash from the Atitlan caldera can be found in the rock strata as far north as Florida and as far South as Panama and most of Central America would have experienced a darkening of the skies as the ash rose into the atmosphere.

The author of this article, Matt Purvis, is a geologist who recently started working with the local guides taking people up the volcano. He explains how Lake Atitlan formed, talks about the geology of Guatemala, and acts as a translator for the local guide regarding the local flora and fauna. Along the way, Matt also discusses Guatemala’s three active volcanoes: Pacaya, Fuego and Santiaguito.