Museum at Posada de Don Rodrigo Hotel

Map of Lake Atitlan

Scale Model of Lake Atitlan

Lake Atitlan's Geology

Lake Atitlan's Geology, 2nd Poster

Los Chocoyos Eruption, final phase

Past and Present formation of volcanoes

[Note: Because the charts are hard to read, and are mostly in Spanish, I'm providing relevant information similar to the charts from other sources.]
Other Sources:
Wikipedia on Lake Atitlan
Volcanic activity began in the Lake Atitlan area about 11-12 million years ago. The present-day stratovolcanoes and caldera represent the most recent of four periods of volcano growth and caldera collapse. This recent period of activity began about 1.8 million years ago. A large explosive eruption about 84,000 years ago formed the most recent Atitlan caldera. Lake Atitlan fills part of the caldera.
The 3535-m-high summit of Atitlán directly overlies the inferred margin of the Pleistocene Atitlán III caldera and is the highest of three large post-caldera stratovolcanoes constructed near the southern caldera rim. The volcano consequently post-dates the eruption of the voluminous, roughly 85,000-year-old rhyolitic Los Chocoyos tephra associated with formation of the Atitlán III caldera.
The explosion at Mt. St. Helen released 2 cu. kilometers of rock and ash, Pompeii blew out 6 cu. kilometers. Atitlan discharged over 180 cu. kilometers of hot ash and rock. An immense hole was left in the earth's crust, causing Atitlan to be one of the deepest lakes in Central America.
The caldera-forming eruption is known as Los Chocoyos eruption. The lake is shaped by deep escarpments which surround it and by three volcanoes on its southern flank.

Guatemala Interlude
About 150,000 years ago, a large magma chamber developed at modest depth (Batholith de los Chocoyos) from which volcanoes began to appear in the region. Then, about 85,000 years ago, there was one "super-duper" eruption (like a pressure cooker exploding) causing the whole batholith to discharge over a short period. Volcanic ash was spread all over Mexico and as far south as Costa Rica. In the central region the ash deposits were one to three meters thick. A cauldron some 18 kilometers in diameter formed and all previous volcanoes collapsed or disappeared. The huge depression soon filled with water to form Lago Atitlán, whose area has since been modified somewhat by the eruption of new volcanoes around the periphery (e.g. volcanoes San Pedro, San Marcos, Cristalina, Tolimán and Atitlán).
The wonderful lake vista that we see today is thus due to a cataclysmic volcanic explosion which, no doubt, caused appreciable global climate change for many years afterwards. The Los Chocoyos super-eruption is named after the chocoyos parakeets, which nest in the holes left behind from charred tree trunks buried in the volcanic ash deposits. Such a geological event happens all too frequently. In New Zealand, in the volcanic region of the North Island, beautiful Lake Taupo (full of trout) formed in the same way, and only 1,800 years ago!

The Origins of Lake Atitlan
Three large calderas have formed in the Atitlan region in the past 14 million years. The modern Atitlan III Caldera’s story began 150,000 years ago, when a magmatic batholith – a huge subterranean pocket filled with liquid magma – formed in the area that is now the lake. By 100,000 years ago, there were at least three volcanoes in the area being fed by the Los Chocoyos Batholith, though these volcanoes are today partially or completely destroyed.
After thousands of years of building up pressure, the Los Chocoyos Batholith finally discharged 84,000 years ago in a massive violent expulsion of over 250 cubic km of magma, ash, and sand. The eruptive column reached heights of 40-60 km. After its collapse, the batholith continued to spew magma and scolding ash, scorching everything it came into contact with as it advanced over a massive area.
Finally, so much magma had been expelled that only an empty cavity was left where the magma had been. Unable to support the weight of the earth above it, the entire area collapsed, forming the 18km-diameter cauldron (known to geologists as a caldera) that became the lake, and collapsing the existing volcanoes (Volcan Pakisis, Volcan Zakilac, and Volcan Santa Cruz) with it. You can see the geological top of the caldera when you look east across the lake at the tops of the cliffs above Panajachel.