COMPREHENSIVE GEOLOGICAL, VOLCANOLOGICAL, ARCHAEOLOGICAL, BIOLOGICAL AND GEOGRAPHIC SURVEY OF THE ATITLÁN CALDERA Region @2026-10-30 06:56:53 - 14.738679, -91.158410

REFERENCE COORDINATES Latitude: 14.738698° N Longitude: −91.158418° W Location: Calle 15 de Febrero, Panajachel, Sololá Department, Guatemala Panajachel elevation: approximately 1,597 m (5,240 ft); some municipal sources report 1,573 m LOCATION AND PHYSICAL SETTING Lake Atitlán occupies the Atitlán III caldera in the Guatemalan Highlands of southwestern Guatemala. The lake is a deep tropical caldera lake within the Central American Volcanic Arc, produced by subduction of the Cocos Plate beneath the Caribbean Plate. Surface area: 125.77 km² Maximum depth: 327.56 m Mean depth: 203.21 m Water volume: 25.46 km³ Shoreline perimeter: 101.67 km Drainage basin: 546.03 km² Caldera dimensions: approximately 15 × 25 km at the surface; the collapse depression itself is nearly circular, about 18 km in diameter, with an area of about 250 km² and an original depth of about 900 m The lake floor is comparatively flat and lies more than 300 m below the lake surface. Of the original 900 m collapse depth, roughly 300 m is now filled with lacustrine sediment and eruptive material from the post-caldera volcanoes; water occupies another roughly 300 m; the remaining roughly 300 m is bedrock. GEOLOGICAL EVOLUTION The Atitlán region records approximately 14 million years of volcanic evolution, with three major cycles of stratovolcano growth, silicic eruption, and caldera formation. The present Atitlán III caldera resulted from the Los Chocoyos eruption, dated to about 84,000 years before present. The Los Chocoyos eruption had a Volcanic Explosivity Index greater than 8, making it the largest known Quaternary eruption in the Central American Volcanic Arc. It produced approximately 1,220 km³ of dispersed rhyolitic ash and unwelded pyroclastic-flow deposits, equivalent to about 730 km³ dense-rock equivalent (DRE). Earlier estimates of 250–300 km³ DRE were superseded by the 2024 USGS Bayesian dating study. The eruption column may have reached 30–50 km in height, and associated deposits extended to a radius of about 100 km around the vent. Ash reached Costa Rica, the Gulf of Panama, and parts of the Florida Strait, covering an area exceeding one million square kilometres. The eruption lasted at least 20–27 days. Pyroclastic flows extended more than 100 km to the northeast and northwest, filling valleys to depths of about 200 m in some locations. Caldera collapse occurred at least several weeks after the climactic eruption, as the magma chamber drained and the roof lost support. The resulting depression began filling with water immediately after collapse; the rivers Quiscab and Panajachel contributed to the filling. POST-CALDERA VOLCANOES Three principal post-caldera volcanoes developed within or on the rim of the collapse structure: Volcán San Pedro: elevation approximately 3,020 m; located inside the caldera; composition andesite to basaltic andesite. It was the first post-caldera volcano to begin forming. Volcán Tolimán: elevation approximately 3,158 m; located within the caldera; composition andesitic lavas and dacitic products. Volcán Atitlán: elevation approximately 3,537 m; located on the southern caldera rim; the youngest, tallest, and most active of the three; composition andesite to basaltic andesite. Only Volcán Atitlán has documented historical eruptions: reports exist for activity shortly before the Spanish conquest, 1717–1721, 1826, 1827, and 3 May 1853. A fourth, smaller post-caldera feature is Cerro de Oro, a parasitic lava dome on the northern flank of Tolimán, near the lake shore. Its flow entered Lake Atitlán and may be only a few thousand years old, making it the most recent volcanic feature in the basin. MINERALS, ROCKS, AND GEOCHEMISTRY Dominant volcanic materials around Atitlán are rhyolitic ignimbrite, pumice, ash, dacite, andesite, and basaltic-andesite lavas. The Los Chocoyos deposits are unwelded, poorly sorted, and rich in pumice. Rhyolite is the principal silicic rock of the Los Chocoyos ignimbrite; it is silica-rich and commonly contains quartz, sanidine/feldspar, biotite, and volcanic glass. Plagioclase and zircon are the minerals used in modern dating of Los Chocoyos ash; zircon is especially useful because it survives erosion and preserves crystallization ages. Rhyolitic terrain tends to produce relatively acidic soils because of high silicon and low calcium and magnesium; this helps explain pine tolerance on some ignimbrite surfaces. The lake is not a saline mineral lake; its principal contemporary water-quality issue is nutrient enrichment and cyanobacteria, not economically significant dissolved-mineral accumulation. LAKE HYDROLOGY AND LIMNOLOGY Lake Atitlán is a deep, concave tropical lake with strong depth and volume relative to its surface area. AMSCLAE, Guatemala’s Lake Atitlán basin authority, conducts routine physical, chemical, and biological monitoring at five lake sampling stations. NASA and AMSCLAE use satellite observations plus field water samples to track cyanobacteria concentrations and nutrient inputs carried by rainfall into tributaries. A 15-year dataset (2010–2024) revealed a warming trend of 0.34 ± 0.14 °C per decade, intensified during warmer phases of the El Niño–Southern Oscillation. Increased air temperatures and reduced wind speeds have been related to shallower thermocline depths and stronger stratification, which prolong anoxia and deteriorate deep-water quality during stagnation. The thermocline typically lies at 35–40 m depth. Oxygen concentrations above about 40 m oscillate between roughly 7 and 8.5 mg/L; below 40 m they drop to between roughly 2 and 4 mg/L. Years with complete mixing release phosphorus to the surface, creating low dissolved inorganic nitrogen-to-phosphorus ratios associated with cyanobacteria blooms, notably in 2011–2013, 2015, and 2018. Only about 55% of households in the basin are connected to sewage systems, a factor in nutrient loading. BIOLOGY The lake’s fish fauna has been heavily altered. Only two native fish species are documented in the lake: Cichlasoma nigrofasciatum and Poeciliopsis gracilis. Numerous introduced species are present, including largemouth bass (Micropterus salmoides), smallmouth bass (Micropterus dolomieu), bluegill (Lepomis macrochirus), black crappie (Pomoxis nigromaculatus), and common carp (Cyprinus carpio). Largemouth bass were introduced in 1958 for sport fishing; smallmouth bass followed in 1960. The introduction of bass is directly linked to the extinction of the Atitlán giant grebe (Podilymbus gigas), a flightless waterbird endemic to Lake Atitlán and known locally as “La Poc.” The population declined to about 30 individuals by 1983, and the species was extinct by 1986. ARCHAEOLOGY Lake Atitlán contains a confirmed submerged Maya cultural landscape dated to the Late Preclassic period, approximately 400 BCE to 250 CE. The best-known site is Samabaj, located in the southern part of the lake at depths generally reported between 12 and 17 m, with some sources citing about 15 m. Archaeological evidence includes temples, plazas, houses, and stelae, indicating a complex settlement rather than a single drowned building. Intact ceramics at the site suggest inhabitants left in haste. A second submerged site, Chiutinamit, has also been explored. A frequent popular claim is that Samabaj or the submerged landscape sank in a single sudden catastrophe. Archaeological and geological evidence instead supports a history involving past lake-level fluctuations; a single one-day inundation event is not established. Samabaj may have been an island before water levels rose. LOCAL POPULATION AND LANGUAGES Panajachel, at the supplied coordinates, is on the northern shore. The name derives from Kaqchikel: “Pan” (locative), “aj” (cane or reed), and “achel” (matasano, the white sapote tree), meaning “place of reeds and matasanos”. The town was probably founded in the mid-16th century, around the same period as Sololá and Santiago Atitlán (1547). In 1579, Spanish authorities tested a boat launched at Panajachel. Tourism began in the area around 1885 with the establishment of the Hotel Tzanjuyú; the first steamship, the General Barillas, was introduced in 1888. By 1936, Panajachel had about 1,200 inhabitants. The population is predominantly indigenous Kaqchikel; one source reports 71.6% indigenous, mostly Kaqchikel. The lake basin includes both Kaqchikel and Tz’utujil Maya communities; language distribution varies by municipality and is not uniform around the shore. Municipality populations (2018 census): Santiago Atitlán: 42,267 (Tz’utujil) San Lucas Tolimán: 31,031 (Tz’utujil / Kaqchikel border zone) San Antonio Palopó: 15,362 (Kaqchikel) Panajachel: 15,277 (Kaqchikel / mixed) San Andrés Semetabaj: 14,161 (Kaqchikel) San Juan La Laguna: 12,541 (Tz’utujil) San Pedro La Laguna: 11,122 (Tz’utujil) San Pablo La Laguna: 7,835 (Tz’utujil) Santa Cruz La Laguna: 7,640 (Kaqchikel) Concepción: 6,906 (Kaqchikel) Santa Catarina Palopó: 4,740 (Kaqchikel) San Marcos La Laguna: 2,836 (Tz’utujil / mixed) COMMON MISCONCEPTIONS “Lake Atitlán is a crater lake.” It is a caldera lake: the basin formed by collapse after a major silicic eruption, not by the simple crater of one cone. “The three volcanoes created the lake.” The caldera formed first; San Pedro, Tolimán, and Atitlán grew afterward within or on the rim of the collapsed basin. Cerro de Oro is an even younger dome. “Los Chocoyos erupted only about 300 km³.” That was an earlier DRE estimate; the 2024 USGS study estimates about 730 km³ DRE, with roughly 1,220 km³ of dispersed deposits. “The lake is bottomless.” Its maximum measured depth is 327.56 m. “Clear water means safe water.” Cyanobacteria and nutrient pollution can be present even when water appears acceptable; monitoring uses satellite data and field samples. Oxygen depletion below 40 m and internal phosphorus release during mixing are documented. “The submerged Maya city was destroyed by one sudden flood.” Research supports a submerged cultural landscape shaped by lake-level fluctuations over time, not a proven one-day inundation event. “All lakeside communities speak the same Maya language.” The basin includes both Kaqchikel- and Tz’utujil-majority or mixed municipalities. “The lake’s native fish are still dominant.” Only two native fish species remain documented; introduced bass and other species dominate, and the introduction of bass is linked to the extinction of the endemic Atitlán giant grebe. SCIENTIFIC SUMMARY Lake Atitlán is a flooded caldera produced by the approximately 84,000-year-old Los Chocoyos VEI > 8 super-eruption. The collapse depression is about 18 km in diameter, with an original depth of about 900 m, of which roughly 300 m is now sediment and eruptive fill, roughly 300 m is water, and roughly 300 m is bedrock. The lake holds 25.46 km³ of water with a measured maximum depth of 327.56 m. The modern landscape consists of post-caldera volcanoes (San Pedro, Tolimán, Atitlán, and the younger Cerro de Oro dome), rhyolitic ignimbrite deposits, pumice-rich pyroclastic materials, a stratified tropical water column with documented climate-driven warming and anoxia, a heavily altered fish fauna with only two native species remaining, and a submerged Late Preclassic Maya cultural landscape including Samabaj and Chiutinamit. Panajachel, at the reference coordinates, is a Kaqchikel-majority town at approximately 1,597 m elevation, founded in the mid-16th century, with a documented history of colonial boat testing, nineteenth-century steamship introduction, and twentieth-century tourism development.