Fuego and Acatenango Volcanoes - Past EventsVolcanism at the Fuego-Acatenango volcano complex has occurred intermittently for more than 230,000 years, and historical observations of eruptions date back nearly 500 years. Fuego has erupted more than 60 times historically, but Acatenango has erupted only twice in the 20th century. Most of the information about Acatenango's past behavior and some information about Fuego's behavior comes from studies of deposits produced by prehistoric events. Eruptive behavior has varied from vigorous explosions, accompanied by tephra falls and pyroclastic flows, to effusive lava flows. At least two voluminous debris avalanches occurred in the last 80,000 years.
The volcano massif mostly grew in the past 84,000 years. About 84,000 years ago the Los Chocoyos ash erupted from Lake Atitlán caldera. It comprises an underlying, tephra deposit (unit H) and an overlying ignimbrite. The unit is widespread and serves as a stratigraphic marker in the vicinity of Fuego and Acatenango. After the Los Chocoyos ash fell eruptions of Acatenango Antiguo emplaced numerous lava flows. Activity at Acatenango Antiguo culminated sometime before 43,000 years ago, with a huge debris avalanche that is now exposed near La Democracia.
Between 43,000 years before present (BP) and about 5,000 years BP, Yepocapa cone grew at the site of Ancient Acatenango. Eruptions produced lava flows, pyroclastic flows, and numerous tephra-fall deposits. The last known eruption from Yepocapa was about 5,000 BP, but Yepocapa should be considered potentially active.
Beginning by about 20,000 years ago, eruptions began from the Pico Mayor de Acatenango vent. Eruptions have produced widespread tephra layers, lava flows, and pyroclastic flows. The last known pyroclastic flows erupted about 2,300 years BP. The last widespread tephra deposits erupted about 1,900 years BP. Historical accounts suggest an eruption of Acatenango in AD 1661. Phreatic eruptions occurred in AD 1924 to 1927 and in December 1972.
Volcanism at the approximate location of Meseta volcano began 230,000 years ago. Numerous lava flows were emplaced, but no known deposits of pyroclastic flows or tephras have been correlated to Meseta. One of the youngest lava flows is less than 30,000 years BP. After emplacement of this lava, but before about 8,500 years BP, an enormous edifice collapse occurred at Meseta. The resulting debris avalanche deposit now underlies more than 300 square kilometers of the Pacific coastal plane south of Escuintla and has a volume of more than 9 cubic kilometers. A magmatic eruption probably accompanied the edifice collapse, but, if so, its deposits have not yet been detected. No subsequent volcanism is known at Meseta.
With an historical record of activity that includes more than 60 eruptions since AD 1524, Fuego is one of the most active volcanoes in the world. These eruptions are typically short-lived (hours to a few days), violent vulcanian eruptions that commonly include pyroclastic flows. The historical record shows four 20-to-50-year-long clusters of activity and sporadic intermittent activity. Widespread pyroclastic deposits east of Fuego with 14 C ages of 980 +/- 50 BP, 1050 +/- 70 BP, two at about 1350 BP (1330 +/- 60, 1375 +/- 45), and 3530 +/- 70 BP suggest that more voluminous eruptions, which fill barrancas and spread widely, sometimes occur. Extrapolating the historical volume rate of eruption suggests that the entire edifice of Fuego volcano could have been constructed in 8,500 years. An age determination on one of the youngest Meseta lavas constrains the age of Fuego to be less than 30,000 years old.
The most recent eruption of Fuego occurred in May 1999, and the most recent large eruption occurred in 1974. The 1974 eruptions spread 20 cm or more ash more than 50 kilometers to the southwest. Both eruptions produced pyroclastic flows that moved up to 10 kilometers down valleys and traveled 60 kilometers per hour. Subsequent rains, especially during the summer monsoons, mobilized lahars that traveled up to 10 kilometers beyond pyroclastic flow termini (as much as 20 kilometers from the summit). A significant volume of loose volcaniclastic debris remains stored in steep upper reaches of the barrancas that head on Fuego.
Although there have been few historical eruptions at Acatenango, heavy rainfall, especially during the rainy season, could trigger landslides and debris flows on the steep slopes of the volcano during quiescent periods. In October 1998 at Casita volcano in Nicaragua, torrential rain triggered an avalanche and debris flow that swept down the volcano, spread out on the apron of the volcano, destroyed two towns and killed more than 2,500 people. Steep dissected slopes, partly affected by hydrothermal alteration of the rocks, made Casita more susceptible to such an event than were younger more active volcanoes nearby. Acatenango volcano also has steep slopes and rocks that have been partly weakened by hydrothermal alteration. It is thus susceptible, in the event of torrential rain, to an avalanche and a debris flow much like that which occurred at Casita.
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