Amatitlan, an Actively Resurging Cauldron 10 Km South of Guatemala City

A 14 x 16 km diameter collapse Pacaya, and other composite cones are part of this caldera has been recognized 10 km south of volcanic front, while Amatitlan and many other Guatemala City, Guatemala. The caldera is north Central American silicic centers are north of the of the presently active volcano Pacaya and west of volcanic front [Rose et al., 1981]. Agua, a large stratovolcano. The caldera was not The region south of Guatemala City (Figure 2) previously recognized because its eastern and has been mapped at a scale of 1:50,000 by Eggers western margins coincide with faults that outline [1971, 1975] and Koch and McLean [1975]. These the Guatemala City graben and because the northern maps, together with work by Bonis et al. [1970], margin of the caldera is buried by pyroclastic Williams [1960], Reynolds [1977, 1980], Brown et rocks. The existence of the northern caldera al. [1971], Carr et al. [1969], and Durgin et al. margin is now established by gravity data and a [1970], were used to make a geologic map (Figure variety of geological observations including 2). The extensive rhyolitic pyroclastic deposits circumferential faults, hot springs, well-log of the area were described by Koch and McLean data, and lithological changes in sedimentary [1975], who concluded that these pyroclastics had rocks. A sequence of nine silicic pyroclastic their source region in the area south of Guatemala deposits, totaling a volume of more than 70 km 3 of City. Eggers and Williams both concluded that the dense rock were erupted from the caldera. The Areatitian area was the site of a "volcano-tectonic ages of these eruptions are mainly between about depression." 300,000 years B.P. to less than 23,000 years B.P. Recently, important geophysical (gravity, The rocks erupted at the caldera and its seismic and electrical resistivity) and well-log associated vents consist of domes and nonwelded data in the Areatitian area were collected by Munoz pyroclastic flow, surge, and fall deposits, mainly et al. [1978]. Based on these earlier studies and of rhyolitic to dacitic composition. Successive on a variety of our field observations, we pyroclastic eruptions have generally become summarize the evidence that Areatitian is a smaller in volume and more silicic with time. moderate-size caldera: Major and minor element chemistry distinguish 1. The arcuate depression around Lake Amatitlan pyroclastics from those of other nearby Amatitlan is structurally and morphologically calderas. The caldera lies at the intersection of suggestive of a caldera. An arcuate scalloped an offset of the volcanic chain (the Palin Shear) and the faults along the volcanic front (Jalpatagua fault zone). The caldera has a heavily faulted resurgent dome crosscut by an impressive longitudinal graben. The graben's alignment with the Jalpatagua fault zone suggests a genetic relationship. The longitudinal graben and resurgent dome are morphologically youthful and are the sites of many young silicic vents. Available seismic data show a heavy concentration of epicenters over the northern part of the resurgent dome, near a young silicic intrusion. The caldera is active and will probably erupt again. Over 1 million people live within 20 km topographic margin is visible on Landsat photos [Wunderman, 1983; Hahn, 1976], as well as numerous arcuate concentric normal faults (downward displacement toward the caldera's center) around the caldera's structural margin. Buried caldera faults on the floor of the Guatemala City Basin are revealed by geophysical techniques, and a 5-10 mgal gravity low is centered over the caldera [Munoz et al., 1978]. 2. The presence of a caldera in the region is consistent with the large volumes of recent silicic pyroclastic rocks. Recognizing the source of the pyroclastics was south of, but not far from, Guatemala City, some earlier workers and would be threatened in the event of a moderate suggested that many of the pyroclastic flow and eruption. Suggestions for future research focus fall deposits were from Pacaya [Koch, 1970; on hazard assessment and forecasting. McLean, 1970]. This is unlikely since it is now known that voluminous repeated eruptions of this Introduction type cause consequent surface collapse [Smith and Bailey, 1968]. No other appropriately sized The Guatemalan Highlands lie on the continental depression is present nearby. We conclude that side of a convergent plate boundary that trends most of the voluminous silicic pyroclastics and north-northwest. Active subduction takes place in domes were erupted from sources within the the Middle America Trench off the southwestern caldera. coast (Figure 1). The active "volcanic front" 3. A variety of other geologic features forms the southern border of the highlands [Cart, commonly observed in calderas is also present. 1974]. The front rises from a coastal lowland and These include present-day hot spring and fumarolic trends roughly parallel to the coast. Agua, activity, surge deposits within the caldera, the mapped extent of lake sediments that follows (and helps delineate) the caldera's approximate Copyright 1984 by the American Geophysical Union. northern extent, a breached caldera rim to the south, evidence of resurgence, and possible Paper number 4B0543. indications of continuing seismic activity within 0148-0227/84/004B-0543 $05.00 the caldera.