After 200 yr of repose, Pacaya Volcano resumed Strombolian activity in 1961 and has remained active until the time of this writing (2013). A three-dimensional map of 50 yr of nearly continuous activity of Pacaya depicts an accumulation of homogeneous, crystal-rich high-Al basalt on the west side of a preexisting cone. The material erupted is loose and welded spatter, volcanic ash, and 249 pahoehoe and a‘a lava flows, most of which were extruded in a few days, and most have extended less than 2 km in length from vents near the 2500-m-high summit down slopes of 20°–33°. The configuration of lava flows makes up a rigid, web-like network that welds the asymmetrical, steep western slope of an expanded Pacaya cone. The vents have fed the lava flows, forming a sieve-like pattern where lava leaks out. The cone contains a complex network of intrusive feeders, which fill and empty with lava, degas, and drain back. The volcano has shown explosive lava fountaining and effusive periods of activity and often exhibits both, as summit eruptions occur while lava drains from the cone. Lava flows and pyroclastic units from collapse-related avalanches and tephra fall tend to alternate. The overall length of lavas is limited, so that inhabited areas below the cone on most flanks are unlikely to be reached by flows, although topographic barriers, which blocked the flow of lava to the closest villages north of Pacaya, are now filled, so that lavas of moderate length (~2 km) could reach towns to the north under some conditions. The volcano is known to have experienced catastrophic explosive collapse in the last few thousand years. The current cone itself may be unstable because the new material has mostly asymmetrically loaded the west side of an old cone, and collapse to the west may be more likely because of mass imbalance and because of persistent activity that opens paths and accumulates on that side. Collapse to the west would threaten significant populations. Pacaya's past eruptions lasted centuries, with repose intervals of similar length, so the current activity may continue for another century or more. Overall, Pacaya is a complex of overlapping basaltic cones, and its pattern of activity provides insight into the early stages of composite cones such as nearby Agua, Fuego, Atitlán, and Santa María, all larger and older cones on the volcanic front of Guatemala with Pacaya.
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