Volcanology

The impact of volcanic activity at San Pedro Aguacatepeque

San Pedro Aguacatepeque is located on the SE flank of Volcán de Fuego, atop of and partially covered by a sequence of volcanic deposits that have been produced by a variety of processes. Some of the deposits were produced by eruptions that happened during the last few centuries; the largest of these eruptions must have had a severe impact on the people living at the site. In an interdisciplinary effort to understand how the site was affected by eruptive activity we have done joint preliminary fieldwork at the site, to look at the relationship of eruptions and the human groups that inhabited the site, from both volcanological and archaeological points of view. From this work it seems that the site has been repeatedly hit by air fall tephra, but surprisingly not by pyroclastic flows, at least in the most recent past. The question of whether or not volcanic activity contributed to the abandonment of the site is at this point still open.

Antonio de Ciudad Real⁽*⁾ mentions that an eruption on January 14^th of 1582 from Fuego volcano destroyed the town of San Pedro Aguacatepeque, but fortunately the inhabitants of the town had left the site due to fear that the eruption could harm them. The description that follows in the account by Ciudad Real could fit with a suite of volcanic hazardous phenomena, but two types of hazards were considered as more likely a priori: pyroclastic flows and air fall tephra. Later eruptions of similarly large magnitude, e. g. in 1717, 1880, and 1932, could have had a similar impact on the site, and their deposits are most likely preserved at that location.

Volcanological research conducted thus far has focused on the analysis of volcanic deposits at the site and its vicinity, and on the processes that originated them. From this analysis a preliminary model of the general geology of the area has been defined (see figure 1).

The predominant types of deposits are from pyroclastic flows, which are not only very common but also very voluminous. Intercalated lahar, fluviatile, lava flow, and air fall tephra deposits form a relatively smaller part. The uppermost and therefore youngest part of the sequence is made of air fall tephra and associated reworked material, and may represent several centuries to a few thousands of years of volcanic activity.

The existence of the uppermost tephra sequence (UTS) implies that pyroclastic flows have not swept over the terrain at this site in recent times, or otherwise the upper deposit would be that of a pyroclastic flow. This suggests that the destruction of San Pedro Aguacatepeque in 1582 may have been caused by air fall tephra. However, given the extension of the human occupation area at site, it is possible that other areas considered part of the site may have been affected by pyroclastic flows during that time frame. Ongoing research will be aimed to try to answer this and other related questions.

Underneath the UTS is an older pyroclastic and laharic sequence (OLPS) that most likely represents a few thousands of years of volcanic activity (see figure 2).

At this point it is not clear whether the different sequences represent differences in the style, explosivity or magnitude of the eruptions, or are otherwise the result of the complex interaction between the different types of flows with the topography, and the influence of the wind on the deposition of air fall tephra.

Ongoing work on radiocarbon dating of vegetation burned by the pyroclastic flows will hopefully clarify some of the questions about the time scale involved in the formation of both the UTS and the OLPS. Additional work on numerical modeling of the pyroclastic flows is also showing encouraging results that could help to better interpret the series of events that generated the UTS and OLPS.

See below for a pyroclastic flow simulation approximating the volume of flows from the 1582 eruption. Note the location of Aguacatepeque and its apparent impact by pyroclastic flows [in yellow and red]. However, to date no evidence of pyroclastic flows impacting the site have been discovered, although ample evidence of tephra deposition has been identified. Simulation and video by Rudiger Escobar Wolf

*Ciudad Real , A. de, 1873, Relacion breve y verdadera de algunas cosas de las muchas que sucedieron al Padre Fray Alonso Ponce en las provincias de la Nueva España, siendo comisario general de aquellas partes. Tomo I. Imprenta de la Viuda de Calero. Madrid.

All text and content contributed by:

Rudiger Escobar Wolf
Ph. D. Candidate
Department of Geological Engineering and Sciences
Michigan Technological University