The Ilopango Tierra Blanca Joven (TBJ) eruption, El Salvador: volcano-stratigraphy of a major Holocene event of Central America and hazards implicationsDario Pedrazzi (1), Iván Sunyé-Puchol (2), Gerardo Aguirre-Díaz (2), Antonio Costa (3), Victoria Smith (4), Pablo Davila Harris (5), Walter Hernández (6), and Eduardo Gutiérrez (6)(1) ICTJA, CSIC, Group of Volcanology, SIMGEO UB-CSIC, Institute of Earth Sciences Jaume Almera, Lluis Sole i Sabaris s/n, 08028 Barcelona, Spain (dpedrazzi@ictja.csic.es), (2) Centro de Geociencias, Universidad Nacional Autónoma de México, Blvd. Juriquilla 3001, Campus UNAM, Querétaro, 76230, México, (3) Istituto Nazionale di Geofisica e Vulcanologia, INGV-Bologna, Via Donato Creti, 12, 40100 Bologna, Italia, (4) Research Laboratory for Archaeology and the History of Art University of Oxford 1-2 South Parks Road Oxford OX1 3TG, U.K., (5) División de Geociencias Aplicadas, Instituto Potosino de Investigación Científica y Tecnológica, San Luis Potosí, 78216, México, (6) Gerencia de Geología del Observatorio Ambiental, Ministerio de Medio Ambiente y Recursos Naturales MARN San Salvador, 76230, El Salvador

GEOLOGICAL SETTING AND GENERAL CHARACTERISTICS OF ILOPANGO CALDERA

STRATIGRAPHY

INTRODUCTION REGIONAL IMPACT AND HAZARDS IMPLICATIONS

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DISCUSSION AND CONCLUSIONS

- El Salvador is located in North Central America, on the Pacific margin of the Ca-ribbean Plate, near its northwestern border.

-The chain of volcanoes, CAVA, associated with the subduction of the Cocos Plate beneath the Caribbean extends for more than 1000 km from the southeast of Mexico to the central valley of Costa Rica.

-The Volcanic Arc of El Salvador (VEES), which is part of the CAVA, includes 21 active volcanoes, three of which have erupted in the last century (Santa Ana, San Salvador and San Miguel; Siebert and Simkin, 2002).

- El Salvador is mostly made up of volcanic rocks of ages ranging from the Ceno-zoic to the present. Modified from Hernández, 2004

- The Ilopango Caldera is a 12 by 16 km volcano-tectonic structure filled by an intra-caldera lake (Mann et al., 2004).

- According to Lexa et al., 2011, the geomorphology of the topogra-phic edge of the IC with several semicircular bays could be an evi-dence of the multiple collapse events.

Modified from Alonso-Henar et al., 2015

- The Ilopango Caldera (IC) is located in El Salvador and is one of the active volcanoes of the Central American Volcanic Arc (CAVA), which in turn forms part of the Pacific Ring of Fire.

- The caldera had four large explosive eruptions during the last 57,000 years: TB4, TB3, TB2 and TBJ (Hernández, 2004; Mann et al., 2004).

- Apart from the last four large eruptions (TB4, TB3, TB2, and TBJ), there are at least nine pre-57 ka deposits of large, mas-sive and very compact silicic ignimbrites (Hernández, 2004; Hernández et al., 2010; Lexa et al., 2011; Aguirre Díaz et al., 2017).

- The last large explosive eruption of the IC was the TBJ (Tierra Blanca Joven), which is estimated to have erupted ~ 84 km3 of magma about 1500 years ago, between 430 and 535 AD (Dull et al. 2001; 2010). This eruption is considered to be one of the most catastrophic in Central America at least during the Holocene (Hernández, 2004).

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GEOCHEMISTRY AND PETROGRAPHY

- Basically, all the geological studies carried out so far on the volcanic activity from IC have focu-sed on the TBJ.

- Recent studies have focused on small events, the eruption of a dacitic dome that formed the Islas Quemadas in Ilopango Lake in 1897 (Richer et al., 2004), and a submarine eruption in the lake (Mann et al, 2004).

-We have conducted a detailed field revision of the TBJ deposits with the aim of reconstructing the eruptive sequence.

- We here present the field descriptions for the TBJ deposits and discuss briefly transport/depositional mechanisms, the corres-ponding eruption dynamics, regional impacts and the possible hazards implications .

Composite columnof TBJ deposits

- The TBJ was a cataclysmic eruption (Rolo et al., 2004) and is considered to be the largest in Central America since the eruption of Los Chocoyos-Guatemala, which occurred approximately 84,000 years ago. C. (Dull et al., 2010).

- It was one of the most catastrophic eruptions of the Central American continent, where its large eruptive products, consi-derably affected the Mayan populations living in Salvadoran and nearby territories at that time (Dull et al., 2001; Hernán-dez, 2004).

- The TBJ may have been the cause of the collapse of the economic and cultural progress of the Maya empire at the end of the Classic Period, since the last C14 dating of Dull et al. (2010) dates the eruption to AD 536, which coincides with a Mayan period known as "hiatus", in which, the civilization appears to have not progressed (no architectural, artistic or cultu-ral advance) (Demarest 1988).

Tazumal Archaeological site (Chalcuapa, ILO-289) San Salvador City

- On the other hand, the eruption of the TBJ seem to coincide with a sudden decrease in global temperature between 535-555 AD, recorded in ice nuclei with high sulfur content in Antarctica (Pratt, 2012).

- Ash and aerosols of H2SO4 emitted during large eruptions spread throughout the terrestrial atmosphere acting like a solar barrier and creating a period of volcanic winter, which lower temperatures and can cause the decrease in crop yields and consequent famine of its people.

- The Ilopango caldera, a 11x17 km volcano-tectonic structure filled by an intracalderic lake, is located in El Salvador, and is one of the active volcanoes of the Central American Volcanic Arc, which in turn forms part of the Pacific Ring of Fire.

- The TBJ unit, of dacitic composition, is characterised by a bright/white color. The eruptive sequence is made of several sub-units. Few cm of fallout deposits, found mainly close to the Ilopango caldera, represent the beginning of the eruption. Subsequently, dense and dilute PDCs of hydromagmatic and magmatic origin filled the depressions near the Ilopango lake with thicknesses of up to at least 70 m and reached distances of at least 40-50 km from the vent, covering completely what today is the actual city of San Salvador. Ash deposits of the last stage of the eruption were found along the whole El Salva-dor with significant thicknesses.

- The TBJ eruption was one of the greatest Quaternary eruptions of Central America, where its large eruptive products, considerably affected the Mayan populations living in Salvadoran and near territories at that time.

- Therefore, a complete study of this eruption was carried out to help in the understanding its evolution and related hazards and posible future impact over the population of El Salvador and nearby countries.

- Aguirre-Diaz, G., Sunyé Puchol, I., Davila-Harris, P., Pedrazzi, D., Hernandez, W., Gutierrez, E., 2017. Cordilleran Section - 113th Annual Meeting.- Alonso‐Henar, J., Schreurs, G., Martinez‐Díaz, J. J., Álvarez‐Gómez, J. A., Villamor, P., 2015. Neotectonic development of the El Salvador Fault Zone and implications for deformation in the Central America Volcanic Arc: Insights from 4‐D analog modeling experiments. Tectonics, 34(1), 133-151.- Demarest, A. A.,1988. Political evolution in the Maya borderlands: The Salvadoran frontier. The Southeast Classic Maya Zone, 335-94.- Dull, R.A., Southon, J.R., Sheets, P., 2001. Volcanism, ecology, and culture: A reassessment of the Volcán Ilopango TBJ eruption in the southern Mayan realm: Latin American Antiquity, vol. 12, no. 1, p. 25-44.- Dull, R., Southon, J., Kutterolf, S., Freundt, A., Wahl, D., Sheets, P., 2010. Did the IlopangoTBJ Eruption Cause the AD 536 Event?, Meeting of American Geophysical Union 2010.- Dull, R. A., Southon, J. R., Sheets, P., 2001. Volcanism, ecology and culture: a reassessment of the VolcánIlopangoTBJ eruption in the southern Maya realm. Latin American Antiquity12, 25-44.- Hernández, E.W., 2004. Características geotécnicas y vulcanológicas de las tefras de Tierra Blanca Joven de Ilopango, El Salvador: Tesis de maestría, Univ. Politécnica de El Salvador, San Salvador, 115 p.- Hernández W., Ferrés, D., Delgado Granados, H., Pullinger, C., Gutiérrez de Henríquez, E., 2010. The last 40 ka eruptive cycle of Ilopango caldera deposits: a settlement for the San Salvador Metropolitan Area (El Salvador). Abstract volume, International Conference Cities on Volcanoes, Fundación Canaria ITER, Tenerife, pp 56-- Lexa, J., Šebesta, J., Chávez, J.A., Hernandez, W. & Pécskay, Z., 2011. Geology and volcanic evolution in the southern part of the San Salvador Metropolitan Area.- J. Geosci. 56: 105–140.- Mann, C.P., Stix, J., Vallance, J.W., and Richer, M., 2004. Subaqueous intracaldera volcanism, Ilopango Caldera, El Salvador, Central America, in Rose, W.I., Bommer, J.J., López, D.L., Carr, M.J., and Major, J.J., eds., Natural hazards in El Salvador: Boulder, Colorado, Geological Society of America Special Paper 375, pp. 159–174.- Pratt, S.E., 2012. Eruption of El Salvador’s Ilopango explains…A.D. 536 global cooling: Earth Magazine, February 25, 2012 issue, 5 p.- Richer, M., Mann, C.P., Stix, J., 2004. Mafic magma injection triggers eruption at Ilopango Caldera, El Salvador, Central America: In Rose, W.I., et al., eds., Natural hazards in El Salvador: Boulder, Colorado, Geological Society of America Special Paper 375, pp. 175–189.- Rolo, R., Bommer, J. J., Houghton, B. F., Vallance, J. W., Berdousis, P., Mavrommati, C., Murphy, W., 2004. Geologic and engineering characterization of Tierra Blanca pyroclastic ash deposits. SPECIAL PAPERS-GEOLOGICAL SOCIETY OF AMERICA, 55-68.-Siebert, L., Simkin, T., 2002. Volcanoes of the World: an Illustrated Catalogue of Holocene Volcanoes and their Eruptions. Smithsonian Institution, Global Volcanism Program Digital Information Series, GVP-3.

This study was financed by CONACYT-CB grant 240447 and logistically supported by MARN-El Salvador. We thank the Policia Nacional Civil de El Salvador for the logistic support.

The TBJ eruption can be divided in 8 units that were labeled alphabetically from base to top (A0 to G)

ACKNOWLEDGEMENTS

- Geochemical analysis: pyro-clastic deposits from TBJ erup-tions correspond to dacites.

- Pumice clasts from the TBJ units are highly vesicular.

- The mineralogy of the TBJ samples co-rresponds to euhe-dral to subhedral pla-gioclase and horn-blende.

- Pyroxene, apatite, oxides and zircons were observed in the matrix.

Map of representative outcops of TBJ eruption

Northern view of Ilopango Caldera