The Ilopango Tierra Blanca Joven (TBJ) eruption, El Salvador. Volcano-stratigraphy and physical characterization of the major Holocene event of Central America

The Ilopango caldera is the source of the large Tierra Blanca Joven (TBJ) eruption that occurred about 1.5 ka years ago, between ca. AD270 and AD535. The eruption dispersed volcanic ash over much of the present territory of El Salvador, and pyroclastic density currents (PDCs) extended 40 km from the volcano. In this study, we document the physical characteristics of the deposits from all over El Salvador to further constrain the eruption processes and the intensity and magnitude of the different phases of the eruption. The succession of deposits generated by the TBJ eruption is made of 8 units. The eruption started with PDCs of hydromagmatic origin (Unit A0), followed by fallout deposits (Units A and B) that are <15 cm thick and exposed in sections close to the Ilopango caldera (within 10–15 km). The eruption, then, transitioned into a regime that generated further PDCs (Units C–F), these range from dilute to dense and they filled the depressions near the Ilopango caldera with thicknesses up to 70 m. Deposits from the co-ignimbrite plume (Unit G) are the most widespread, the deposits are found in Guatemala, Honduras, Nicaragua, Costa Rica and the Pacific Ocean and cm-thick across El Salvador. Modelling of the deposits suggests that column heights were 29 km and 7 km for the first two fallout phases, and that the co-ignimbrite phoenix plume rose up to 49 km. Volumes estimated for the fallout units are 0.15, 0.8 and 16 km3 dense rock equivalent (DRE) for Unit A, B and G respectively. The PDCs deposits volumes were estimated to be ~0.5, ~3.3, ~0.3 and ~9.1 km3 DRE for Units C, D, E and F, respectively. The combined volume of TBJ deposits is ~30 km3 DRE (~58 km3 bulk rock), indicating that it was one of largest Holocene eruptions from Central America. This eruption occurred while Mayan populations were living in the region and it would have had a significant impact on the areas within tens of kilometres of the vent for many years to decades after the eruption.