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Aggregation of airborne particles is an important way in which the atmosphere is cleansed of fi ne dust particles, such as following explosive eruptions and meteorite impacts. We identify successive stages in the growth history of particle aggregates based upon well-preserved ash aggregate-bearing pyroclastic layers on Tenerife. The layers are persistently organized into couplets made up of a lower ignimbrite layer and an upper , widespread coignimbrite ash-fall layer. The upper part of each ignimbrite contains whole and fragmented concentric-laminated accretionary lapilli, whereas the overlying coignimbrite ash-fall layer lacks accretionary lapilli and is composed of frameworksupported smaller and nonlaminated ash pellets , sometimes slightly deformed or partly disaggregated. The pellets resemble the cores of the larger accretionary lapilli in the underlying ignimbrite layer. These fi eld relations are repeated numerous times in several different successions, and they indicate that ash pellets, not accretionary lapilli, form within the coignimbrite ash plumes. Some pellets fell directly to the ground, producing coignimbrite ash-fall layers, but others settled into pyroclastic density currents, where they accreted successive concentric laminations of fi ne ash as they circulated through the variously turbulent levels of the stratifi ed current, and heat of the lower part of the current dried and partly lithifi ed them into brittle accretionary lapilli. The fully formed whole and broken accretionary lapilli were then deposited from the current along with ash and pumice lapilli. Numerous ignimbrite veneers on Tenerife have the form of ash layers, a few centimeters thick, that drape topography and locally contain matrix-supported accretionary lapilli. Most volcanoes lack laterally continuous fi eld exposure, and such accretionary lapilli-bearing layers might be mistaken for ash-fall deposits. We highlight the value of careful distinction between different types of ash aggregate facies when interpreting the origin of pyroclastic deposits, for example , during hazard assessments.

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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 A 0 ), followed by fallout deposits (Units A and B) that are b15 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 km 3 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 km 3 DRE for Units C, D, E and F, respectively. The combined volume of TBJ deposits is 30 km 3 DRE (~58 km 3 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.

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The Tierra Blanca Joven (TBJ) eruption from Ilopango volcano deposited thick ash over much of El Salvador when it was inhabited by the Maya, and rendered all areas within at least 80 km of the volcano uninhabitable for years to decades after the eruption. Nonetheless, the more widespread environmental and climatic impacts of this large eruption are not well known because the eruption magnitude and date are not well constrained. In this multifaceted study we have resolved the date of the eruption to 431 ± 2 CE by identifying the ash layer in a well-dated, high-resolution Greenland ice-core record that is >7,000 km from Ilopango; and calculated that between 37 and 82 km3 of magma was dispersed from an eruption coignimbrite column that rose to ∼45 km by modeling the deposit thickness using state-of-the-art tephra dispersal methods. Sulfate records from an array of ice cores suggest stratospheric injection of 14 ± 2 Tg S associated with the TBJ eruption, exceeding those of the historic eruption of Pinatubo in 1991. Based on these estimates it is likely that the TBJ eruption produced a cooling of around 0.5 °C for a few years after the eruption. The modeled dispersal and higher sulfate concentrations recorded in Antarctic ice cores imply that the cooling would have been more pronounced in the Southern Hemisphere. The new date confirms the eruption occurred within the Early Classic phase when Maya expanded across Central America.

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