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Abstract

The eruption of El Jorullo (1759–1774) in Guanajuato, Mexico, generated substantial (100–300 m high) pyroclastic cones, an extensive ash blanket and a flow field of thick lavas. The cones have the aspect of scoria cones that result from Strombolian eruptions, but the ash blankets consist predominantly of sub millimetre-sized particles (comprising ≥80 wt% beyond 1 km from the vent). This combination of cones, fine deposit grain size, and moderate dispersal area has previously been attributed to ‘violent Strombolian’ eruptions. The ash blanket at El Jorullo comprises c. 40% of the erupted volume and contains hundreds of strictly parallel laminae, evidence for deposition by fallout from a great number of explosions such as those observed during the eruption of nearby Parícutin volcano (1943–1952). The high degree of fragmentation could have resulted from hydromagmatic activity, but the deposit mostly lacks evidence for significant involvement of external water. We consider that the predominantly fine grain size was probably produced by a combination of a high yield strength and viscosity of the erupting magmas, possibly high juvenile water content, and recycling and milling of pyroclasts within the vent. The lava flow field at El Jorullo constitutes c. 40% of the erupted volume. Eight major flows vary from 10 to 50 m thick. Flow thickness and yield strength (calculated from flow profiles) increased with time from c. 30 000 Pa for the earliest flows to c. 200 000 Pa for the latest.

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