Reconstruction of the AD 1600 Huaynaputina eruption based on the correlation of geologic evidence with early Spanish chronicles

The largest historical eruption (VEI 6) in the Andes began on February 19 and continued until March 6 or 15, AD 1600 at Huaynaputina, a dacitic stratovolcano located on a high volcanic plateau in south Peru. Tephra falls, pyroclastic flows and surges disrupted life in an area of ∼4900 km² around the volcano, and ash-fall was reported 200-500 km away in south Peru, west Bolivia, and north Chile. The aftermath of the large-scale eruption was severe and protracted for the people and colonial economy of south Peru. By linking up the series of events inferred from Spanish chronicles with the lithofacies and composition of the tephra (bulk volume 11.4-12.1 km³, dense rock equivalent (DRE) 4.6-4.95 km³), we distinguish five eruptive phases. (1) During the plinian phase, a sustained plinian column 27-35 km high on February 19-20 delivered a dacitic pumice fall of ∼3.1 km³ DRE volume. The plinian pumice formed a widespread lobe of ∼95 000 km² within the 1-cm isopach; strong winds carried fine ash >500 km to the west, and west-northwest into the Pacific Ocean. The computed volumetric eruption rate was in the range of 5.4-6.6×10⁴ to 1×10⁵ m³/s and the mass eruption rate 1.3-1.6×10⁸ kg/s. The onset and high discharge of the sustained plinian eruption was fueled by the disruption of an active hydrothermal system enclosed in the pre-AD 1600 amphitheater. The plinian column shut off as the vent was choked when the fragmentation focus deepened to beneath the weathered bedrock, >1600 m below the vent area. (2) During the second phase, a dwindling column sent ash-falls on proximal to medial areas and possibly pyroclastic surges on proximal slopes. (3) During the third ignimbrite-forming phase with interspersed hydromagmatic events, pyroclastic flows 1.5-2 km³ in volume were channeled into the Río Tambo canyon and tributaries. The flows with interbedded base-surge deposits in proximal tributaries probably produced vigorous columns over high, rugged relief around the Huaynaputina plateau. Winds winnowing the columns dispersed a widespread co-ignimbrite ash, probably mixed with co-plinian ash, over an area of ∼265 000 km². (4) During the fourth phase, an unusual crystal ash-fall was deposited when the residual magma with a crystal content as high as 80% was tapped near the end of the eruption. (5) During the fifth phase, ash-flows produced surge deposits and lag-fall breccias near vent, small-volume ash-flow deposits in proximal catchments, and a thin ash-fall layer in medial to distal areas. The proximal deposits were also produced by diluted flows able to surmount ridges 1000 m high as far as 15 km east from the vent. The ignimbritic and hydromagmatic phases greatly modified the ≤400-m-diameter plinian vent. Tapping of the crystal-rich magma and ash flows towards the end of the eruption led to the formation of two youthful vents in domes. Geochemistry and mineralogy of the plinian and post-plinian units point to an unusual zoned magma sequence. The ignimbrite-forming phase tapped a magma batch richer in silica than the less differentiated plinian magma. The crystal-rich magma of unit 4 was fed by 'crystal mush' in a layered magma reservoir and (or) from two magma reservoirs at distinct depths. The geochemical and mineralogical trend throughout the eruption, and preliminary measurements of geobarometers suggest a complex model linking a shallow (6-7 km) magma reservoir to a deeper (∼15 km) reservoir. The total DRE volume (4.6-4.95 km³) of erupted tephra did not lead to caldera collapse. Ring fractures cutting multiple vents are associated with a dyke swarm intruding the weathered volcanic bedrock. This suggests the onset of a funnel-type or piecemeal collapse.