ERUPTION OF A MAJOR HOLOCENE PYROCLASTIC FLOW AT CITLALTEPETL VOLCANO(PICO-DE-ORIZABA), MEXICO, 8.5-9.0 KA

Multiple volcanic eruptions occurred between 8500 and 9000 yr. B.P. fr om the central crater of Citlaltepetl Volcano generating a series of p yroclastic flows that formed a deposit with a total volume of about 0. 26 km(3) (D.R.E.). The flows descended in all directions around the cr ater, but they were mostly controlled by topography and deposited in v alleys or local topographic depressions up to about 30 km from vent, A lthough the flows were apparently emplaced without much violence, some features of the deposits reveal local turbulent conditions and an exp anded fluidization that can be related to moderate flow velocities. Th e deposit has two members: the lower one consists of multiple flow uni ts, and the upper one includes a single flow unit and a thin basal pum ice-fall. Both members are lithologically similar and dominated by den se, andesitic scoriae with minor amounts of different pumice types (an desitic, dacitic, and banded), and lithics in a silty matrix. The erup tion probably had a low-pressure 'boiling-over' mechanism and was poss ibly triggered by mixing of dacitic and andesitic magmas. Juvenile mat erial in the pyroclastic-flow deposit is compositionally similar to th at of Holocene lava hows at Citlaltepetl, which have apparently result ed from magma homogenization of mafic and silicic end members. Because the system is continuously injected with new basaltic-andesitic magma , a recurrence of explosive activity is possible in future eruptions.