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. from the central crater of Citlaltepetl Volcano generating a series of pyroclastic 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 crater, but they were mostly controlled by topography and deposited in valleys or local topographic depressions up to about 30 km from vent, Although the flows were apparently emplaced without much violence, some features of the deposits reveal local turbulent conditions and an expanded fluidization that can be related to moderate flow velocities. The deposit has two members: the lower one consists of multiple flow units, and the upper one includes a single flow unit and a thin basal pumice-fall. Both members are lithologically similar and dominated by dense, andesitic scoriae with minor amounts of different pumice types (andesitic, dacitic, and banded), and lithics in a silty matrix. The eruption probably had a low-pressure 'boiling-over' mechanism and was possibly triggered by mixing of dacitic and andesitic magmas. Juvenile material in the pyroclastic-flow deposit is compositionally similar to that of Holocene lava hows at Citlaltepetl, which have apparently resulted 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.