The latest cycle of volcanism on Tenerife has involved the construction of two stratovolcanoes, Teide and Pico Viejo (PV), and numerous flank vent systems on the floor of the Las Canadas Caldera, which has been partially infilled by magmatic products of the basanite-phonolite series. The only known substantial post-caldera explosive eruption occurred similar to 2ka sp from satellite vents at Montana Blanca (MB), to the east of Teide and at PV. The MB eruption began with extrusion of approximate to 0.022 km(3) of phonolite lava (unit I) from a WNW-ESE fissure system. The eruption then entered an explosive subplinian phase. Over a 7-11 hour period, 0.25 km(3) (DRE) of phonolitic pumice (unit II) was deposited from a 15 km high subplinian column, dispersed to the NE by 10 mis winds. Pyroclastic activity also occurred from vents near PV to the west of Teide. Fire-fountaining towards the end of the explosive phase formed a proximal welded spatter facies. The eruption closed with extrusion of small volume domes and lavas (approximate to 0.025 km(3)) at both vent systems. Geochemical, petrological data and Fe-Ti oxide geothermometry indicate the eruption of a chemically and thermally stratified magma system. The most mafic and hottest (approximate to 875 degrees C) unit I magma can yield the more evolved and cooler (755-825 degrees C) phonolites of units II and III by between 7 and 11% fractional crystallization of an assemblage dominated by alkali feldspar. Analyses of glass inclusions from phenocrysts by ion microprobe show that the pumice was derived from the water-saturated roof zone of a chamber containing 3.0-4.5 wt.% H2O and abundant halogens (F approximate to 0.35wt.%). Hot ter, more mafic tephritic magma intermingled with the evolved phonolites in banded pumice, indicating the injection of mafic magma into the system during or just before eruption. Reconstruction ot the event indicates a small chamber chemically stratified by in situ (sidewall) crystallization at a depth of 3-4 km below PV. Although phonolite is the dominant product of the youngest activity of the Teide-PV system, there has been no eruption of phonolitic magma for at least 500 years from Teide itself and for similar to 2000 years from the PV system. Therefore there could be a large volume of highly evolved, volatile-rich magma accumulating in these magma systems. An eruption of fluorine-rich magma comparable with MB would have major damaging effects on the island.
