VOLCAN QUIZAPU, CHILEAN ANDES

Quizapu is a flank vent of the basalt-to-rhyodacite Holocene stratocone, Cerro Azul, and lies at the focus of a complex Quaternary volcanic field on the Andean volcanic front. The Quizapu vent originated in 1846 when 5 km3 of hornblende-dacite magma erupted effusively with little accompanying tephra. Between approximately 1907 and 1932, phreatic and strombolian activity reamed out a deep crater, from which 4 km3 of dacite magma identical to that of 1846 fed the great plinian event of 10-11 April 1932. Although a total of > 9 km3 of magma was thus released in 86 years, there is no discernible subsidence. As the pre-plinian crater was lined by massive lavas, 1932 enlargement was limited and the total plinian deposit contains only approximately 0.4 wt % lithics. Areas of 5-cm and 1-cm isopachs for compacted 1932 fallout are about half of those estimated in the 1930's, yielding a revised ejecta volume of approximately 9.5 km3. A strong inflection near the 10-cm isopach (downwind approximately 110 km) on a plot of log Thickness vs Area 1/2 reflects slow settling of fine plinian ash - not of coignimbrite ash, as the volume of pyroclastic flows was trivial (< 0.01 km3). About 17 vol.% of the fallout lies beyond the 1-cm isopach, and approximately 82 wt% of the ejecta are finer than 1 mm. At least 18 hours of steady plinian activity produced an exceptionally uniform fall deposit. Observed column height (27-30 km) and average mass eruption rate (1.5 x 10(8) kg/s) compare well with values for height and peak intensity calculated from published eruption models. The progressive "aeolian fractionation" of downwind ash (for which Quizapu is widely cited) is complicated by the large compositional range of 1932 juvenile pumice (52-70% SiO2). The eruption began with andesitic scoria and ended with basaltic scoria, but > 95% of the ejecta are dacitic pumice (67-68% SiO2); minor andesitic scoria and frothier rhyodacite pumice (70% SiO2) accompanied the dominant dacite. Phenocrysts (pl > hb approximately opx > mt > ilm approximately cpx) are similar in both abundance and composition in the 1846 (effusive) and 1932 (plinian) dacites. Despite the contrast in mode of eruption, bulk compositions are also indistinguishable. The only difference so far identified is a lower range of delta-D values for 1846 hornblende, consistent with pre-eruptive degassing of the effusive batch.