Temporal variations in column height and magma discharge rate during the 79 A.D. eruption of Vesuvius

The 79 A.D. plinian eruption of Vesuvius ejected approx 4 km 3 of phonolitic magma over a period of approx 19 hr. A change in magma composition during the eruption is marked by a sharp transition from white, evolved phonolitic pumice to denser, overlying gray pumice, at mid-level within the fall deposit. Deposition of the upper, gray pumice fall was interrupted six times by the emplacement of pyroclastic surges and flows. Reverse size grading is conspicuous in the fall deposit. Measurements of maximum pumice and lithic diameters have been used to construct isopleths for eight chronostratigraphic levels within the fall deposit. The variation in pumice density is attributed primarily to differences in volatile content of two magmas which were tapped and mixed in varying proportions during ascent and eruption. -from Authors