Origin and emplacement of a pyroclastic flow and surge unit at Laacher See, Germany

Abstract

Within the pyroclastic ejecta sequence of Laacher See volcano, West Germany, are many repeated phreatomagmatic depositional cycles. A complete cycle begins with a thin, low aspect ratio, fines-depleted breccia layer, followed by a sequence with several coarse- to fine-grained, cross-bedded units, in turn overlain by a massive pyroclastic flow layer. Each sequence is interpreted in terms of water/magma interaction and effects upon the eruption column. The breccia represents the triggering eruption with a high eruption column where fines were winnowed out and fallback from high altitudes produced a low aspect ratio layer. The cross-bedded sequence containing more fine-grained ash resulting from an increase in amount of water, represents a fluctuating eruption column of sufficient height to produce large antidunes within 700 meters of the rim. The massive pyroclastic flow layer which contains more fine-grained ash than the underlying layers, represents the culmination of the eruption where an increase in amount of water interacting with the magma resulted in a dense and very low eruption column producing a dense, laminar moving pyroclastic flow.