Large Plinian Eruptions Research Articles

Sedimentation from gravity currents generated by turbulent plumes

ABSTRACTSedimentation from radially spreading gravity currents generated at the top of ascending sediment‐laden plumes is described by a model which assumes that sediment is dispersed homogeneously by turbulence in the gravity current, resulting in an exponential decrease in the concentration of sediment with time as particles settle out of the lower boundary of the current. For radial spreading this model predicts a Gaussian distribution of sediment accumulation away from the source with an exponential constant, B, which depends on flow rate, Q, and particle settling velocity, v (B=nv/Q). In the experiments described, sedimentation occurs from gravity currents generated by ascent of buoyant, particle‐laden plumes of fresh water in a tank of salty water. The sediment accumulation shows close agreement with the theoretical model, and the Gaussian decay constant, B, can be determined from a maximum in the accumulated mass of sediment per unit distance and from the slope of the line In(S/S0) = ‐Br2, where r is the radial distance, S is the sediment mass flux per unit area and S0 is the value of S at r=0. Data from the dispersal of volcanic ejecta from a large (c. 24 km high) plinian eruption column in the Azores also show good agreement with the theory, confirming that it is general and independent of scale and the nature of the fluid. The experimental data also show a change in sedimentation behaviour at distances from the source corresponding to the corner of the plume where it diverts into a lateral gravity current and there is an abrupt decrease in vertical velocity. Sedimentation of coarse grain sizes, between the source and the corner, occurs from the inclined plume margins and does not behave as predicted by the theoretical model.

Formation of the Green Tuff, Pantelleria

The strongly peralkaline Green Tuff, Pantelleria, is an example of a thin, densely welded air-fall tuff which mantles an area of at least 85 km2. Offshore the tuff is correlated with the Y-6 ash layer in the central Mediterranean Sea, and the total volume of the eruption is estimated at 7 km3 D.R.E. New petrological data suggests that the tuff was erupted from a zoned magma chamber containing a cooler, more fractionated upper zone relative to be bulk of the magma. Analysis of the distribution of accessory lithic fragments in terms of existing models of eruption dynamics indicates emplacement by a plinian-type eruption. It is shown that, due to the low viscosity of pantelleritic ejecta, dense welding can occur at moderate tephra accumulation rates and a rate of the order of 1 cm/minute is suggested for the Green Tuff; this yields an estimate for the eruption duration of rather less than one day. It is predicted that welded tuff should be formed during large plinian eruptions of pantelleritic magma, and therefore that welded airfall tuffs should be common in areas of peralkaline volcanism.

The Granadilla pumice deposit of Southern Tenerife, Canary Islands

The Granadilla pumice, the product of a large Plinian eruption, covers over 800 km2 of southern Tenerife. It is, for the most part, reversely graded, and is divided into four fall-units which characterise specific phases in the eruption. Isopach and isograde maps indicate a north-westerly palaeo-wind direction. The results of the granulometric analysis of 170 samples are presented, together with criteria which allow pyroclastic-fall and pyroclastic-flow deposits to be differentiated. The Granadilla eruption took place over 32,000 years B.P. after a considerable repose period. It began with intermittent pumice showers and increased to a continuous high intensity gas blast. Reduced activity during the early part of the eruption allowed rain-flush beds to form while the volcano was quietly venting large volumes of fine ash. The eruption concluded with a pyroclastic-flow which is absent higher on the volcano, having failed to adhere to the steep slopes. The deposits are composed of pumice lapilli, lithic fragments, and a small proportion of sanidine crystals. Each component shows better sorting characteristics than complete samples, the frequency curves of which are asymmetrically skewed by the admixture of varying percentages of lithic fragments. In the finer grades, high lithic percentages are the result of wind drift removing the fine-grained pumice from the eruption cloud. The Granadilla pumice and associated pyroclastic deposits originated in the caldera of Las Canadas to the north-east. The formation of this caldera cannot be explained simply by northward Iandslipping as has been suggested previously. The southward dispersal of many large volume ignimbrites undermines the theory for the formation of Las Canadas by simple landslipping, while the total volume of all pyroclastic deposits in southern Tenerife fully accounts for the volume of the caldera and the pre-Teide volcano. It is therefore believed the formation of the caldera was gradual, in response to the progressive emptying of the magma chamber during a series of largescale Plinian eruptions.