8 Distribuzione depositi di flussi piroclastici (colata e surge)
9 Fall Flow Surge Tripartite Classification of Pyroclastic Materials Emplacement MechanismAreal DispersalDeposit TexturesGrain-Size CharacteristicsEruption MechanismClassFallBallistic, aerodynamic drag modified; suspensionSymmetrical along wind vectors; relatively wide-spreadMantles topography; normally and reversely (Plinian) graded bedsWell sorted by terminal velocity; coarse near vent, fines at distanceAllFlowSteady, lateral movement over substrate by grain flow, saltation, suspensionDirected, radial from vent, following drainages up to tens of kilometersMassive; confined to topographic lows; fine base with reverse pumice grading; some bedded intervalsPoorly sorted fine to coarse ash with near vent brecciaPlinian, VulcanianSurgeUnsteady, lateral blast over substrate by pulsating saltation, suspension, and grain flow locally accelerated by shocksDirected, partially confined by drainages (some mantling), up to several vent radii from sourceThinly bedded, showing variety of bedforms: dunes, plane beds, massive beds, wet sediment deformationsPoor to moderate sorting of fine to coarse ash; zones of fine ash depletionVulcanian, Surseyan, Plinian, phreatic, hydrothermal
29 Illustration of the "energy-line" concept for a Plinian eruption from a hypothetical composite cone. The gas and convective thrust regions of the eruption column (Wilson, 1976; Sparks et al ., 1978) are shown with a pyroclastic flow that is initiated by collapse near the top of the gas thrust region (hc ) with the vent at a height of h(0). The initial potential velocity of the pyroclastic flow is constrained by v(0) = [2gD h(0)]1/2 , where D h(0) = hc - ho . Flow acceleration a(i) and runout are a function of the local topographic slope [q (i)] and the Heim coefficient (µh ) shown by Eq. (1-17). The general slope of the energy line (q e ) is given by arctan (hc /Lf ) for which Lf = the distance from the vent where v(i) vanishes (that is, where the energy line intersects the topographic surface). (Adapted from Sheridan, 1979.)
30 Ignimbriti LARI HARI Saldatura Co-ignimbrite breccia - depositi di colata piroclastica fortemente saldati e con strutture a fiamme.- depositi di colata piroclastica di grande volume ed estensione areale- depositi di colata piroclastica, saldati o non saldati, formati essenzialmente da pomici e ceneri (Encyclopedia of Volcanoes,1999)
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