Processi di messa in posto dei prodotti piroclastici Lezione del 16 Aprile 2012
I prodotti piroclastici (Tephra) Juvenili Accessori (Cognate) Accidentali Ceneri Lapilli Bombe e Blocchi Pomici Scorie Litici
Bombe e Blocchi
Lapilli e Ceneri
Cenere vulcanica
{ { Blocchi e bombe (> 64 mm) Breccia Piroclastica (blocchi) Agglomerato piroclastico (bombe) 50 50 Tufo-breccia { Litico Cristallino Vetroso Tufo Lapillistone Tufo a lapilli Lapilli tuff Lapilli 50 Ceneri - (64-2 mm) (< 2 mm)
Meccanismi primari di deposizione piroclastica
Distribuzione depositi di flussi piroclastici (colata e surge)
Fall Flow Surge Tripartite Classification of Pyroclastic Materials Emplacement Mechanism Areal Dispersal Deposit Textures Grain-Size Characteristics Eruption Mechanism Class Fall Ballistic, aerodynamic drag modified; suspension Symmetrical along wind vectors; relatively wide-spread Mantles topography; normally and reversely (Plinian) graded beds Well sorted by terminal velocity; coarse near vent, fines at distance All Flow Steady, lateral movement over substrate by grain flow, saltation, suspension Directed, radial from vent, following drainages up to tens of kilometers Massive; confined to topographic lows; fine base with reverse pumice grading; some bedded intervals Poorly sorted fine to coarse ash with near vent breccia Plinian, Vulcanian Surge Unsteady, lateral blast over substrate by pulsating saltation, suspension, and grain flow locally accelerated by shocks Directed, partially confined by drainages (some mantling), up to several vent radii from source Thinly bedded, showing variety of bedforms: dunes, plane beds, massive beds, wet sediment deformations Poor to moderate sorting of fine to coarse ash; zones of fine ash depletion Vulcanian, Surseyan, Plinian, phreatic, hydrothermal
Depositi di caduta
Depositi di caduta hawaiiani
Depositi di caduta hawaiiani
Depositi di caduta hawaiiani (Coni di scorie saldate o Spatter cones)
Depositi di caduta stromboliani
Deposito di caduta stromboliano (Cono di scorie)
Depositi di caduta stromboliani
Ecuador's Chimborazo volcano
Depositi di caduta vulcaniani, pliniani, freatopliniani Pliniana Vulcaniana
Colonna eruttiva
Depositi di caduta pliniani e subpliniani
Depositi di caduta pliniani e subpliniani
Colate piroclastiche
200 300 400 500 100 600 700 Raggio boc,ca (m) Velocità (m/s) Tasso eruttivom (m3/s) 107 108 109 1010 Acqua essolta (peso%) 1.0 4.0 2.0 3.0 Colonna convettiva collassante Wilson et al., 1981
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.)
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)