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)
{
50
Lapillistone
Tufo-breccia
Tufo a lapilli
Breccia Piroclastica (blocchi)
Agglomerato piroclastico (bombe)
50
Tufo
Lapilli tuff
Lapilli
(64-2 -mm)
50
Ceneri
(< 2 mm)
{
Litico
Cristallino
Vetroso
Meccanismi primari di deposizione piroclastica
Distribuzione depositi di flussi
piroclastici (colata e surge)
Tripartite Classification of Pyroclastic Materials
Emplacement
Mechanism
Areal
Dispersal
Deposit
Textures
Grain-Size
Characteristics
Eruption
Mechanism
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
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
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
Class
Fall
Flow
Surge
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
Acqua essolta (peso%)
1.0
2.0
3.0
4.0
500
700
600
1010
Raggio boc,ca (m)
400
109
300
108
200
Colonna
convettiva
Tasso eruttivom (m3/s)
Colonna
collassante
500
100
107
0
200
300
400
500
Velocità (m/s)
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)