Eruzioni vulcaniche
Lezione del 29 Marzo 2012
Eruzioni sottomarine
Frammenti
di ceneri
idrovulcaniche
e
pliniane
In this summary of hydrovolc
anic experiments, four basic i
nteraction phenomena, consi
sting of melt
fountaining, unsteady blasts,
steady production of steam a
nd ejecta, and the nonexplosi
ve quenching of
melt into globular shapes (blo
bs), are correlated to volcanic
activities and the water:melt
mass ratio (Rm ).
Scaled kinetic energy relative to the initial melt thermal energy is shown as a function of
water:magma mass ratio. Ranges are indicated for Strombolian, Surtseyan (hydrovolcanic), and
submarine activity as well as for corresponding landform and deposit bedforms.
(Adapted from Wohletz and McQueen, 1984.)
D = area compresa
nell’isopaca 1/100
di quella massima
F = % piroclastiti <
1 mm
all’intersezione tra
asse di dispersione
e isopaca 1/10
della massima
Volcanic Explosivity Index (VEI)
VEI
Description
Plume Height
Volume
Classification
How often
Example
0
non-explosive
<100 m
1000s m3
Hawaiian
daily
Kilauea
1
gentle
100-1000 m
10,000s m3
Haw/Strombolian
daily
Stromboli
2
explosive
1-5 km
1,000,000s m3
Strom/Vulcanian
weekly
Galeras, 1992
3
severe
3-15 km
10,000,000s m3
Vulcanian
yearly
Ruiz, 1985
4
cataclysmic
10-25 km
100,000,000s m3
Vulc/Plinian
10's of years
Galunggung, 1982
5
paroxysmal
>25 km
1 km3
Plinian
100's of years
St. Helens, 1981
6
colossal
>25 km
10s km3
Plin/Ultra-Plinian
100's of years
Krakatau, 1883
7
super-colossal
>25 km
100s km3
Ultra-Plinian
1000's of years
Tambora, 1815
8
mega-colossal
>25 km
1,000s km3
Ultra-Plinian
10,000's of years
Yellowstone, 2 Ma
Morfologie vulcaniche
Poligeniche
Positive
Monogeniche
Negative
Morfologie monogeniche
Coni di scorie saldate (Spatter cone)
Coni di scorie stromboliane (Cinder cone)
Coni di tufo, anelli di tufo
Duomi lavici e spine
Plateau monogenici
Crateri
Maar
Caldere monogeniche
Coni di scorie saldate – Spatter cone
Cono di scorie saldate – Spatter cone
Cono di scorie stromboliane – Cinder cone
Cono di scorie stromboliane – Cinder cone
Coni di scorie stromboliane
Tuff cone, Main Ethiopian Rift
Characteristics of Tuff Rings, Tuff Cones, and Cinder Conesa
Character
Tuff Rings
Tuff Cones
Cinder Cones
Height-Width Ratio
1:10 to 1:30
1:9 to 1:11
1:9 to 1:11
Lithology
Mostly sideromelane tuff
and lapilli-tuff, with
substantial amounts of
palagonitic tuff breccia
containing blocks of lava
and sediments; some
accretionary lapilli
Palagonitic and sideromelane
tuff and tuff breccia, possibly
scoria and lava within the vent,
abundant accretionary lapilli
Tachylitic cinders and
scoria, traces of
sideromelane ash, lava
flows
Bedding
Well-defined, relatively thin
beds, massive bedding
where base was below local
water level
Poorly defined, relative thick
beds, some thin beds at the base
and top
Massive bedding, poorly
defined
Sedimentary
Structures
Well-developed graded
bedding with dune,
massive, and plane-parallel
structures, impact sags
around ballistic fragments,
soft sediment deformations
Graded bedding at base and
top, bulk of cone is nongraded
massive beds, soft-sediment
deformations
Crude inverse grading in
avalanche structures
Water Source
Water from aquifer or
shallow lake
Water from deep aquifer, deep
lake, or marine environment
Little or no water
(magmatic)
Mechanism of
Ejecta Dispersal
Pyroclastic surge, fallout,
and slumping
Pyroclastic surge and flow,
fallout, and lahars
Ballistic fallout and
avalanching
a
From Heiken (1971).
Plateau lavico
monogenico
A
B
C
• Duomi lavici
Endogeni
Esogeni
Duomo lavico, Mount St. Helens
St. Helens
Mount St. Helens
Plug dome e Spina
Lipari,
Duomo esogeno, Castello d’Ischia