Eruzioni vulcaniche

Lezione del 29 Marzo 2012

Eruzioni sottomarine

Frammentidi ceneri

idrovulcaniche

epliniane

In this summary of hydrovolcanic experiments, four basic interaction phenomena, consisting of melt fountaining, unsteady blasts, steady production of steam and ejecta, and the nonexplosive quenching ofmelt into globular shapes (blobs), 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 ofwater:magma mass ratio. Ranges are indicated for Strombolian, Surtseyan (hydrovolcanic), andsubmarine 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

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

Volcanic Explosivity Index (VEI)Volcanic Explosivity Index (VEI)

Morfologie vulcaniche

Poligeniche Monogeniche

Positive Negative

Morfologie monogeniche

Coni di scorie saldate (Spatter cone)Coni di scorie stromboliane (Cinder cone)Coni di tufo, anelli di tufoDuomi lavici e spinePlateau monogeniciCrateriMaarCaldere 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 ofEjecta 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

EndogeniEsogeni

Duomo lavico, Mount St. Helens

St. Helens

Mount St. Helens

Plug dome e Spina

Lipari,

Duomo esogeno, Castello d’Ischia