ISNS 4359 EARTHQUAKES AND VOLCANOES
Spring 2005
Steve Bergman, Instructor
Lecture 14. Volcanic Eruption Products & Types, Explosivity Scales
Eruptions are described as Explosive or Effusive based on the degree of violence (explosive most violent).
“non-explosive” Effusive eruptions-Central Vent (pipe-like) and Fissure eruptions (fracture/sheet-like)
Three Main Volcanic Explosion types:
Phreatic explosions occur when ground or surface water is vaporized by hot (700-1200 ºC) magma.
Magmatic explosions result from the expansion of juvenile gases from a decompressing, cooling, and/or
crystallizing magma.
Phreatomagmatic explosions result from the interaction of juvenile material with ground or surface
water (Surtseyan).
Six Explosive Magmatic Eruption Styles:
Hawaiian / Icelandic - (non-explosive: VEI<0.1) (basalt) gusher-like lava fountains (~meters to 1 km
high), red-hot lava rivers and flows; Icelandic- construct volcanic plateau & small shield volcanoes;
Hawaiian- construct large shield volcanoes with spatter cones.
Strombolian - (Low explosivity: VEI=1-3) (basalt/andesite- like bursting bubbles in boiling chocolate);
arching, incandescant ejecta; construct Scoria Cones.
Vulcanian - (Moderate explosivity: VEI=2-5) (rhyolite-basalt) cycles of infrequent phreatic explosions,
with mingled magma, ash, and lithics, finally thick viscous lavas; construct Scoria cones &
Stratovolcanoes
Plinian - (High: VEI=3-8) (rhyolite-andesite, rarely basalt) ash columns injected <10-50 km into the
atmosphere; construct Stratovolcanoes & Calderas
Caldera – (High: VEI=6-8) (rhyolite-andesite) most explosive-largest volume; magma chamber roof
collapses & partially empties into huge ash columns; forms deep depressions-“negative volcanoes”
Peleean - (nuée ardente-pyroclastic flows) (rhyolite-andesite) glowing avalanches, limited airfall.
Pyroclastic Rock-(Greek: fire-broken) A clastic rock or deposit formed by volcanic ejection from a
vent, produced by the fragmentation of magma (± country rocks). Tephra- any pyroclastic deposit.
Pyroclast classification (size): ash (<2 mm), lapilli (2-64 mm), and blocks and bombs (>64 mm);
corresponding pyroclastic rocks: tuff, lapillistone and breccia.
Pyroclastic deposit classification based on transport process:
Pyroclastic Fall deposit: vertical settling from a low concentration cloud of ash injected into the
atmosphere; blanket topography; generally well sorted.
Pyroclastic Surge deposit: turbulent surface flow of an expanded, low concentration gas-solid mixture;
generally thickest in depressions; show well-developed sedimentary bed forms and structures and are
generally well sorted.
Pyroclastic Flow deposit: laminar flow of high concentration gas-solid mixture (fluidized and hot) along
the surface; generally channelized (restricted to valleys and depressions) and poorly sorted (ignimbrite).
Epiclastic rocks: pyroclastic rocks reworked by surficial sedimentary processes.
Lahar: water-saturated mud-rich debris flow from a volcano (sudden melting snow & ice, rain, flank collapse).
Eruption Size: Magnitude (mass of erupted material=kg) and Intensity (mass eruption rate=kg/sec).
VEI-Volcanic Explosivity Index: 0 -non explosive [<104 m3 ejecta] to 8 [>103 km3 ejecta]
Damage Summary: 0- quiet, effusive eruptions of lava; typically only a threat to local property
1-3 - (105-7 m3) progressively more violent explosive eruptions capable of local damage
4-5 - (108-9 m3) moderate explosive eruptions capable of regional damage and disruption
6-7 - (1010-11 m3) large to gigantic explosive eruptions capable of global climate modification
8 - (>1012 m3 =103 km3) super-eruptions capable of severe global climate modification
Most volcanic energy is dissipated as heat (range-1-2 MJ/kg for rhyolite to basalt at 800-1200C)