Explosive Eruption Dynamics 
 
Volcanology Background 

Volcanology is fundamentally the science of volcanoes.  Just as the term "physics" can apply to subdisciplines as diverse as solid state physics, cosmology, and, yes, fluid dynamics, volcanology encompasses various aspects of geophysics, petrology, geochemistry, sedimentology and, in recent years, fluid dynamics. 

This site focuses on the physical aspects and fluid dynamics of explosive eruptions.  However, to better understand these aspects, a primer in basic volcanology is necessary.  A proper vocabulary is also useful. 

MAGMA -- the multi-phase system of melt and crystals (+/- volatile phase) which can either crystallize at depth, as an intrusion, or erupt from volcanoes as LAVA or PYROCLASTIC rocks 

LAVA -- magma erupted effusively (as flows) at the surface; incidentally, "lava" rhymes with "java," not "have a. . ." 

PYROCLASTIC ROCKS -- magma erupted explosively at the surface; this is really a catch-all term for pumice, volcanic ash, scoria, and bombs, all of which are discussed in the later section "Magma Rising to the Surface" 

PLINIAN ERUPTIONS -- explosive eruptions whose dominant phase is a buoyant column of ash and pumice rising to heights up to 30+ km above sea level; see "Buoyant Regime" and "Neutral Buoyancy and Spreading of Plume"

PYROCLASTIC FLOWS -- pyroclastic density currents formed through non-buoyant behavior/collapse of eruption columns, or collapse of lava domes; these currents flow downhill at speeds of X to Y km/h, and are either dilute (termed pyroclastic surges) or concentrate (termed pyroclastic flows) 

Volcanic eruptions can be broadly classified as either effusive or explosive. The former erupt lava flows and domes, whereas the latter erupt tephra, bombs, and ash, producing plumes and pyroclastic density currents.

The primary control on behavior is composition and volatile content. Low-silica magmas, such as basalts, are generally hotter than and have much lower viscosities than high-silica magmas. Additionally, rhyolites can dissolve more water and other volatiles than basalts (Wallace and Anderson, 2000). It is the vesiculation of magma, and escape, or containment, of volatile phases that controls explosivity. Bubbles in rhyolites cannot easily move through the magma; as a result, bubble expansion fragments the magma.

Rock type Liquidus T (C) %SiO2 Viscosity (Pas)
Basalt 1200 <50 10-100
Andesite 1100 ~60 100-10^4
Rhyolite 900 ~70 10^7-10^10

 

(Values from Rogers and Hawkesworth, 2000; and Spera, 2000)

Some very large eruptions in history

Eruption/volcano Style of eruption Volume (DRE, km^3)
Mt St. Helens, 1980 Plinian 0.8
Pinatubo, 1991 Plinian/caldera 4
Katmai, 1912 Plinian/caldera 12
Ksudach, 240 A.D. Plinian/caldera 8-9
Krakatau, 1883 Plinian/caldera 10
Taupo, 1.8 ka Plinian/caldera 35
Santorini, 3.6 ka Plinian/caldera 25
Crater Lake, 7.7 ka Plinian/caldera 55
Toba, 75 ka Plinian/caldera 1500
Yellowstone, 600 ka Plinian/caldera 1000-2000
Long Valley, 760 ka Plinian/caldera >600

From Lipman, 2000

1822 eruption of Vesuvius, from Stromboli Online

Eruption Hazards

Volcanic eruptions can produce some of natures most spectacular hazards and phenomena -- imagine standing near what is now Long Valley Caldera 760,000 years ago. . . 700 km^3 of magma erupting and falling over the course of 3 days (REF) would certainly ruin your day. To put a volume that big in perspective, if spread evenly over New York City, the deposits would be 1 km thick. . . If spread over the ENTIRE state of Texas, all 261,797 suare miles of it, the deposits would be 3 feet thick. . . Of course the same deposits in Alaska would be less than 18 inches thick. Apparently size DOES matter. . .

Type of eruption

Submarine

Lava Flow

 

 

Plinian

 

Pyroclastic Flow

 

Dome Growth

Lahars

 

Hazard

Very little hazard

Destruction of property, loss of life possible, though generally not as direct result of lava immolation

Roof collapse; Significant hazard to airplanes -- engines flame out

Substantial hazard proximally, particularly in valleys

Growth may lead to collapse and Pyroclastic flows

Substantial hazard in low areas (valleys), potentially no precursor activity

The Alaska Volcano Observatory, AVO, is one agency that monitors and assesses the hazards of volcanoes. AVO is a cooperative effort of the University of Alaska Fairbanks Geophysical Institute, the Alaska State Division of Geological and Geophysical Surveys, and the United States Geological Survey.