Earth Sciences 240A - Lecture 20 - Volcanoes II:
Subduction Volcanism
Introduction
Magma generation and evolution
Island arcs
Magma similar to MORB; called IAB
High Rb, Ba, (K)
Compatible – incompatible elements
Fractional crystallization
Continental margins
Enriched in incompatible elements
Slow ascent through various magma
chambers
Fractional crystallization
Assimilation
Very viscous; very gaseous
Magma mixing
Upper chamber
More evolved magma
Lower chamber
Less evolved
Example
Basalt absent if crust >60 km
Andesite to rhyolite most common
Subduction zone igneous bodies
Composite volcanoes
Others
Underplated sheets
Batholiths
Subsidence
volcanoes or calderas
Eruption summary
Melt moves into upper chambers
Within ~2 km of surface, eruption
inevitable (confining pressure lowered)
Volatiles exsolve
Rapid upward movement
Gas pressure increases
Top blown; pulverized magma and top
rocks
Pyroclastic products
If chamber top collapses, secondary
explosion
Later stages quiet and chamber
emptied
Top seals, volcano becomes dormant
Pyroclastic products
(See table )
Ignimbrite to nuée
ardent
Surges (flows of lower density)
Lighter than
pumice
Example:
Preliminary:
First small earthquake
Next 16 years: continued small
quakes
Very rich agriculture around
volcano
No association of volcano with
quakes
Structural repairs quickly made
The Event (Pliney’s story):
‘Plinian
eruption’ noted by ‘Pliney the Elder’
Rescue mission to Stabiae
Noted high S
smell; darkness; tsunami; ash
Top of Vesuvius
gone
Massive destruction; precarious
civilization
Herculanium
buried by flows, lahars
Min. 20 m thick cover
Ercolanium built on top
Min. 3 m ash cover
Not rediscovered
until 1595
Valuables
plundered (by the Brits!)
Vesuvius eruption history
Since 79, erupts roughly every 100
yrs
Big ones are:
650 BC, 79 AD, 203, 472, 512, 685,
993, 1036, 1049, 1138, 1631, (1944-minor)
Intervals: 729, 124, 269, 40, 173, 308, 43,
13, 89, 493, (313)
Currently an active subduction zone
Every reason to expect another
eruption
2002 study: magma ‘field’ beneath Vesuvius measures 400 km2 and is 8 km beneath top
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