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

Glass Mountain: Rhyolite (upper) + basalt (lower); Explosive dacite

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: Mount Vesuvius

Preliminary: 5 February 62 AD

First small earthquake

Next 16 years: continued small quakes

Roman Empire at its height

Very rich agriculture around volcano

No association of volcano with quakes

Structural repairs quickly made

The Event (Pliney’s story): 24 August 79

Plinian eruption’ noted by ‘Pliney the Elder’

Rescue mission to Stabiae

Noted high S smell; darkness; tsunami; ash

Top of Vesuvius gone

Pompeii and other cities gone (buried)

Massive destruction; precarious civilization

Pompeii and Herculanium

Herculanium buried by flows, lahars

Min. 20 m thick cover

Ercolanium built on top

Pompeii 9 km downwind

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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