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Chapter 7

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Carleton University
Earth Sciences
ERTH 2415
Claire Samson

Chapter 7Volcanic Eruptions Plate TectonicsMagmaVolcanoes deal out overwhelming doses of energy no human can surviveThe most famous of all volcanoes probably is Vesuvius in Italy and the most famous of all eruptions must be those of 79 CE o It was then that the cities of Pompeii and Herculaneum were buried and forgotten for over 1500 yearsA reminder of the natural hazards near Vesuvius arrived on 5 February 62 CE when a major earthquake destroyed much of Pompeii and caused serious damage in Herculaneum and Neapolis NaplesEarthquakes as not as large were a common occurrence for the next 17 yearsPompeii has been a centre of commerce for centuriesIn 79 CE the city had a population of about 20000 people 8000 of whom were slavesIt was described as an average city inhabited by average people by Robert Etienne th Vesuvius blew out 4 km3 of volcanic material on the 24 of AugustAbout half of the old volcanic cone was destroyedIn Pompeii great clouds of hot gas and volcanic ash flowed across the city killing the people who had not fledToday the excavated city is a major tourist attractionThe Hazards of Studying VolcanoesThe dangers of volcanic eruption are obvious but the quiet spells between active volcanism are seductiveSome people are lured to volcanoes like moths to a flame even those who should know better o January 14 1993 volcanologists attending a workshop in Colombia as part of the international decade of natural disaster deduction hiked into the summit crater of Galeras Volcano to sample gases and measure gravity o They were looking for ways to predict imminent eruptions o The volcano has been quiet since July 1992 but during their visit and unexpected gaspowered secondary eruption killed six in the scientific partyfour Colombians a Russian and an Englishman o Their deaths were not an unusual eventthey serve as a small scale example of the larger drama played out when a volcano suddenly buries an entire cityDuring long periods of volcanic quiescence people tend to build cities near volcanoesFor example 400000 people live on the flanks of Galeras Volcano delaying the inevitability of a large life snuffing eruptionAn individual volcano may be active for millions of years but its eruptive phases are commonly separated by centuries of inactivity lulling some into a false sense of securityAround 410 BCE Thucydides wrote History repeats itselfWe know that those who do not learn the lessons of history are doomed to repeat themHow We Understand Volcanic Eruptions There are two primary building blocks of knowledge that are paramount to understanding volcanic eruptions 1 Plate tectonics 2 Magma a Varies in its chemical composition ability to flow easily gas content and volumeThese variations govern whether eruptions are peaceful or explosive Plate Tectonic Setting of VolcanoesOver 90 of volcanism occurs at the edges of tectonic plates more specifically at spreading centres and subduction zonesOver 80 of magma is extruded through volcanism is found at oceanic spreading centresSolid but hot and ductile mantle rock rises upward into regions of lower pressure where up to 3040 of the rock can melt and easily flow to the surfaceMost other volcanism occurs above hot spots accounting for the remaining 10 of all extruded magmaVirtually all volcanic activity occurs below sea level and thus is difficult to viewSubduction ZonesCause the tall and beautiful volcanic mountains we see at the edges of continents but the volume of magma released at subduction zones is small compared to that of spreading centres Transform faults and continentcontinent collision zones have little or no association with volcanism o This is because the two plates simply slide together in a horizontal sense and at all times keep quite an effective lid on the hot athenosphere some 100 km below o At continentcontinent zones continental rock stacks into extra thick masses that deeply bury the hot mantle rock which makes it difficult for magma to rise to the surfaceOceanic volcanoes are relatively peaceful whereas subductionzone volcanoes are explosive and dangerousChemical Composition of MagmaWhile there are 92 naturally occurring elements only 8 make up more than 98 of the Earths quest o Oxygen and Silicon are so abundant that their percentages dwarf all the other elements o Oxygen atoms carry negative charges and silicon atoms are positively chargedWhen magma cools silicon and oxygen atoms will bond If magma cools and solidifies below the surface it crystallizes as plutonic rocks named for Pluto the Greek god of the underworldIf magma reaches the surface it forms volcanic rocks named for Vulcan the Roman god of fireViscosity Temperature and Water Content of Magma
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