GEOL 106 - LEC 5

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Queen's University
Geological Sciences and Geological Engineering
GEOL 106

Read Chapter 3 in Textbook (pages 72 to 107) • Do Risk Analysis and Risk Management Volcano “Zones” 1. Constructive (ie. Divergent) Plate margins 2. Destructive (ie convergent) Plate margins 3. Hot Spots • most commonly not at plate boundaries (eg. Hawaii) • both oceanic and continental Dealing with volcanic risk • We first need to consider the nature of Magma of different compositions Chemical composition • Magma differ in the proportion of elements o Results in rocks of different colours o And magmas with different properties More Si+O More Fe+Mg Less Fe+Mg Less Si+O Rock is Whiter/Pinker -> Felsic Rock is blacker -> Mafic Eg. Rhyolite Eg. Basalt Magmas Felsic Intermediate Mafic High Viscosity Intermed Viscosity Low Viscosity High Si Intermed Si Low Si High H20 (>5%) Intermed H2O (1 to Low H2O (<1%) 5%) Viscosity • A measure of the ease with which a fluid flows • Low Viscosity -> flows easily eg. Water • High Viscosity -> does not flow easily eg. Tar or molasses IMPORTANT NOTE • Magma contains dissolved gases (eg. H20 (water), CO2 (carbon dioxide) ,SO2 (sulphur dioxide) ,F (fluorine) ,Cl (chlorine)) • H2O is the dominant one Magma • When it comes out of the ground will be in one of the two forms o 1. Lava (liquid) o 2. Pyroclastic material (hot particles) • Reaches the surface by coming through either o 1. Fractures (fissures)  ie. Cracks (where a dyke reaches surface)  mostly involve mafic lave (ie. Basalt)  results in extensive lava sheets  due to low viscosity (eg. Iceland)  These are calm eruptions o Dangers of fissure eruptions  Generally quite safe  But major eruptions can generate atmospheric effects from SO2 gas and CO2 gas release (eg. Laki eruption in Iceland)  In particular, enormous flood basalt events may be catastrophic  Linked to mass extinctions (eg, 65 mya)(eg. 250 mya) • 2. Volcanic cones o pipe-like conduit and vent bringe magma to surface  eruptions range from calm to violent  depends on magma composition  3 Main Types  1. Cinder Cone (kind of material = Pyroclastic) not really important and not really big  2. Stratovolcano (composite cone) ( lava + pyroclastic , felsic to intermediate material ) much bigger  Shield Volcano (lava) mafic material (calm eruptions) Magmas Felsic Mafic High viscosity and sticky and high Low viscosity and fluid and low water content (more than 5%) water content less than 1% Which type of Why? because their magma compositiong is felsic or intermediate therefore the magma has • A) high viscosity • B) high water content This leads to • A) high, steep sided volcanoes o Because of the pyroclastic ash that piles up with steep- sides o And because the sticky magma flows slowly down the sides maintaining steep slopes of the volcanoe • B) volcanoes that erupt violently o The magma is water-rich and sticky, as magma rises to the surface, there is a decrease in pressure. The gas (steam) bubbles try to expand and get out. but the sticky magma prevent this. Eventually , explosive expansion occurs o Also, stratovolcanoes occur along continental margins where people like to live (ie. Subduction zones . destructive plate margines) Dangers of stratovolcanoes • 1. Air-fall “ash” (Tephra) and ash clouds o a) air-fall tephra (bombs, pumice ,ash) can bury cities (eg. Mount Vesuvius, 79AD ) buried roman city o b) Ash clouds -> ash and sulphuric acid can block suns ray, lower global temperatures for years (eg. Tamboro , 1815 AD)  Effects of Tambora. In 1816 AD :  Famine in Europe Ash Hazards • 1. Airborne ash affects airplanes • 2. This ash-fall layer (10-15cm thick) can cause roof collapse • 3. Very thin ash-fall layer (<1CM) can disrupt agriculture • 4. Intake of ash affects industry (eg. Hydroelectric and nuclear power stations) • 5. Human health hazard (inhalation, suffocation) • 6. Atmospheric effects Effects if Tambora Eruption in 1815 AD In 1816 AD • 1. Famine in Europe • 2. Mary Shelley Woolstencraft writes Frankenstein • 3. Byran composes his most miserable poem ( darkness) • 4. Joseph Turner paints dramatic sunsets • 5. Massive emigration to California ( the “mackerel year”) 2 Types of Ashflow (pyroclastic flow) • a) nuée ardente -> glowing cloud o travel downslope at up to 150 km/hr o can travel more than 10 km  eg. Mt Pelée 1902 AD -> 30,000 killed • b) Pyroclastic surge or blast (lateral blas
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