Lecture%20Outlines%20Natural%20Disasters,%207th%20edition

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Lecture OutlinesNatural Disasters, 7th edition
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Volcanic Eruptions Plate Tectonics and Magma
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Vesuvius, 79 C.E.

• Cities of Pompeii and Herculaneum buried by
  massive eruption which blew out about half of Mt.
  Vesuvius
• Similar to 1991 eruption of Mt. Pinatubo in
  Philippines
• Clouds of hot gas (850oC), ash and pumice
  enveloped city
• Many tried to escape near sea, but were buried by
  pyroclastic flows

Figure 8.1
Figure 8.3
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Vesuvius, 79 C.E.

• Vesuvius was inactive for 700 years before 79 CE
  eruption
• People lost fear and moved in closer to volcano
• After 79 CE, eruptions in 203, 472, 512, 685,
  993, 1036, 1049, 1138-1139
• 500 years of quiet, then 1631 eruption killed
  4,000 people
• 18 cycles of activity between 1631 and 1944,
  nothing since then
• 3 million people live within danger of Vesuvius
  today 1 million people on slopes of volcano

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The Hazards of Studying Volcanoes

• Eruptive phases are often separated by centuries
  of inactivity, luring people to live in vicinity
  (rich volcanic soil)
• 400,000 people live on flanks of Galeras Volcano
  in Colombia
• Many people killed each year by volcanoes,
  sometimes including volcanologists
• Volcanoes may be active over millions of years,
  with centuries of inactivity

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How We Understand Volcanic Eruptions

• Understand volcanoes in context of plate
  tectonics
• Variations in magmas chemical composition,
  ability to flow, gas content and volume
  determines whether eruptions are peaceful or
  explosive

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Plate-Tectonic Setting of Volcanoes

• 90 of volcanism is associated with plate
  boundaries
• 80 at spreading centers
• About 10 at subduction zones
• Remaining 10 of volcanism occurs above hot spots

Figure 8.5
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Plate-Tectonic Setting of Volcanoes

• Subduction carries oceanic plate (with water-rich
  sediments) into hotter mantle, where water lowers
  melting temperature of rock
• Rising magma melts continental crust it passes
  through, changing composition of magma

Figure 8.6
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Plate-Tectonic Setting of Volcanoes

• No volcanism associated with transform faults or
  continent-continent collisions
• Oceanic volcanoes are peaceful
• Subduction-zone volcanoes are explosive and
  dangerous
• Subduction zones last tens of millions of years
• Volcanoes may be active any time, with centuries
  of quiet

Figure 8.6
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Chemical Composition of Magmas

• Of 92 naturally occurring elements
• Eight make up more than 98 of Earths crust
• Twelve make up 99.23 of Earths crust
• Oxygen and silicon are by far most abundant
• Typically join up as SiO4 tetrahedron, that ties
  up with positively charge atoms to form minerals

Figure 8.7
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Chemical Composition of Magmas

• Mineral formation in magma crystallization
• Order of crystallization of different minerals in
  magma can be determined
• Iron and magnesium link with aluminum and SiO4 to
  form olivine, pyroxene, amphibole and biotite
  families
• Calcium combines with aluminum and SiO4 until
  calcium replaced by sodium, to form plagioclase
  feldspar family calcium and sodium are later
  replaced by potassium, to form potassium feldspar
  and muscovite families finally only Si and O
  remain, forming quartz

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Chemical Composition of Magmas
Figure 8.8
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Chemical Composition of Magmas

• Elements combine to form minerals
• Minerals combine to form rocks
• Different compositions of magma result in
  different igneous rocks
• If magma cools slowly and solidifies beneath
  surface ? plutonic rocks
• If magma erupts and cools quickly at surface ?
  volcanic rocks

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Viscosity, Temperature and Water Content of Magmas

• Viscosity internal resistance to flow
• Lower viscosity ? more fluid behavior
• Water, melted ice-cream
• Higher viscosity ? thicker
• Honey, toothpaste
• Viscosity determined by
• Higher temperature ? lower viscosity
• More silicon and oxygen tetrahedra ? higher
  viscosity
• More mineral crystals ? higher viscosity
• Magma contains dissolved gases volatiles
• Solubility increases as pressure increases and
  temperature decreases

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Viscosity, Temperature and Water Content of Magmas

• Consider three types of magma basaltic,
  andesitic and rhyolitic
• Basaltic magma has highest temperatures and
  lowest SiO2 content, so lowest viscosity (fluid
  flow)
• Rhyolitic has lowest temperatures and highest
  SiO2 content, so highest viscosity (does not
  flow)
• Basaltic makes up 80 of magma that reaches
  Earths surface, at spreading centers, because it
  forms from melting of mantle
• Melted mantle at subduction zones rises through
  continental crust before reaching the surface,
  incorporating continental high SiO2 rock as it
  rises, to become andesitic or rhyolitic in
  composition before it erupts

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Viscosity, Temperature and Water Content of Magmas
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Viscosity, Temperature and Water Content of Magmas

• Water is most abundant dissolved gas in magmas
• As magma rises, pressure decreases, water becomes
  steam bubbles
• Basaltic magma has lower water content ?
  peaceful, safe eruptions
• Rhyolitic magma has higher water content and high
  viscosity ? many steam bubbles form and can not
  escape through thick magma, so explode out ?
  violent, dangerous eruptions

Figure 8.10
Figure 8.9
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Plate-Tectonic Setting of Volcanoes Revisited

• Spreading centers have abundant volcanism
  because
• Sit above hot asthenosphere
• Asthenosphere has low SiO2
• Plates pull apart so asthenosphere rises and
  melts under low pressure, changing to
  high-temperature, low SiO2, low volatile, low
  viscosity basaltic magma that allows easy escape
  of gases ? peaceful eruptions

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Plate-Tectonic Setting of Volcanoes Revisited

• Subduction zones have violent eruptions because
• Magma is generated by partial melting of the
  subducting plate with water in it
• Melts overlying crust to produce magmas of
  variable composition
• Magma temperature
• decreases while SiO2,
• water content and
• viscosity increase ?
• violent eruptions

Figure 8.11
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How a Volcano Erupts

• Begins with heat at depth
• Rock that is superheated (heated to above its
  melting temperature) will rise
• As it rises, it is under less and less pressure
  so some of it melts (becomes magma)
• Volume expansion leads eventually to eruption
• Three things will cause rock to melt
• Lowering pressure
• Raising temperature
• Increasing water content
• Lowering pressure is most common way to melt rock
  ? decompression melting

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How a Volcano Erupts

• Magma at depth is under too much pressure for gas
  bubbles to form (gases stay dissolved in magma)

• As magma rises toward surface, pressure decreases
  and gas bubbles form and expand, propelling the
  magma farther up
• Eventually gas bubble volume may overwhelm magma,
  fragmenting it into pieces that explode out as a
  gas jet

Figure 8.12
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How a Volcano Erupts

• Eruption Styles and the Role of Water Content
• Concentration of water in magma largely
  determines peaceful or explosive eruption
• Basaltic magma can erupt violently with enough
  water
• Rhyolitic magma usually erupts violently because
  of high water content, high viscosity (secondary
  role)
• Styles of volcanic eruptions
• Nonexplosive Icelandic and Hawaiian
• Somewhat explosive Strombolian
• Explosive Vulcanian and Plinian

Figure 8.14
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How a Volcano Erupts

• Some Volcanic Materials
• Low-water content, low-viscosity magma ? lava
  flows
• High-water content, high-viscosity magma ?
  pyroclastic debris

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How a Volcano Erupts

• Nonexplosive eruptions
• Pahoehoe smooth ropy rock from highly liquid
  lava
• Aa rough blocky rock from more viscous lava

Figure 8.15
Figure 8.16
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How a Volcano Erupts

• Explosive eruptions
• Pyroclastic debris broken up fragments of magma
  and rock from violent gaseous explosions,
  classified by size
• May be deposited as
• Air-fall layers (settled from ash cloud)
• High-speed, gas-charged pyroclastic flow

Figure 8.18
Figure 8.17a
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How a Volcano Erupts

• Explosive eruptions
• Very quick cooling
• Obsidian volcanic glass forms when magma cools
  very fast
• Pumice porous rock from cooled froth of magma
  and bubbles

Figure 8.19
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Side Note How a Geyser Erupts

• Geyser eruption of water superheated by magma
• Can only exist in areas of high heat flow
  underground
• Water boils (becomes gas) at 100oC unless it is
  under pressure no room for expansion to gas
  state
• Water can be heated to higher than boiling
  temperature ? superheated

• When superheated water reaches point of lower
  pressure, it flashes to steam violently, and
  erupts out of the ground

Figure 8.20
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The Three Vs of Volcanology Viscosity,
Volatiles, Volume

• Viscosity may be low or high
• Controls whether magma flows easily or piles up
• Volatile abundance may be low, medium or high
• May ooze out harmlessly or explode
• Volume may be small, medium or large
• Greater volume ? more intense eruption

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The Three Vs of Volcanology Viscosity,
Volatiles, Volume

• By mixing different values for the three Vs, can
  forecast different eruptive styles for volcanoes

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The Three Vs of Volcanology Viscosity,
Volatiles, Volume

• By mixing different values for the three Vs, can
  define different volcanic landforms

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The Three Vs of Volcanology Viscosity,
Volatiles, Volume

• Shield Volcanoes Low Viscosity, Low Volatiles,
  Large Volume
• Basaltic lava with low viscosity and low
  volatiles flows to form gently dipping, thin
  layers
• Thousands of layers on top of each other form
  very broad, gently sloping volcano like Mauna Loa
  in Hawaii
• Great width compared to height

Figure 8.22
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The Three Vs of Volcanology Viscosity,
Volatiles, Volume

• Hawaiian-type Eruptions
• Curtain of fire lines of lava fountains up to
  300 m high
• Low cone with high fountains of magma
• Floods of lava spill out and flow in rivers down
  slope
• Eruptions last days or years, usually not
  life-threatening but destroy buildings and roads

Figure 8.24