VOLCANOES AND VOLCANIC ERUPTIONS (Chapter 6)

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VOLCANOES AND VOLCANIC ERUPTIONS(Chapter 6)
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A volcano produces

1. Lava flow (molten rock)
2. Pyroclastic debris (solid chunks of rocks and
   ash)
3. Volcanic gas and AEROSOLS (when volcanic gases
   condense to form tiny liquid droplets)

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Ash, gas and aerosols combine to form the smoke
above a volcano

• Volcanic behavior depends on
• VISCOSITY ("stickiness") of the magma
• GAS CONTENT
• TEMPERATURE

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More viscous (less fluid) magmas cause more
explosive eruptions (Text page 176, table 6.2)
Gases cannot easily escape from viscous magmas
High pressure
Explosive eruptions
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Viscosity of magma depends on

• Composition of the magma (how much silica it
  contains)
• This is the major factor that controls viscosity
  and explosivity of volcanoes
• Temperature (Hotter magmas are more fluid)

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• Volcanoes along mid-ocean ridges (e.g. Iceland)
  and hot spots (e.g. Hawaii) produce BASALTIC
  magma (pages 179-180)
• Basaltic magma is less viscous (low silica
  content, 50) and least dangerous
• It usually forms shield volcanoes (table 6.2)

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• When Basaltic magma rises through continental
  crust, it melts the crustal rocks and form FELSIC
  (or rhyolitic) magma (silica gt70)
• Felsic magma is highly viscous and usually forms
  volcanic domes (table 6.2)
• This type of magma causes highly explosive
  eruptions

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• Mixing of felsic and Basaltic magma, as well as
  melting of ocean sediments produces intermediate
  magma (ANDESITE, silica 55)
• Intermediate magma is most common in subduction
  zones and form composite volcanoes (or
  stratovolcanoes, table 6.2)
• Intermediate magma is highly explosive

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IMPACTS OF VOLCANIC ERUPTIONS

• Lava flows (Page 185)
• Non-explosive, gentle eruptions
• Will burn anything on its path
• Is possible to control/divert lava flows (page
  186)

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• Pyroclastic flows (Page 186)
• Mixture of HOT, mostly poisonous gases, ash and
  rocks
• Can travel at speeds of 100-300 kmph (60-90 mph)
• Can cover a large area in a very short time
• Causes instant death on contact, flatten
  buildings and forests (e.g. Mt. St. Helens)

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• Ash falls (Page 187)
• Hot burning ash is fatal close to the volcano
  (e.g. Pompeii)
• Windborne ash can cause respiratory problems,
  bury crops and spread toxic chemicals over large
  areas
• Hazard to airplanes
• Ash, dust and aerosols in the stratosphere can
  cause global cooling (similar to nuclear winter)

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• Gases (pages 188-189)
• Toxic sulfurous gases are harmful
• Carbon dioxide can cause suffocation (e.g. Lake
  Nyos, Cameroon, West Africa, Fig. 6.22)
• Water vapor from volcanic eruptions started the
  hydrologic cycle on the earth

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• SECONDARY IMPACTS
• Landslides and lahars (page 190)
• Earthquakes
• Tsunamis

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WHY DO PEOPLE STILL LIKE TO LIVE NEAR VOLCANOES?

• Nutrient-rich soil
• View (Travel and tourism, outdoor activities)
• Geothermal energy