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Volcanism - PowerPoint PPT Presentation

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Volcanism. Any activity that includes the movement of magma towards or onto Earth’s surface. http://dsc.discovery.com/tv-shows/discovery-presents/videos/understanding-volcanoes-lava-flow.htm. Volcano:. A vent or fissure in Earth’s surface through which magma and gases are expelled.

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Any activity that includes the movement of magma towards or onto Earth’s surface



A vent or fissure in Earth’s surface through which magma and gases are expelled

- Subduction - Rifting - Hotspots

Volcanoes are formed by:


Pacific Ring of Fire

An area around the Pacific Ocean characterized by volcanoes and seismic activity

Hotspot volcanoes



Magma that flows onto the Earth’s surface

Liquid rock produced under the Earth’s surface

  • What is magma?
    • Magma is a mixture of molten rock, mineral grains, and dissolved gases formed deep beneath the earth’s surface at high temperatures
  • Where does it come from?
    • From the upper part of the mantle directly beneath the lithosphere
what factors affect the formation of magma
What factors affect the formation of magma?
  • Temperature -- temps need to be between 800˚C and 1200˚C
  • Pressure – as pressure decreases, the temperature at which a substance melts also decreases
  • Water content of the rocks – rocks melt at lower temperature in the presence of water
  • Composition of the rocks – different types of rock melt at different temperatures
what is viscosity
What is Viscosity?
  • A substance’s resistance to flow is called its viscosity.
  • Ex: cold syrup is more viscous than hot syrup (cold syrup does not flow easily)
  • Ex: basaltic magma has lower silica therefore has lower viscosity, so it flows more quickly than Rhyolitic magma which has higher viscosity
gases in magma
Gases in Magma
  • Most important: Water vapor, carbon dioxide, and sulfur
  • Also has: hydrogen (which can combine with oxygen to form steam)
  • Amount of gas determines type of eruption
    • Large amt of gases = more explosive eruptions
  • VISCOSITY – (Very Important Term!!!)
    • Definition: Resistance to Flow
      • Viscous Magma – like pudding – doesn’t flow well
        • Determined byHigh Silica Content - FELSIC
      • Non-Viscous Magma – like maple syrup – flows well
        • Determined byLow Silica Content – MAFIC
  • A little different than magma b/c some gas escapes, some new materials added
  • Also felsic and mafic
    • Mafic: gases escape quickly & lava comes out smoother
      • Pahoehoe – fluid, “ropy” lava
    • Felsic: gases can’t move easily, explosive eruption
      • Aa – blocky, chunky, lava that topples over itself
lava fragments
Lava Fragments
  • Tephra: solid fragments of lava produced by explosive eruptions
    • Smallest pieces: ash
    • Larger pieces: lapilli
    • Largest pieces: blocks (solid), bombs (liquid then harden)





volcanic hazards
Volcanic Hazards
  • Pyroclastic Flows – clouds of ash, dust, and gases (AKA – NueeArdente – “Glowing Cloud”)
    • Move at speeds of up to 200 km/h
      • Denser than surrounding air – so it stays close to ground
      • Poisonous
      • Superheated (5X the temperature of boiling water-~7000C)
  • Lahar – heated snow covered mountaintop – snow melts  super fast mudslide – more dangerous than nueeardente (more common)
  • Lava Flows – obviously dangerous due to heat – but can be diverted or chilled Ash – threat of suffocation
  • Poisonous gases – kill due to toxicity – but the most common and dangerous is CO2 (drives away oxygen)


types of volcanoes
Types of Volcanoes
  • Type of Volcano depends on:
    • Type of material that forms it and type of eruptions that occur
  • We will focus on (3) most common
    • Shield Volcano (Mauna Loa)
      • Mountain with broad, gently sloping sides
      • Generally non-explosive
    • Cinder-Cone Volcano (Izalco – El Salvador)
      • Material that is ejected high into the air – piles back up around the vent
      • Steep sides, but generally small in size
      • Water and silica laden magmas, and full of gas – somewhat explosive
    • Composite Volcano (Mount St. Helens)
      • Large amounts of water, silica, and gases
      • VERY LARGE – Due to their large size – pose the greatest threat
      • These can blow their tops completely off forming a CALDERA – Crater Lake Oregon

Just to put the size of the Mount Saint Helens eruption in perspective, this figure compares the size of that eruption (as measured by the volume of ejected material) to some bigger ones in the past. Notice how small Mt. St. Helens was compared to the others!!


living vs dead volcanoes
Living vs. Dead Volcanoes
  • Active Volcano: A volcano that is erupting. Also, a volcano that is not presently erupting, but that has erupted within historical time and is considered likely to do so in the future.
  • Dormant Volcano: Literally, "sleeping." The term is used to describe a volcano which is presently inactive but which may erupt again. Most of the major Cascade volcanoes are believed to be dormant rather than extinct.
  • Extinct Volcano: A volcano that is not presently erupting and is not likely to do so for a very long time in the future
volcano prediction
Volcano Prediction
  • Tectonic Activity
    • The volcanic mountain will “rumble” prior to an eruption
      • Magma is working its way up the vent or volcanic pipe, and pushes stuff around during its ascent
  • Topographic “Bulges”
    • The magma can push up/out on the land as it rises
  • Gas emissions – volcanic “burps” of sulfur, water vapor, and carbon dioxide
  • Temperature Spike – the temperature around and inside the volcano will rise
  • ~Questionable~: Animals will bug out of the area –