Gentle Giant or Fire Breathing Dragon

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Gentle Giant or Fire Breathing Dragon
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Volcanoes

• Do all liquids flow at the same rate? Why or Why
  Not?
• Do you think all Volcanic eruptions are the same?
  Why or Why Not?

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VISCOSITY

• Viscosity is an internal property of a fluid that
  offers resistance to flow

High viscosity fluids move slowly (molasses) Low
viscosity fluids move rapidly (water)
Higher temperature fluids have lower viscosity
than lower temperature fluids
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What IS A VOLCANO?

• Volcanoes are the landforms created when magma
  (molten rock) escapes in an opening (Vents) and
  include any deposits immediately surrounding the
  vent.
• Volcanism is the geologic process that results in
  the expulsion of magma at the Earths surface
• Volcanic type and location is related to Plate
  boundaries (tectonic activity)

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What Causes Volcanism?

• Magma Eruptions are dependent upon
• Amount of Pressure
• Viscosity of the Magma

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• What determines the Viscosity of the magma?

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• Viscosity of magma is dependent upon
• Silica Content (SiO2)
• Temperature

• High silica magmas (rhyolite, granite) are very
  viscous and erupt explosively
• Low silica magmas (basalt) have low viscosity
  and erupt effusively
• More Silica More Viscous
• Higher temperature lavas are less viscous than
  lower temperature

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Silica Content of Magma
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How does the silica content affect the viscosity
of the magma?

• Silica through the processes of re-melting
  igneous rock form groupings of tetrahedra
  (polymerized, strongly bonded Si-O) that are
  irregular rather than regular patterns formed in
  crystallized minerals), even in a molten state,
  the greater silica content, the more groupings of
  tetrahedra. This increases the magmas
  resistance to flow!

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Types of Magma
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Volcanic Explosiveness

• VISCOSITY PRESSURE
• Do higher or lower viscous magma allow gas
  trapped under pressure to be released more or
  less easily?
• How does viscosity affect the release of
  volatiles trapped under pressure?

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Low viscous magma is usually associated with gas
that bubbles out gently, not very explosive.
Whereas higher viscosity or thick pasty magma
holds the gas in until the pressure becomes high
enough to break free and when it breaks free, it
often does explosively
The more gas there is to escape, the higher the
viscosity, the more likely it is to be explosive
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Shield vs. CompositeVolcanoes

• Composite
• Explosive
• High Silica
• High Viscosity
• Andesitic Granite magmas

• Shield
• Effusive
• Low Silica
• Low viscosity
• Basaltic magma

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SHIELD VOLCANOES

• Largest Volcanoes on Earth
• Form the Hawaiian Islands
• Composed mostly of Basalt
• High in Iron and Magnesium, low in silica
• Lava is thin, relatively fluid
• Lava flows down the sides of the volcano building
  up over time

Do Shield Volcanoes have high or low viscous Lava
flows?
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CompositeA volcano built of alternating layers
of cinders and lava.

• Eruptions of Andesite (60 silica), more viscous
  than basalt
• 50 lava and 50 pyroclastic materials
• Explosive because gas pressure builds up due to
  viscous lavas

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What type of volcano is this?
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Mount St. Helens a composite volcano.

• Over the last 10,000 years, Mt. Shasta has
  erupted on average once every 800 years. During
  the 3,500 years the volcano has erupted about
  once every 300 years. The most recent eruption
  may have occurred in 1786 A.D.

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What type of volcano is this?
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Kohala is a shield volcano.
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SoWhat Factors Control Volcano Explosiveness?
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Silica Content determines the viscosity
Viscosity of Magma
Lower or Higher Temperatures affect
viscosity Viscosity affects the ability for
volatiles to escape