Chapter 9: Volcanoes

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Chapter 9 Volcanoes

• 9.1 How and Where Volcanoes Form
• 9.2 Magma and Erupted Materials
• 9.3 Volcanic Landforms
• 9.4 Extraterrestrial Volcanism

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9.1 How and Where Volcanoes Form

• Volcano-Refers to both the opening in Earth's
  crust through which molten rock, gases and ash
  erupt and to the landform that develops around
  this opening.

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9.1 How and Where Volcanoes Form

• Magma Formation
• 1. Three main conditions for magma to form
• A decrease in pressure can lower melting
  temperatures.
• An increase in temperature can cause materials in
  the Asthenosphere to melt.
• An increase in the amount of water can lower
  melting temperatures.

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9.1 How and Where Volcanoes Form

• 2. Most volcanoes are found along mid-ocean
  ridges and along Subduction boundaries.
• 3. Most magma is less dense than the
  surrounding rocks and therefore rises to the
  surface.

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9.1 How and Where Volcanoes Form

• At Subduction Boundaries
• Oceanic-Oceanic and Oceanic-Continental. See
  diagram on page 195.
• Create Volcanoes.
• At Divergent Boundaries
• Plates split apart and magma oozes up, creating
  ridges.
• Mostly occurs in the middle of oceans.

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9.1 How and Where Volcanoes Form

• D. Over Hot Spots
• Hot spot- Areas of volcanic activity that result
  from plumes of hot solid material that have risen
  from deep with the Earth's mantle.
• Hot spots stay in the same place as the plates
  continue to move. See page 197.
• Volcanic islands forming over a hot spot.

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9-2 Magma and Erupted Materials

• Types of Magma
• Viscosity-resistance to flow. Determined by
  silica content. Higher the silica, slower the
  flow.
• Gases can create explosive volcanoes.
• See summary on page 199.
• Lava Flows
• Lava-Magma that reaches the surface
• Basaltic lava flows are usually slow and
  non-explosive. There are two main types
  Pahoehoe (ropey) and AA (sharp).
• Pillow Lava- rounded lumps of lava formed
  underwater.

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9-2 Magma and Erupted Materials

• Ash and Rock Fragments
• Pyroclastic Material- When gases are released
  solid fragments of this material are ejected.
• These materials are classified by size. Ash is
  less than 2 mm. Lapilli ranges in size from 2-64
  mm and blocks or bombs are greater than 64 mm.
• Pyroclastic Flow- A dense superheated cloud that
  travels downhill with amazing speed (100 kph).

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9.3 Volcanic Landforms

• Shield Volcanoes
• Low viscosity lava that flows for long distances.
• Lava builds up in layers and forms broad bases
  and gentle slopes.

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9.3 Volcanic Landforms

• B. Cinder Cones
• The simplest of the volcanoes occurs when lava is
  thrown from the volcano.
• Lava hardens before it hits the ground and
  accumulates forming a cone shaped mound.

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9.3 Volcanic Landforms

• Composite Cone
• Develops when layers of material from several
  volcanic events accumulate around a vent.
• The materials include both hardened lava and
  pyroclastic material.
• Lahar-A fast moving mudflow created by a volcanic
  eruption.

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9.3 Volcanic Landforms

• D. Caldera
• When magma beneath a volcano is released after
  the top of the volcano collapses, forming a large
  crater shaped basin.
• Examples are Crater Lake in Oregon and
  Yellowstone NP. See page 204.
• E. Lava Plateaus
• Basaltic lava pouring from fissures and spreading
  across the land.
• Volcano
• Volcanic Landforms

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9.4 Extraterrestrial Volcanism

• The Moon
• Dark areas on the moon are basaltic lava flows.
• Volcanism began about 3-4 billion years ago.
• Mars
• Home to Olympus Mons, the largest volcano in the
  solar system.
• It measures 600 km across and 27 km above the
  surrounding terrain

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9.4 Extraterrestrial Volcanism

• Venus
• Has 1600 large volcanoes
• Some volcanoes may still be active

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9.4 Extraterrestrial Volcanism

• D. Io (Jupiters 3rd moon)
• One of the most volcanically active areas in the
  solar system.
• Io is caught in a tug of war, which creates
  friction and then heat. That heat drives the
  volcanic activity.