Volcanoes

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Chapter 3

• Volcanoes

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What is a volcano?

• A volcano is a weak spot in the crust where magma
  comes to the surface.
• Most volcanoes occur at plate boundaries. (such
  as mid-ocean ridge or in subduction zones and
  edges of oceans)

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What is lava?

• When magma reaches the surface, it is called
  LAVA.
• Volcanoes can be found at
• 1) Diverging Plate Boundaries
• 2) Mid-ocean ridge
• 3) Subduction Zones

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Converging Plate Boundaries

• What happens when two oceanic plates collide?
• It forms an ISLAND ARC.

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Island Arc Formation
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Hot Spot

• An area where magma from deep within the mantle
  melts through the crust like a blow torch.

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How did the Hawaiian Islands Form?

• A hotspot volcano in the ocean floor gradually
  formed a series of volcanic mountains.

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Volcano Anatomy (use page 95 in textbook)
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Factors that Determine the Force of a Volcanic
Eruption

• Amount of gas dissolved in the magma
• How thick or thin the magma is
• Temperature
• Silica content

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Silica

• The more silica the magma has the thicker it will
  be.

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Volcanic Rock

• Rhyolite has the same composition as granite
  (formed from High Silica Lava)
• Obsidian Forms when high-silica lava cools very
  quickly
• Pumice Forms when gas bubbles are trapped in
  cooling lava (High Silica as well)
• Basalt Forms from low-silica lava

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Image of Rocks
Obsidian
Pumice
Rhyolite
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Different Types of Lava (Low Silica)

• Hot fast-moving lava is called pahoehoe. The
  surface of this lava looks like wrinkles or
  coils.
• Relatively cooler, slow-moving lava is called aa.
  When it cools it forms a rough surface consisting
  of jagged lava chunks.

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Pahoehoe
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aa
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Different Sizes of Lava

• Volcanic Ash Fine rocky particles as small as a
  grain of sand
• Cinders Pebble-sized particles
• Bombs Particles ranging from the size of a
  baseball to the size of a car

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Pyroclastic Flow

• A pyroclastic flow is an explosive eruption of
  ash, cinders, bombs and gases.

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Calderas

• A caldera collapse is usually triggered by the
  emptying of the magma chamber beneath the
  volcano, as the result of a large volcanic
  eruption. If enough magma is erupted, the emptied
  chamber will not be able to support the weight of
  the volcanic edifice (the mountain) above.

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Volcanic Neck
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Dikes
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Caldera Image
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Batholith and Dome Mountains

• Batholith is a mass of rock formed when a large
  body of magma cools inside the crust.
• Dome mountain, a dome mountain forms when rising
  magma is blocked by horizontal layers of rock.
  Eventually the rock above the dome wears away
  leaving it exposed.