Volcanoes

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Volcanoes

• Volcanoes Plate Tectonics

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What is a Volcano?

• A volcano is a weak spot in the crust where
  molten material, or magma, comes to the surface
• Magma reaching the surface is called lava

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Location of Volcanoes

• 600 active volcanoes worldwide
• Many more are found beneath oceans
• Most occur in belts that extend across continents
  and oceans

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• A major belt is the Ring of Fire which
  encircles the Pacific Ocean

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• Most volcanoes occur along diverging plate
  boundaries such as the mid-ocean ridge, or in
  subduction zones, around edges of oceans

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• Hot Spot volcanoes form where magma from deep
  within the mantle melts through the crust like a
  blow torch
• Examples
• Hawaiian Islands
• Yellowstone National Park

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Volcanoes

• Volcanic Activity

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Inside a Volcano

• All volcanoes have a pocket of magma beneath the
  surface and one or more cracks through which
  magma forces its way through
• Magma pocket is called a magma chamber

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• A pipe is a long tube in the ground that connects
  the magma chamber to the earths surface
• Molten rock and gas leave the volcano through an
  opening called a vent

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• A lava flow is the area covered by lava as it
  pours out of a vent
• A crater is a bowl-shaped area that may form at
  the top of a volcano around the central vent

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Types of Volcanic Eruptions

• The silica content of magma helps to determine
  whether the volcanic eruption is quiet or
  explosive
• Silica is the material formed from the elements
  silicon and oxygen

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• Silica is one of the most abundant materials in
  Earths crust and mantle
• The more silica that magma contains, the thicker
  it is

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Quiet Eruptions

• Magma flows easily the gas dissolved in the
  magma bubbles out gently
• Examples
• Hawaii
• Iceland

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• Produce two types of lava
• Pahoehoe - fast moving, hot lava
• Surface looks like a solid mass of wrinkles,
  billows, and rope-like coils

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• Aa - cooler, slower-moving lava when hardens,
  forms a rough surface consisting of jagged lava
  chunks

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Explosive Eruptions

• Magma is thick and sticky
• Magma slowly builds up in the volcanoes pipe
• Dissolved gases cannot escape
• Trapped gasses build up pressure until they
  explode

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• A pyroclastic flow occurs when an explosive
  eruption hurls out ash, cinder, bombs, and gasses

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Stages of a Volcano

• Active - is erupting, or has shown signs that it
  may erupt in the near future
• Dormant - does not show signs of erupting in the
  neat future
• Extinct - unlikely to erupt

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Related Volcanic Activities

• Hot Spring - groundwater heated by a nearby body
  of magma rises to the surface and collects in a
  natural pool

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• Geyser - forms when rising hot water and steam
  become trapped underground in a narrow crack
• Pressure builds until the mixture suddenly sprays
  upward, clearing the crack

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Volcanoes

• Volcanic Landforms

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Types of Volcanoes

• Shield Volcano
• Thin layers of lava pour out of a vent and harden
  on top of previous layers
• Example Hawaiian Islands

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• Cinder Cone Volcano
• Form when cinders from a vent, pile up around the
  vent, forming a steep- cone-shaped mountain

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• Composite Volcano
• Layers of lava alternate with layers of ash,
  cinders, and bombs

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Related Volcanic Landforms

• Caldera
• A large hole at the top of a volcano formed when
  the roof of a volcanos magma chamber collapses

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Formation of a Caldera
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• Dike
• A slab of volcanic rock formed when magma forces
  itself across rock layers
• (vertical rock formation)

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• Sill
• A slab of volcanic rock formed when magma
  squeezes between layers of rock
• (horizontal rock formation)

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• Batholith
• A mass of rock formed when a large body of magma
  cooled inside the crust

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• Dome Mountains
• Rising magma within the crust is blocked by
  layers of rock
• Magma forces layers of rock to bend upward into a
  dome shape
• Example Black Hills

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