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Plate Tectonics and Volcanoes. Earth Science Chapter 18. Volcanoes. Volcanoes Hills or mountains made from hardened magma. Volcanoes. Magma Molten rock that is below the surface of the earth Located in the mantle Forms when rocks melt due to subduction Lava

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plate tectonics and volcanoes

Plate Tectonics and Volcanoes

Earth Science

Chapter 18

  • Volcanoes
    • Hills or mountains made from hardened magma
  • Magma
    • Molten rock that is below the surface of the earth
    • Located in the mantle
    • Forms when rocks melt due to subduction
  • Lava
    • Molten rock that has reached the surface of the earth
    • Forms when volcanoes erupt
  • Magma/lava is made of:
    • Silica
    • Water vapor
    • Magnesium
    • Iron
  • 3 Types of magma/lava
    • Felsic (Rhyolitic)
      • Made of 60% or more silica
    • Intermediate (Andesitic)
      • Made of 50-60% silica
    • Mafic (Basaltic)
      • Made of 50% or less silica
  • Properties of Felsic (Rhyolitic) magma
    • Light in color
    • High silica (SiO2) content
    • Contains a lot of water vapor
    • Does not flow easily
    • Highly viscous (thick)
    • Often solidifies before reaching the earth’s surface
    • High amount of gases (bubbles stuck in thick liquid)
  • Properties of Mafic (Basaltic) magma
    • Dark in color
    • High in iron
    • Low silica content
    • Flows easily
    • Does not contain a lot of water vapor
    • Low amount of gases (bubbles escape easily)
  • 2 Manners of Eruptions

1. Pyroclastic

      • Highly explosive
      • Felsic lava (trapped gases build up pressure)
      • Lots of poisonous gases and tephra, little magma erupted
      • Pyroclastic flow – hot stream of gases and tephra that flows down the volcano
  • Types of Tephra:
    • Bombs
      • Extremely large (> 64 mm) chunks erupted from volcano
      • Ejected as liquid, harden as they fall
    • Blocks
      • Erupted as solid pieces (> 64 mm)
    • Lapilli
      • Smaller than bombs ( up to 64 mm)
    • Ash
      • Fine grained/very small chunks (< 2 mm)

2. Non-Pyroclastic

  • Non-explosive
  • Mafic lava (very little trapped gases)
  • Lava erupts more continuously, but flows out quietly
  • Mid-Atlantic Ridge is best example
  • 3 Types of Volcanoes
    • Shield cone
      • Broad or wide
      • Mafic lava
      • Mellow eruptions
      • Hawaii (caused by a hot spot)
  • Cinder cone
    • Narrow
    • Tall (not as tall as composites)
    • Felsic lava
    • Pyroclastic eruptions
      • Lots of tephra and gases, not much lava
    • Paricutin, Mexico
  • Composite Cone
    • Alternating layer of lava and cinders (ash)
    • Alternates pyroclastic and non-pyroclastic eruptions
    • Tall, snow-capped peaks
    • Mount St. Helen’s
volcanoes and boundaries
Volcanoes and Boundaries
  • Subduction Boundaries
    • Pyroclastic eruptions
      • Felsic lava
      • Slabs are pushed down into the mantle
      • Pressure and temp rise – water turns to steam
      • Hot fluids melt mantle rock & magma migrates upward.
      • Cinder and composite cones have pyroclastic eruptions
volcanoes and boundaries1
Volcanoes and Boundaries
  • Subduction Volcanoes
    • Ocean/Ocean
      • Chain of volcanoes called an island arc
    • Ocean/Continent
      • Volcanic arc
      • Continental crust has higher silica content
      • Magma varies in composition (what it’s made of)
volcanoes and boundaries2
Volcanoes and Boundaries
  • Subduction Volcanoes
volcanoes and boundaries3
Volcanoes and Boundaries
  • Divergent Boundary
    • Non-pyroclastic
      • Mafic lava
      • Magma produced during sea floor spreading
      • Magma rises to fill in rift where plates have separated
      • Sometimes called basaltic (most of the sea floor is made of basalt)
volcanoes and boundaries4
Volcanoes and Boundaries
  • Big Idea:
    • Plate motions provide the mechanism by which mantle rocks melt to generate magma.
plutonic structures
Plutonic Structures
  • There is much more magma under the surface of the Earth than we see with volcanoes.
  • That magma forms other Igneous structures (plutons) that we can identify.
  • It solidifies in the Earth’s crust, hardening in other rocks.
plutonic structures1
Plutonic Structures
  • Types:
  • Batholiths
    • Hardened magma that forms the cores of many mountain ranges
    • Largest type of intrusion
    • Sometimes exposed at the surface due to erosion
plutonic structures2
Plutonic Structures
  • Stock
    • Hardened magma exposed at the surface due to erosion
    • Small batholith
plutonic structures3
Plutonic Structures
  • Dike
    • Magma cools inside a fracture
      • A fracture is a break in the crust with no movement
    • Cuts vertically across the layers of rock
plutonic structures4
Plutonic Structures
  • Sill
    • Magma goes into rock layers horizontally and hardens
plutonic structures5
Plutonic Structures
  • Laccolith
    • Goes into layers horizontally, but the stiff magma is unable to spread to form a sill
    • Instead it pushes the land up to form a dome
plutonic structures6
Plutonic Structures
  • Volcanic Neck
    • Plug of hardened magma left in the vent
    • Cone is completely eroded