Plate Tectonics and Volcanoes

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Plate Tectonics and Volcanoes

• Earth Science
• Chapter 14

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Volcanoes

• Volcanoes
• Hills or mountains made from hardened magma

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Volcanoes

• 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

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Volcanoes

• Magma/lava is made of
• Silica
• Water vapor
• Magnesium
• Iron

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Volcanoes

• 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

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Volcanoes

• 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 earths
  surface
• High amount of gases (bubbles stuck in thick
  liquid)

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Volcanoes

• 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)

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Mafic Lava
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Volcanoes

• 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

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Volcanoes

• 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 (

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Volcanoes

• 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

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Volcanoes

• 3 Types of Volcanoes
• Shield cone
• Broad or wide
• Mafic lava
• Mellow eruptions
• Hawaii (caused by a hot spot)

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Shield Cone
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Volcanoes

• Cinder cone
• Narrow
• Tall (not as tall as composites)
• Felsic lava
• Pyroclastic eruptions
• Lots of tephra and gases, not much lava
• Paricutin, Mexico

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Cinder Cone
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Volcanoes

• Composite Cone
• Alternating layer of lava and cinders (ash)
• Alternates pyroclastic and non-pyroclastic
  eruptions
• Tall, snow-capped peaks
• Mount St. Helens

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Composite Cone
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Volcanoes and Boundaries

• Subduction Boundaries
• Pyroclastic eruptions
• Felsic lava
• Slabs are pushed down into the mantle
• Pressure and temp rise - gets rid of water as
  steam
• Hot Fluids melt mantle rock magma migrates
  upward.
• Cinder and composite cones have pyroclastic
  eruptions

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Ring of Fire
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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 its made of)

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Volcanoes and Boundaries

• Subduction Volcanoes

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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)

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Volcanoes and Boundaries

• Big Idea
• Plate motions provide the mechanism by which
  mantle rocks melt to generate magma.

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Volcanic Structures

• There is much more magma under the surface of the
  Earth than we see with volcanoes.
• That magma forms other volcanic structures
  (plutons) that we can identify.
• It solidifies in the Earths crust, hardening in
  other rocks.

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Volcanic 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

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Volcanic Structures

• Stock
• Hardened magma exposed at the surface due to
  erosion
• Small batholith

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Volcanic 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

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Volcanic Structures

• Sill
• Magma goes into rock layers horizontally and
  hardens

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Volcanic 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

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Volcanic Structures

• Volcanic Neck
• Plug of hardened magma left in the vent
• Cone is completely eroded