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Volcanoes. Chapter 7. Volcano – weak spot in the crust where a mountain forms when layers of lava and volcanic ash erupt and build up and push through

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Volcanoes

Chapter 7


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Volcano – weak spot in the crust where a mountain forms when layers of lava and volcanic ash erupt and build up and push through

Magma - less dense than the rock around it so it is very slowly forced upward toward Earth’s surface made up of gases, water molten rock forming stuff

Lava – magma that reaches the earths surface

Volcanoes and Plate Tectonics


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Where Volcanoes Occur?

  • Volcanic Belts form along boundaries of Earth’s crust, one major is the Pacific Ring of Fire

  • 1. Divergent Boundary – plates moving apart – seafloor spreading

    • Example: N. Atlantic Ocean. The North American plate is moving away from Eurasia and African plates.

    • Iceland formed by the separation of these plates at the rift

  • 2. Convergent Boundary – 2 plates collide, rumple up and cause mountains instead of volcano

    • Juan de Fuca Plate is converging with the N. American plate


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  • Convergent Plates (cont)

    • Magma that is forced upward forms volcanoes of the Cascade Mountains

    • Pacific Ring of Fire – area around the Pacific Plate where there are earthquakes and volcano’s

  • Hot Spots – middle of a plate, area’s in Earth’s mantle that are hotter than neighboring areas

  • Magma forms and rises towards the crust

  • Example: Hawaiian islands, plates move and form islands when it gets over the hot spot


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Types of Eruptions – depend on the pressure build up and material

  • 2 factors:

  • 1. Water Vapor and other gases trapped within the magma,

    • if they escape easily – quiet eruptions

    • Gas trapped under high pressure eventually escapes – explosive eruptions

  • 2. Magma is either basaltic or granitic

    • Basaltic magma – contains less silica, is very fluid and produces quiet, non-explosive eruptions magma runs down the side like in Hawaii


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  • Granitic magma – thick and contains lots of silica and has a high water content, it gets trapped in vents, causing pressure to build up so when it erupts the gases expand rapidly carrying pieces of lava


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3 Forms of Volcanoes

  • 1. Shield Volcano – Quiet eruptions that spread out basaltic lava in broad, flat layers Hawaiian Islands (pg 399)

  • 2. Cinder Cone Volcano – Explosive eruptions that throw lava into the air forms steep sided consolidated volcano ( pg 400) Paricutin

    • Tephra – various sizes of this lava after it has cooled

      • Ash – fine

      • Bombs – large heavy rocks


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  • 3. Composite Volcano – formed at convergent plate boundaries above subduction zone, this type of volcano alternates between quiet and explosive tephra layer alternating with a lava layer Mt. St. Helens (pg 400)

  • Subduction Zone – area where an ocean plate descends into the upper mantle


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Volcano Features

  • Magma that cools underground forms igneus rock (most magma does not reach the surface)

  • Balholith – largest igneous rock bodies, hundreds of km wide and thick Yosemete has exposed one

  • Dike – magma squeezed into a vertical crack and hardened

  • Sill – magma squeezed into a horizontal crack

  • Laccolith – dome of rock

  • Volcanic Neck – cone of the volcano erodes and leaves the igneous rock Ship Rock New Mexico

  • Caldera – formed after eruption when the top collapses down into magma chamber Crater Lake


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Vent – opening in the surface which magma reaches the earths surface through (1000 yrs)

Crater – opening at the top of a volcano’s vent


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Geothermal Energy from Volcanoes

  • Geothermal energy – utilizing the heat from magma and converting it to electricity by heating water which produces steam that turns the turbines that run the generators to make electricity (fig 15-5)

  • Geothermal energy used in 20 countries and US

  • Hawaii wants to heat by geothermal energy of Kilauea

  • Many people are against this because some of the rain forest must be cleared for roads, drilling etc, they say it will cut down cultural trees


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