volcanoes volcanoes & plate tectonics 6.e.2.2 explain how heat flow and volcanoes reflect forces...
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Volcanoes
Volcanoes & Plate Tectonics
6.E.2.2 Explain how heat flow and volcanoes reflect forces within the earth.
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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 is a molten mixture of rock-forming substances, gases, and water from the mantle.
• When magma reaches the surface, it is called lava.
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Location of Volcanoes
• There are about 600 active volcanoes on land.
• 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
Volcanic belts form along the boundaries of Earth’s plates.
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Volcanoes often form where two or oceanic plates collide or where an oceanic plate collides with a continental plate. In both situations, an oceanic plate sinks beneath a trench. Rock above the plate melts to form magma, which then erupts to the surface as lava.
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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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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 chambermagma chamber
• A pipepipe is a long tube in the ground that connects the magma chamber to the earth’s surface
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• Molten rock and gas leave the volcano through an opening called a ventvent
• A lava flowlava flow is the area covered by lava as it pours out of a vent
• A cratercrater is a bowl-shaped area that may form at the top of a volcano around the central vent
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A Volcanic Eruption
• When a volcano erupts, the force of the expanding gases pushes magma from the magma chamber through the pipe until it flows or explodes out of the vent.
• Once magma escapes from the volcano and becomes lava, the remaining gases bubble out.
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Types of Volcanic Eruptions
• Geologists classify volcanic eruptions as quiet or explosive.
• 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 Earth’s crust and mantle
• The more silica that magma contains, the thicker it is
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Quiet Eruptions:
• A volcano erupts quietly if its magma is low in silica.
• Magma flows easily; the gas dissolved in the magma bubbles out gently
• Examples:
– Hawaii– Iceland
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Quiet eruptions produce two types of lava:
1. Pahoehoe1. Pahoehoe - fast moving, hot lava;
Surface looks like a solid mass of wrinkles, billows, and rope-like coils.
2. Aa2. Aa - cooler, slower-moving lava; when hardens, forms a rough surface consisting of jagged lava chunks.
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Explosive Eruptions:
• A volcano erupts explosively if its magma is high in silica.
• 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 flowpyroclastic flow occurs when an explosive eruption hurls out ash, cinder, bombs, and gasses
• Pebble-sized particles are called cinders.
• Larger pieces, called bombs, may range from the size of a baseball to the size of a car.
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Stages of a Volcano
• ActiveActive - is erupting, or has shown signs that it may erupt in the near future
• DormantDormant - does not show signs of erupting in the neat future
• ExtinctExtinct - unlikely to erupt
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Related Volcanic Activities
• Hot SpringHot Spring - groundwater heated by a nearby body of magma rises to the surface and collects in a natural pool
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• GeyserGeyser - 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 VolcanoShield Volcano: nonexplosive eruptions, fluid basaltic lava, gentle broad slopes
– Thin layers of lava pour out of a vent and harden on top of previous layers
– Example: Hawaiian Islands
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Shield VolcanoShield Volcano
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• Cinder Cone VolcanoCinder Cone Volcano: explosive eruptions, small but steep slopes, pyroclastics
– Form when cinders from a vent, pile up around the vent, forming a steep- cone-shaped mountain
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Cinder Cone VolcanoCinder Cone Volcano
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• Composite VolcanoComposite Volcano: alternating between lava and pyroclastics, explosive and nonexplosive eruptions, steep and tall
– Layers of lava alternate with layers of ash, cinders, and bombs
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Composite VolcanoComposite Volcano
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Related Volcanic Landforms
• CalderaCaldera:
– A large hole at the top of a volcano formed when the roof of a volcano’s magma chamber collapses
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Formation of a Caldera
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• DikeDike:
– A slab of volcanic rock formed when magma forces itself across rock layers
– (vertical rock formation)
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DikeDike
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• SillSill:
• A slab of volcanic rock formed when magma squeezes between layers of rock
• (horizontal rock formation)
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SillSill
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• BatholithBatholith:
• A mass of rock formed when a large body of magma cooled inside the crust
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BatholithBatholith
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• Dome MountainsDome 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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Dome MountainsDome Mountains
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Questions
When magma reaches the surface, it is called?
A. Larva
B. Lava
C. Lagoon
D. Lump
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What is a magma pocket called?
A. Magma chamber
B. Hot pocket
C. Hot chamber
D. None of the above
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What type of lava is fast moving?
A. Aa
B. Naa
C. Pahoehoe
D. HoHoHo
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The End