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The Good Earth/Chapter 6: Volcanoes and Other Mountains
As a scientifically literate citizen, what 3 questions would you ask about this volcano if you moved to the city in the foreground (Tacoma, Washington)?
Volcanoes and Other Mountains
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The Volcano Commandos
• 1,500 active volcanoes worldwide
− a third have records of previous eruptions
− 2 or 3 eruptions per decade are major disasters
− 500 million people live near active volcanoes
− Fewer than 200 volcanoes have instruments to assess potential for eruption
The Good Earth/Chapter 6: Volcanoes and Other Mountains
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What exactly is a Volcano?
Location where molten rock and
other mantle materials, are
released, to the surface.
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Parts of the Volcano
• Magma:– Molten rock – Less dense than crust
• Lava:– Magma that has
reached the surface and erupted out of the volcano
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Crater
Lava
Central vent
Pipes
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Parts of the Volcano
• Magma Chamber– Underground pools of magma
• Pipes– Pathways for magma
• Vents– Area where magma reaches the surface
• Crater– Depression at the top of a volcanic form after an
eruption
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2 Types of volcanoes
1. ShieldMade from fast flowing basaltic lava
(Basaltic: A hard, dense, dark volcanic rock)
Large and flat due to movement of the lava
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Hawaiian Islands formed this way
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http://www.history.com/shows/how-the-earth-was-made/videos/how-was-hawaii-formed -
how-was-hawaii-formed
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Volcano types
2. Strato-volcanoesmajority of the worlds volcanoes are these most explosive and destructive
Associated with subduction zones
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Where do they occur?
• All boundaries except transform
Famous Volcanoes / areas of volcanoes
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The Mount St. Helens Eruption
The Good Earth/Chapter 6: Volcanoes and Other Mountains
• Cascade Mountains – volcanic arc in Pacific Northwest− Major cities within 100 km of
active volcanoes− Mount St. Helens eruption of
May 18, 1980
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The Mount St. Helens Eruption
The Good Earth/Chapter 6: Volcanoes and Other Mountains
• Cascade Mountains − Volcanoes formed above
subduction zone where Juan de Fuca plate slides beneath North America
− Mount St. Helens is most active volcano in conterminous US
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The Mount St. Helens Eruption
The Good Earth/Chapter 6: Volcanoes and Other Mountains
Prior Activity
• Early (March) unrest featured− Minor eruptions
− Earthquakes
− Release of volcanic gases
• Followed by change in shape of cone (bulge on North flank)
• Increasing frequency of earthquakes
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The Mount St. Helens Eruption
The Good Earth/Chapter 6: Volcanoes and Other Mountains
May 18 Eruption
• A moderate earthquake triggered a massive landslide (debris avalanche) on the North side of the volcano
− Debris clogged streams
− Pressure released on near-surface magma
− Lateral blast produces an initial sideways eruption to North
− Later vertical eruption
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The Good Earth/Chapter 6: Volcanoes and Other Mountains
• Eruption of Mount St. Helens reduced height of volcano by 400 meters
• Features near volcano were blown over or carried away by products of eruption
Geologist David Johnston (right) died at this site (Johnston’s Ridge) located 10 km from the volcano. (this is 6.25 miles or from here to the pro-football hall of fame.
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Products of Volcanic Eruptions
The Good Earth/Chapter 6: Volcanoes and Other Mountains
Major products of volcanic eruptions:• Airborne – lateral blast, tephra, volcanic gases• Flows on land – lava, pyroclastic flows, lahars
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Products of Volcanic Eruptions
The Good Earth/Chapter 6: Volcanoes and Other Mountains
• Rare lateral blasts can destroy objects up to 12 km away and knock down trees more than 25 km distant
− Effect of lateral blast only seen on North flank of Mount St. Helens
Airborne Eruption Products
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Products of Volcanic Eruptions
The Good Earth/Chapter 6: Volcanoes and Other Mountains
• Tephra represents particles blasted into air by eruption
− Volcanic bombs and ash are found near and far from eruption source, respectively
Airborne Eruption Products
Blobs of magma solidify to form lava bombs
Wind can transport fine volcanic ash for hundreds of kilometers downwind
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Products of Volcanic Eruptions
The Good Earth/Chapter 6: Volcanoes and Other Mountains
• Volcanic gases (water vapor, sulfur dioxide, carbon dioxide) may affect climate patterns
− Sulfur dioxide may block insolation, temporarily (up to 1 year) reducing global temperatures
− Widespread release of carbon dioxide and higher temperatures due to faster rates of volcanic activity approximately 120-80 million years ago
Airborne Eruption Products
Trees killed by excessive carbon dioxide released by magma under Mammoth Mountain, California.
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Products of Volcanic Eruptions
The Good Earth/Chapter 6: Volcanoes and Other Mountains
Eruption Products on Land
• Low viscosity lava can flow up to 50 km from its source
− Lava transported to front of lava flows in long lava tubes
− Lava flows build up in a series of layers
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Products of Volcanic Eruptions
The Good Earth/Chapter 6: Volcanoes and Other Mountains
Eruption Products on Land
• Low viscosity lava can flow up to 50 km from its source
− Lava transported to front of lava flows in long lava tubes
− Lava flows build up in a series of layers
Walter’s Kalapana store, Hawaii, was buried in lava within a few weeks in 1990
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Products of Volcanic Eruptions
The Good Earth/Chapter 6: Volcanoes and Other Mountains
Eruption Products on Land
• Higher viscosity lava remains within volcano crater
− Lava dome formed in crater of Mount St. Helens
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Products of Volcanic Eruptions
The Good Earth/Chapter 6: Volcanoes and Other Mountains
Eruption Products on Land
• Pyroclastic flow – dense cloud formed from combination of tephra and volcanic gases
− Fast moving, up to 700 C
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Products of Volcanic Eruptions
The Good Earth/Chapter 6: Volcanoes and Other Mountains
Eruption Products on Land
• Lahars – mudflows formed when volcanic debris mixes with streams or melting ice
− Often confined to stream channels
Lahar along Muddy River reached depths of 20 meters following Mount St. Helens eruption
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Ring of fire area surrounding the pacific
plate with VERY high activity of Volcanoes
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