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Warm Up
Why was the Mt. St. Helens Eruption so destructive? What about the explosion was not predicted by most scientists?
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VOLCANOES
Earth Science
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What is a Volcano? Volcano – A
structure which moves magma onto the surface of the Earth
Magma – liquid rock formed from heat inside the Earth
Lava – Magma that erupts onto the Earth’s surface
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Critical Thinking #1
Considering the prior definitions, explain how a volcano first forms.
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How Volcanoes Form 1. Plate
Boundaries – Subduction and Seafloor Spreading
2. Hot Spots – Extremely hot intraplate regions where mantle rises
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Where are Volcanoes Located?
Most located along the “Ring of Fire” - zone of volcanic activity along edge of Pacific Ocean
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Critical Thinking #2
Why would oceanic-continental convergent boundaries produce a great number of volcanoes?
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Types of Lava
Mafic Lava – forms oceanic crust, dark colored, generally runny
Felsic Lava – forms continental crust, light colored, generally thick
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Two Forms of Mafic Lava
Pahoehoe – means “ropy”, formed when mafic lava cools slowly
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Two Forms of Mafic Lava
AA – jagged chunks, formed when mafic lava cools rapidly
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Critical Thinking #3
What factors would determine the speed at which mafic lava hardens?
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Pyroclastics Pyroclastics – Rock fragments made
of hardened felsic lava, types based on size– Volcanic Dust – Less than 0.25mm in
diameter– Volcanic Ash – 0.25-2mm in diameter
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Pyroclastics
– Lapilli – 2-64mm in diameter– Volcanic Bombs - >64mm in diameter
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Critical Thinking #4
What factors would determine the size of pyroclasts produced?
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Viscosity and Gas Content
Viscosity – Ability to resist flow– Determined by amount of Silica– Low Viscosity = Thin lava– Medium Viscosity = Medium Thickness– High Viscosity = Thick lava
Gas Content – Determines Explosiveness– High Gas Content = Most Explosive– Low Gas Content = Least Explosive
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Critical Thinking #5
What is the average viscosity of mafic and felsic lava? What does this tell you about the silica content of mafic and felsic lava?
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Critical Thinking #6
What is the relationship between gas content and viscosity?
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Three Types of Volcanoes
1. Shield Volcano
2. Cinder Cone 3. Composite
Volcano (Stratovolcano)
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Shield Volcano
Broad, gently slopped (900)m High)
Eruption of fluid low viscosity mafic lava through multiple vents
Ex: Kilauea (Hawaii)
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Cinder Cone
Steep cone-shaped volcano (500m high)
Medium-viscosity Lava fragments (Cinders) ejected from single vent
Ex: Paricutin (Mexico)
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Composite Volcano
Large, often snow-capped peaks (3000m high)
Most explosive eruptions, high viscosity lava and pyroclasts
Ex: Mt. St. Helens (Washington)
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Critical Thinking #7
Which volcano type would likely be the most dangerous? Explain why.
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Craters and Calderas
Crater – Funnel-shaped pit at top of volcanic vent– Formed when material is
blown out of a volcano by explosions
Caldera – Large basin-shaped depression – Formed when magma
chamber is emptied and entire volcanic cone is collapsed
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Critical Thinking #8
In what situation would a crater form? In what situation would a caldera form?
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Mount St. Helens Mount St. Helens –
Composite Volcano (Stratovolcano) in Washington State– Erupted in 1980 after being
dormant – “inactive” for 120 years
– North face of mountain experiences largest ever recorded debris landslide
– Pyroclastic flow from eruption destroyed 230 square miles
– 57 people killed from the blast
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Critical Thinking #9
What factors would create the greatest landslide in a volcanic eruption?
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Critical Thinking #10
What aspect of the eruption of Mount St. Helens caused the greatest damage and casualties?