gg101 lecture 12 volcanoes · volcanoes! 2. compare and contrast 3 common types of magma! 3....
TRANSCRIPT
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What we’ll learn today:!
1. Define the term volcano and explain why geologists study volcanoes!
2. Compare and contrast 3 common types of magma!3. Describe volcanic gases and the role they play in explosive
vs effusive eruptions!4. Identify what gives a shield volcano its distinctive shape!
Learning Objectives (LO)
Lecture 12: Volcanoes Read: Chapter 6 Homework #10 due Thursday 12pm
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“A volcano is any landform from which lava, gas, or ashes, escape from underground or have done so in
the past.”
What is causing this eruption? What factors influence its character?
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From Chapter 5: magma (and lava)
can be felsic, intermediate, or
mafic.
How does magma chemistry
influence the nature of volcanic
eruptions?
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http://www.youtube.com/watch?v=6J6X9PsAR5w
Hawaiian Volcanism
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http://www.youtube.com/watch?v=5LzHpeVJQuE
Indonesian Volcanism
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Viscous: thick and sticky
Viscosity
High viscosity Low viscosity
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Viscosity
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Magma Composition (Igneous Rocks) How does magma chemistry determine lava and eruption characteristics?
Felsic Intermediate Mafic
Mg/Fe content Si/O content
more less less more
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Seven major types of igneous rocks
Composition
Text
ure
Granite
Rhyolite Andesite
Diorite
Basalt
Gabbro Peridotite
The Major 7 Types of Igneous Rocks
Extrusive
Intrusive
Felsic Intermediate Mafic Ultramafic
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Seven major types of igneous rocks
Composition
Rhyolite Andesite Basalt
The Rocks of Volcanoes
Extrusive
Felsic Intermediate Mafic Ultramafic
melt at high temperature
melt at low temperature
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Three Common Types of Magma:
BASALTIC
ANDESITIC
RHYOLITIC
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Three Common Types of Magma:
Basaltic lava flows easily because of its low viscosity and low gas content.
The low viscosity is due to low silica content.
Aa - rough, fragmented lava blocks called “clinker”
Pahoehoe - smooth, shiny, and ropy surface
BASALTIC
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Mount St. Helens, 1980
• Erupts explosively because it has high gas content
• It is viscous and therefore traps gas.
• High viscosity is related to high silica content
ANDESITIC
Three Common Types of Magma:
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Rhyolitic lava flow • Erupts catastrophically because it has high gas content.
• It is viscous and therefore traps gas, builds pressure and explosively erupts.
• High viscosity high silica content
RHYOLITIC
Three Common Types of Magma:
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Comparison of Magma Types
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Fueled by violent release of volcanic gas
Explosive Eruptions
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http://www.youtube.com/watch?v=dYatV_KV-u4
Exploding Trash Can (Trapped Nitrogen Gas) Analogy
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Plinian-Style Eruption
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http://www.youtube.com/watch?v=UK--hvgP2uY
Mt. St. Helens Eruption 1980:
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Pyroclastic Debris Tephra = airborne debris Pyroclastic Flow = Flows downhill due to gravity
Produced by explosive eruptions
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Pompei, Italy (79 AD)
Plymouth, Monserrat (2002)
Aftermath of Pyroclastic Flows
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Types of Pyroclastic Debris
ASH LAPILLI PUMICE
VOLCANIC BOMBS
VOLCANIC BLOCK
WELDED TUFF
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http://www.youtube.com/watch?v=Qgih2TL-9As
Volcanic Bombs
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Effusive Eruptions
Relatively fluid lava flow:
Low viscosity lava High temperature lava Low gas content of lava Long-duration eruptions
Reunion Island
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Gases released from a volcano can be deadly.
Kilauea
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Volcanoes can be classified into 6 major types based on their size, shape, and origin
Large-scale Volcanic Terrains No central vent Network of source material Generally massive
Central Vent Volcanoes Central vent Summit crater Flank eruptions Fissure eruptions
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Six Types of Volcanoes
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Mauna Loa
Shield Volcanoes are a Type of Central Vent Volcano
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Mauna Kea
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Shield Volcano • Low silica, low gas magma originates in the mantle. • Fluid, basaltic lava results in “Aa” and “Pahoehoe”. • Low viscosity creates broad, gentle slopes. • Phreatomagmatic eruptions occur when lava contacts water
(rapid expansion of steam) .
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• Eight main islands are exposed tips of the Hawaiian Ridge. • Age range is modern to ~6 million years old. • Volcanoes develop as the Pacific Plate moves across the Hawaiian Hotspot.
Shield Volcanoes in Hawai‘i
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Lōihi
Mauna Loa Kīlauea
Mauna Kea Hualālai
Kohala
Haleakalā
West Maui
Ni'ihau
Kahoolawe
Wailau
Lāna‘i
Ko’olau Waianae
Kauai
pre-shield
shield
post-shield
erosional
rejuvenated stages of shield volcanism
Lō‘ihi Seamount
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Oahu was originally two shield volcanoes
Ko’olau Shield
Waianae Shield
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Maps of the seafloor indicate that massive landslides have occurred
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Cinder Cones high lava fountains on the vents of shield volcanoes.
composed of pyroclastic debris caused by high gas content
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Diamond Head
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Stratovolcano
Mount Mageik, Katmai National Park
Layers of lava flows and ashfall deposits.
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Features of Stratovolcanoes (composite volcanoes)
• Alternating andesitic lava flows and layers of explosively ejected pyroclastics.
• Magma is intermediate, making the lava viscous and difficult to erupt
• Explosive eruptions due to buildup of gases.
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Mt. Fuji
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Lava Dome – a plug that prevents eruption (until it fails!)
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Lahar: A mudflow resulting
from an eruption
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http://www.youtube.com/watch?v=7Ct7G5lEHpc
Lahar Worry in Seattle
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Mt. Pinatubo – 1991 Dust caused global cooling
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Rhyolite Caldera Complexes are Central Vent Volcanoes
Aniakchak Caldera
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• High-silica, high-gas magmas. • Massive explosions (most explosive of all types).
• Collapse, producing an “inverse volcano”, or Caldera (Spanish for cauldron).
Features of Rhyolite Caldera Complexes
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http://www.youtube.com/watch?v=VR1bg_Yf0T4
Yellowstone Eruption
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Yellowstone ash beds cover much of the western USA
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Large-scale volcanic terrains lack a central vent
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Monogenetic Fields
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Rejuvenated (secondary) volcanism around Hawaii:
A type of monogenetic field
Diamond Head
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Large Igneous Provinces occur on land and in the ocean
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Example: Columbia Flood Basalts mark the beginning of the Yellowstone hotspot
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Large Igneous Province Fed by massive mantle plumes Caused by flood basalts (especially fluid basaltic lavas) Discharge over time through long fissures (cracks) Create large plateaus
Columbia River Basalts
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Mid-ocean Ridges
• Develop at Spreading Centers • Basaltic flow creates global network of interconnected ridges
submarine volcanism
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Mid-ocean Ridges
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Many volcanoes occur on plate boundaries - but sometimes in the middle of plates
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Most volcanoes are associated with spreading center volcanism, arc volcanism, or intraplate volcanism
Arc Volcanism: Andesitic explosive stratovolcanoes; some basaltic types
Spreading Center Volcanism: Fluid basaltic lavas, divergent margins
Intraplate Volcanism: Shield volcanoes and monogenetic fields (basaltic) Rhyolite caldera complexes (rhyolytic)
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What you should know from today:!
1. Define the term volcano and explain why geologists study volcanoes!
2. Compare and contrast 3 common types of magma!3. Describe volcanic gases and the role they play in explosive
vs effusive eruptions!4. Identify what gives a shield volcano its distinctive shape!
Action Items for Thursday, Oct. 8
1. Read Chapter 6 2. Complete homework assignment #10