Download Chapter 4 -Volcanoes

Plumbing System of a Volcano
Fig. 5.1
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Tectonic Setting of Volcanoes
Volcanism Associated with
Plate Tectonics
• Convergent plate boundaries
• Divergent plate boundaries
• Within plate “hotspots”
Fig. 5.30
The World’s Active Volcanoes
Volcanism Associated with
Plumes = Hot Spots
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Volcanic Rocks
Major difference between plutonic
and volcanic rocks is texture, a
reflection of cooling rate.
• Large crystal take a Long time
Material Ejected From Volcanoes
• Lava: magma that has flowed
on the surface of the Earth.
• Tephra: fragments that
solidified in the air during
eruption.
• Small crystals take a Short time
PLUTONIC
VOLCANIC
Granite
coarse-grained
Basalt
fine-grained
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Tephra
Types of Lava
• Pyroclastic Flows
aa
pahoehoe
• Ash Fall
• Ballistic Debris or Bombs
Aa Lava
Tephra
Pahoehoe
Lava
Kim Heacox/DRX
Fig. 5.3
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Volcanic Bomb
Ash Cloud
Science Source/Photo Researchers
Volcanic Breccia
Other Material Ejected From Volcanoes
• Steam (H2O)
• Carbon dioxide (CO2 )
• Hydrogen sulfide (H2S)
• many other constituents
Doug Sokell/Visuals Unlimited
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Types of Volcanoes
Shield Volcano
1. Shield Volcanoes
2. Cinder Cones
3. Volcanic Domes
4. Stratovolcanoes (composite)
5. Lava Floods
6. Submarine Eruptions
1. Shield Volcanoes
• Low-viscosity lava flows
– Low-silica magma (mafic)
– Basalt
»Pahoehoe
»Aa
• Gently sloping flanks
• Tend to be very large
• Spatter cone — minor feature
Shield Volcano
Olympus Mons - Mars
NASA, Viking Orbiter 1
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2. Cinder Cones
•
•
•
•
Formed of pyroclastics only
Steep sides — ~30 degrees
Relatively small
Short duration of activity
Cerro Negro
Cinder Cone
Nicaragua, 1968
Cinder Cone
Cinder Cone
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3. Volcanic Domes
• Forms above a volcanic vent
• Viscous lava — usually silica-rich
(or cooler magma)
• Associated with violent eruptions
4. Composite or Statovolcano
•
•
•
•
•
Alternating pyroclastic layers and lava flows
Slopes intermediate in steepness
Intermittent eruptions over long time span
Mostly andesite
Distribution
– Circum-Pacific Belt (“Ring of Fire”)
– Mediterranean Belt
Composite Volcano
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Distribution of Stratovolcanoes
Mt. Shasta California
Mediterranean
Volcanoes
Mt. St. Helens
Mt. Fujiyama, Japan
Before and After
Raga/The Stock Market
Fig. 5.15
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Fissure Eruptions Form Lava Plateaus
5. Lava Floods
• Fissure Eruptions
• Plateau or Flood Basalts
• Columnar Basalt or
Columnar Jointing
Fig. 5.20
Fissure Eruptions
Fissure Eruption
When low-viscosity (felsic) lava
is issued from cracks in the
Earth tens of kilometers long.
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Plateau or Flood Basalts
When basaltic (mafic) lava
accumulates in horizontal flows.
Victoria
Laki Fissure, Iceland
erupted in 1783
extruding the largest
lava flow in human
history.
Tony Waltham
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Columbia Plateau Basalt Flows
Columnar Basalts
• formed during the relatively quick
cooling of a thick lava flow
• cracks form to accommodate shrinking
in the horizontal direction
• extensive fracture network develops
and results in the formation of columns
Deccan Plateau
Columnar Basalt
Devil's Postpile, California
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Submarine Eruptions
• Vesicular Basalt
• Pillow Basalt
• Phreatic Explosions
Pillow Basalt
Vesicular Basalt
Pillow Basalt
Glen Oliver/Visuals Unlimited
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Volcanic Hazards
• Gas
Phreatic
Explosion
in the
Pacific
– Lake Nyos (Cameroon), 1984 (1700 people killed)
• Ash fall
– Mt. Pinatubo, 1991
• Pyroclastic flows
– Mt. Pelee, 1902 (28,000 killed)
– Mt. Vesuvius, 79 AD (killed)
• Lahars (mudflows)
– Nevado del Ruiz, 1985 (23,000 killed)
• Tsunamis
– Krakatoa, 1883 (36,417 killed)
Maritime Safety Agency, Japan
Volcanic Tsunami
Gas – Lake Nyos
A series of
waves created
when a body of
water is rapidly
displaced on a
massive scale .
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Volcanic Ashfall
Interaction of magma, expanding
steam, and volcanic gases
results in the ejection of mostly
small particles called ASH.
Ashfall - Pinatubo
Pyroclastic Flow
(Nueé Ardente)
Mixture of hot gases, ash, and rocks
forming a super-heated and dense
current capable of moving 150 km/hr.
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Pyroclastic Flow
Volcanic Mudflow (Lahar):
Mt. Pelee 1902
A mixture of water and pyroclastic
material in a concrete-like slurry
capable of moving up to 100km/hr!
Mt. Vesuvius 79 AD
Lahars - Nevado del Ruiz
Nueé Ardente
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Lahar – Mudflow
Xenolith
Other Volcano Related
Features
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Rhyolitic Lava
Dike
Sill
Volcanic Pipe
and Radiating Dikes
Shiprock, New Mexico
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Volcanic Breccia
Crater Lake, Oregon
Calderas
• Depression at top of volcano
produced during an eruption
Wizard Island
• May have younger domes within it
Greg Vaughn/Tom Stack
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