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VOLCANISM &
Igneous Activity
VOLCANISM
• Lava = Magma at earth surface
– Silica content controls “explosiveness”
• Pyroclasts = Fragments of rock due to
explosion- Ash falls (pumice); Volcanic
Bombs (scoria)
• Lava flows
• Extrusive rocks
• Volcano
VOLCANISM
• Lava = Magma at earth surface
– Silica content controls “explosiveness”
• Pyroclasts = Fragments of rock due to
explosion- Ash falls (pumice); Volcanic
Bombs (scoria)
• Lava flows
• Extrusive rocks
• Volcano
Effects on Humans
• Growth of Hawaii
– 1980’s & 90’s 1.5 billion cubic meters
• Geothermal energy- New Zealand; California
• Effect on climate- 1816 “year without summer”
• Volcanic catastrophies
– Mt. St. Helens 1980
– Vesuvius 79 AD
– Krakatoa 1883
– Crater Lake 6,600 y.b.p.
Mount St. Helens
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Northern flank bulged at 1.5m/day
All vegetation stripped for 10km w/in seconds
$100s millions in damage
63 people died
Damage minimized due to prior planning by
USGS and governor
Extrusive Rocks & Gases
• Scientific study of volcanism
• Gases
– Primarily H2O
– Also CO2 , SO2 , H2S, HCl
• Gases & pyroclastics
– Ashfall
– Pyroclastic flow
Extrusive Rocks
• Textures
– Fine-grained (smaller than 1 mm)
– Glassy- Obsidian
– Due to
• rapid cooling (mainly)
• high viscosity
– Porphyritic
• Phenocrysts
– Due to trapped gas
• Vesicles
• Scoria
• Pumice
VOLCANOES
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Volcanoes are cone-shaped
Vent
Crater
Flank eruption
Caldera
Types:
– Cinder Cone, Shield, Composite
VOLCANOES
• Types:
– Cinder Cone,
– Shield,
– Composite
CINDER CONES
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Formed of pyroclastics only
Steep sides- ~30 degrees
Relatively small
Short duration of activity
SHIELD VOLCANOES
• Low viscosity lava flows
– Low silica magma- mafic
– Basalt
• Pahoehoe
• Aa
• Gently sloping flanks- between 2 and 10
degrees
• Tend to be very large
• Spatter Cone- minor feature
COMPOSITE VOLCANO
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Alternating pyroclastic layers & 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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Alternating pyroclastic layers & lava flows
Slopes intermediate in steepness
Intermittent eruptions over long time span
Mostly Andesite
Distribution
– Circum-Pacific Belt (“Ring of Fire”)
– Mediterranean Belt
VOLCANIC DOMES
• Forms above a volcanic vent
• Viscous lava
– Usually silica-rich (or cooler magma)
• Associated with violent eruptions
Lava Flows
• AA
– rubbly surface, broken, jagged
• Pahoehoe
– ropy surface
LAVA FLOODS
• Mafic lava- solidifies to basalt
• Fissure flows
– Plateau basalts
• Columnar structure or jointing
SUBMARINE ERUPTIONS
• Pillow basalt
INTRUSIVE BODIES
(STRUCTURES)
• Bodies that solidified underground-Plutons
• Volcanic neck- shallow intrusion
• Fills cracks (joints)- tabular bodies
– DIKE•
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If no layering in country rock
If country rock is layered- Discordant
SILL- less common
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Concordant- parallel to layering in country rock
INTRUSIVE STRUCTURES
Plutons
• BATHOLITH– Large intrusive body
– Exposed in an area greater than 100 square
Km.
– Coalesced smaller plutons
• smaller bodies are called STOCKS
• Batholiths a gathering of smaller blobs
• Magma moves upward from depth as diapirs