Download IGNEOUS ROCKS

IGNEOUS ROCKS
• EXTRUSIVE (Volcanic)
– Fine-grained
• INTRUSIVE (Plutonic)
– Coarse-grained
MAGMA
• Molten Rock
– Usually with dissolved gases
• Generated at depth
• Eruptions if magma (lava) reaches surface
• If doesn’t reach surface, solidifies underground
– Intrudes country rock
– Intrusive contact
– Chill zone
– Xenolith
VOLCANISM
• Lava = magma at Earth’s surface
– Usually with dissolved gases
– Silica content controls viscosity, explosivity
• Pyroclasts
– fragments of rock created by explosion
• Lava flows
– hot, low viscosity liquid
• Pyroclastic rocks & lavas = Extrusive rocks
– Extrusive rocks commonly form Volcanoes
VOLCANISM
• Lava = Magma at Earth’s surface
– Usually with dissolved gases
– Silica content controls viscosity, explosivity
• Pyroclasts
– fragments of rock created by explosion
• Lava flows
– hot, low viscosity liquid
• Pyroclastic rocks & lavas = Extrusive rocks
– Extrusive rocks commonly form Volcanoes
VOLCANISM
• Lava = Magma at Earth’s surface
– Usually with dissolved gases
– Silica content controls viscosity, explosivity
• Pyroclasts
– fragments of rock created by explosion
• Lava flows
– hot, low viscosity liquid
• Pyroclastic rocks & lavas = Extrusive rocks
– Extrusive rocks commonly form Volcanoes
VOLCANISM
• Lava = Magma at Earth’s surface
– Usually with dissolved gases
– Silica content controls viscosity, explosivity
• Pyroclasts
– fragments of rock created by explosion
• Lava flows
– hot, low viscosity liquid
• Pyroclastic rocks & lavas = Extrusive rocks
– Extrusive rocks commonly form Volcanoes
February 10, 2001
Volcanism: Effects on Humans
• Growth of Hawaii
• Geothermal energy
• Effect on climate
– Tambora 1815; 1816 "year without Summer"
• Volcanic catastrophies
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Mt. St. Helens
Vesuvius
Krakatoa
Nevado del Ruiz
Montserrat, Caribbean
Pinatubo (video & questions next week)
Nevado del Ruiz, Colombia
lahar kills 23,000; November 13, 1985
http://volcanoes.usgs.gov/Hazards/What/Lahars/RuizLahars.html
Eruptive Violence &
Characteristics of Lava
• Function of amount of gas in lava
– Viscosity = resistance to flow
• Temperature
• Silica (SiO2) content
– Silicic lavas- most viscous
– Mafic lavas- least viscous
Extrusive Rocks & Gases
• Scientific study of volcanism
– Better understand eruptions and hazards
• Gases
– Primarily H2O
– Also CO2 , SO2 , H2S, HCl
• Gases & pyroclastics
– Ashfall: gentle dispersal
– Pyroclastic flow: violent gas-rich cloud
• see USGS web site:
http://volcanoes.usgs.gov/Hazards/What/PF/pcflows.html
Pyroclastic
Flow
Deposits
Figure 4-18. Types of pyroclastic flow deposits. After
MacDonald (1972), Volcanoes. Prentice-Hall, Inc., Fisher and
Schminke (1984), Pyroclastic Rocks. Springer-Verlag. Berlin. a.
collapse of a vertical explosive or plinian column that falls back
to earth, and continues to travel along the ground surface. b.
Lateral blast, such as occurred at Mt. St. Helens in 1980. c.
“Boiling-over” of a highly gas-charged magma from a vent. d.
Gravitational collapse of a hot dome
Lascar Volcano, Northern Chile
-1993 Pyroclastic flow
Extrusive Rocks
• Importance of silica content
• Rhyolite- silicic
– Predominantly feldspar and quartz
• Andesite- intermediate
– Plagioclase feldspar &
ferromagnesian minerals
• Basalt- mafic
– Ferromagnesian minerals &
plagioclase feldspar
Extrusive Rocks
• Textures
– Fine-grained (smaller than 1 mm)
– Glassy- Obsidian
– Due to
• rapid cooling (mainly)
• high viscosity
– Porphyritic
• Phenocrysts
Extrusive Rocks
• Textures (continued)
– Due to trapped gas
• Vesicles
• Scoria
• Pumice
– Fragmental
• Pyroclasts
• Dust, ash, cinders
• Blocks & bombs
• Tuff
• Volcanic Breccia
VOLCANOES
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Volcanoes are cone-shaped
Vent
Crater
Caldera
• Active?
• Dormant?
• Extinct?
• See:
http://volcanoes.usgs.gov/Products/Pglossary/volcano.html
VOLCANOES
• Types:
– Shield, Cinder Cone, Composite
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
Taburiente Shield
1.5 Ma to 550 ka
Cumbre Vieja
123 ka to historical
La Palma
Canary Islands
CINDER CONES
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Formed of pyroclastics only
Steep sides- ~30 degrees
Relatively small
Short duration of activity
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
Puyehue Volcano, Southern Andes
Puyehue Volcano, Southern Andes
Composite
Structure:
Alternating
Pyroclastic
and lava
flows
VOLCANIC DOMES
• Forms above a volcanic vent
• Viscous lava
– Usually silica-rich (or cooler magma)
• Associated with violent eruptions
LAVA FLOODS
• Mafic lava- solidifies to basalt
• Fissure flows
– Plateau basalts
• Columnar structure or jointing
SUBMARINE ERUPTIONS
• Pillow basalt
– Common along mid-ocean ridge