VOLCANISM & Igneous Activity
Jan 02, 2016
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
• 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
• Volcanoes are cone-shaped• Vent• Crater• Flank eruption• Caldera• Types:
– Cinder Cone, Shield, Composite
VOLCANOES
• Types:– Cinder Cone, – Shield, – Composite
CINDER CONES
• 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
• 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
• 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- • If no layering in country rock
• If country rock is layered- Discordant
– SILL- less common• Concordant- parallel to layering in country rock
INTRUSIVE STRUCTURESPlutons
• 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