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