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 H 2 O – Also CO 2 , SO 2 , H 2 S, 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

• Types: – Cinder Cone, – Shield, – Composite

VOLCANOES

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

• • AA – rubbly surface, broken, jagged Pahoehoe – ropy surface

Lava Flows

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 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