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VOLCANISM & Igneous Activity

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

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VOLCANISM & Igneous Activity

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  1. VOLCANISM &Igneous Activity

  2. 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

  3. 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

  4. 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.

  5. 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

  6. Extrusive Rocks & Gases • Scientific study of volcanism • Gases • Primarily H2O • Also CO2 , SO2 , H2S, HCl • Gases & pyroclastics • Ashfall • Pyroclastic flow

  7. 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

  8. VOLCANOES • Volcanoes are cone-shaped • Vent • Crater • Flank eruption • Caldera • Types: • Cinder Cone, Shield, Composite

  9. VOLCANOES • Types: • Cinder Cone, • Shield, • Composite

  10. CINDER CONES • Formed of pyroclastics only • Steep sides- ~30 degrees • Relatively small • Short duration of activity

  11. 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

  12. 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

  13. 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

  14. VOLCANIC DOMES • Forms above a volcanic vent • Viscous lava • Usually silica-rich (or cooler magma) • Associated with violent eruptions

  15. Lava Flows • AA • rubbly surface, broken, jagged • Pahoehoe • ropy surface

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