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 – – – – – –

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 • • • •

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 • • • •

Formed of pyroclastics only Steep sides- ~30 degrees Relatively small Short duration of activity

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

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