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30 Cards in this Set
- Front
- Back
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Making Magma
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rocks melts more easily with:
Increasing temperature Decreasing pressure Addition of water |
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Basalt
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Partial melting of asthenosphere (peridotite)
Peridotite to basalt (20% richer in silica) |
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Granite
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Lower melt point
Granitic magma rises to form plutonic rocks |
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Andesite
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intermediate magmas – as with granite
different crust melt or mixing with mantle material |
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Two hypotheses of early continents
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Horizontal tectonics – early continents were akin to overgrown island arcs
Vertical mantle plume tectonics – upwellings drove partial melting |
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Magma Behavior
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Magma rises
Cooling solidifies Lower pressure keeps it liquid Magma composition differences Granitic – 70% silica, up to 10% water Basaltic – 50% silica, 1-2% water Effects of silica Increase silica increases viscosity Effects of water Increased water lowers solidification temperature (dry magma solidifies quicker) Rising magma loses water, hardens quicker |
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Plutons
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Magma solidifies within the crust
A number of structures form Batholith – large pluton, exposed by erosion, bigger than 100 km2 stock – smaller than a batholith Dike – a sheet cutting across layers Sills – a sheet formed between layers |
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volcanoes: Lava
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– magma at the surface, flowing or solid
Pahoehoe – ropy, syrupy lava Aa – rubbly, cindery lava |
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Volcanoes: Pyroclastics
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materials formed by rapid extrusion
Pyroclastic rock Volcanic ash Cinders |
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Volcanoes
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Fissure eruptions – low viscosity lava exuding from cracks
Flood basalt – very large, rapid, fissure eruption Lava (basalt) plateau – many cubic kilometers sized event |
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Types of Volcanoes
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Shield volcanoes
- Fluid basalt – gentle sloped shape - Often quite large - Mauna Loa, Mauna Kea, Iceland Cinder cones - From pyroclastic fragments - Often steep, small, symmetrical - May from abruptly Composite conesaka: stratovolcanoes - Layers of lava and pyroclastics form hard surface, resists erosion - Steep sided profile |
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Volcanic Explosions
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Mt Vesuvius – stratovolcano near Naples, Italy
Pompeii & Herculaneum (79 AD) 5-8 meter ash flow Intermittent activity in early – mid 1900s Magma still underlies Vesuvius Caldera – collapsed, evacuated magma chamber Steep-walled Circular 40+ km in diameter Yellowstone – 3 calderas Last eruptions (1.9 mya) – 2500 km3 of pyroclastic materials (Mt St Helens, 2500smaller), (0.6 mya) – 1000 km3 of ash & debris Yosemite’s Long Valley (Bishop Tuff) Mt St Helens – 170x smaller 1994 CO2 releases denote magma activity |
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Volcanic Explosions (CONT)
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Pinatubo, 1991
Ash & sulfur clouds 2-4% insolation decline Global cooling Permian extinction, 248 mya 90% of marine species, 65% reptile & amphibians Siberian flood basalts |
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Volcanic ash flow
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Ash Flows – column of pyroclastic materials and gas falls to ground
Hot Fast (200 kph) Far-reaching (100 km) Ash flow tuff – post flow the ash may weld / compact into a solid mass |
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Predicting Volcanic Eruptions
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Regional prediction – areas most likely
Subduction zones Spreading centers Hot spots Violence related to crustal domain Continental (granitic/intermediate) – more violent Basaltic – tend to be less violent Short-term prediction – looking for time and place of eruption Changes in shape EQ swarms Gas / ash emission Temperature anomalies |
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1. Melting caused by decreased pressure is called __________ melting.
a. pressure-release b. pressure-decrease c. pressure-cease d. pressure-diminish |
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2. The Ring of Fire, a zone of concentrated volcanic activity encircling the Pacific Ocean basin, is located adjacent to __________.
a. rift zones b. subduction zones c. the Mid-Oceanic Ridge system d. a lava plateau |
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3. Granitic magma has a __________ melting temperature compared to basaltic magma.
a. lower b. higher c. equal d. much higher |
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4. Typical granitic magma contains __________ silica and __________ water than typical basaltic magma.
a. more, less b. less, less c. more, more d. less, more |
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5. Magma with a high water content has a greater tendency to __________ in the crust, compared with magma with a lower water content.
a. solidify b. remain liquid c. metamorphose d. spread out |
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6. Continental crust is typically of __________ composition.
a. basaltic b. granitic c. peridotitic d. magnetic |
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7. Oceanic crust is typically of __________ composition.
a. basaltic b. granitic c. peridotitic d. magnetic |
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8. Granitic magma contains up to __________ percent water.
a. 50 b. 40 c. 25 d. 10 |
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9. Dated rocks and zircon crystals suggest that granitic continental crust had formed by about __________ years ago.
a. 1.6 million b. 4.4 million c. 1.6 billion d. 4.4 billion - |
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10. A __________ is a pluton with an outcrop area of less than 100 square kilometers.
a. batholith b. volcanic neck c. dike d. stock |
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11. __________ is lava with a jagged, broken surface.
a. Pahoehoe lava b. Aa lava c. Cosmic lava d. Plutonic lava |
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12. The most catastrophic volcanic explosions occur when __________ reaches the Earth’s surface.
a. flood basalt magma b. granitic magma c. basaltic magma d. aa magma |
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13. Gentle eruptions of basaltic lava from long fissures create __________.
a. flood basalts b. cinder cones c. shield volcanoes d. calderas |
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14. A/an __________ is a crater formed by the collapse of a magma chamber.
a. ash-flow tuff b. cinder cone c. caldera d. impact crater |
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15. When an ash flow comes to a stop, most of the gas escapes into the atmosphere, leaving behind a chaotic mixture of volcanic ash and rock fragments called __________.
a. ash-flow basalt b. ash-flow tuff c. shield volcano d. submarine volcano |
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