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30 Cards in this Set

  • Front
  • Back
Making Magma
rocks melts more easily with:
Increasing temperature
Decreasing pressure
Addition of water
Partial melting of asthenosphere (peridotite)
Peridotite to basalt (20% richer in silica)
Lower melt point
Granitic magma rises to form plutonic rocks
intermediate magmas – as with granite
different crust melt or mixing with mantle material
Two hypotheses of early continents
Horizontal tectonics – early continents were akin to overgrown island arcs
Vertical mantle plume tectonics – upwellings drove partial melting
Magma Behavior
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
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
volcanoes: Lava
– magma at the surface, flowing or solid
Pahoehoe – ropy, syrupy lava
Aa – rubbly, cindery lava
Volcanoes: Pyroclastics
materials formed by rapid extrusion
Pyroclastic rock
Volcanic ash
Fissure eruptions – low viscosity lava exuding from cracks
Flood basalt – very large, rapid, fissure eruption
Lava (basalt) plateau – many cubic kilometers sized event
Types of Volcanoes
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
Volcanic Explosions
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
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
Volcanic Explosions (CONT)
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
Volcanic ash flow
Ash Flows – column of pyroclastic materials and gas falls to ground
Fast (200 kph)
Far-reaching (100 km)
Ash flow tuff – post flow the ash may weld / compact into a solid mass
Predicting Volcanic Eruptions
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
1. Melting caused by decreased pressure is called __________ melting.
a. pressure-release
b. pressure-decrease
c. pressure-cease
d. pressure-diminish
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
3. Granitic magma has a __________ melting temperature compared to basaltic magma.
a. lower
b. higher
c. equal
d. much higher
4. Typical granitic magma contains __________ silica and __________ water than typical basaltic magma.
a. more, less
b. less, less
c. more, more
d. less, more
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
6. Continental crust is typically of __________ composition.
a. basaltic
b. granitic
c. peridotitic
d. magnetic
7. Oceanic crust is typically of __________ composition.
a. basaltic
b. granitic
c. peridotitic
d. magnetic
8. Granitic magma contains up to __________ percent water.
a. 50
b. 40
c. 25
d. 10
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
10. A __________ is a pluton with an outcrop area of less than 100 square kilometers.
a. batholith
b. volcanic neck
c. dike
d. stock
11. __________ is lava with a jagged, broken surface.
a. Pahoehoe lava
b. Aa lava
c. Cosmic lava
d. Plutonic lava
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
13. Gentle eruptions of basaltic lava from long fissures create __________.
a. flood basalts
b. cinder cones
c. shield volcanoes
d. calderas
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
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