1. Chapter 6: Volcanoes and Igneous Rocks
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Learning Objectives
Volcanoes and volcanic hazards
Contrast the different types of volcanic eruptions and their effects.
How, why, and where rock melts
Describe how temperature, pressure, and water conditions produce magma.
Cooling and crystallization
Compare different types of igneous and volcanic rocks and their formation.
Plutons and plutonism
Describe the types of plutonic rock and plutons and their formation.
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3. How, Why, and Where Rocks Melt
Heat and pressure inside Earth:
Continental crust: temperature rises 30°C/km, then about 6.7°C/km.
Ocean crust: temperature rises twice as rapid.
Figure 6.15 Geothermal gradient
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4. How, Why, and Where Rocks Melt
Effect of temperature and pressure on melting
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5. How, Why, and Where Rocks Melt Heat and Pressure Inside Earth
Fractional melt
A mixture of molten and solid rock
Fractionation
Separation of melted materials from the remaining solid material during the course of melting
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6. How, Why, and Where Rocks Melt
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7. How, Why, and Where Rocks Melt Magma and Lava
Molten rock below surface
Lava
Magma when it reaches the surface
Differs in composition, temperature, and viscosity
Figure 6.18b Two types of lava flows
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8. How, Why, and Where Rocks Melt Magma and Lava
Composition
45% to 75% of magma by weight is silica.
Water vapor and carbon dioxide are usually present.
Temperature
Lavas vary in temperature between 750°C and 1200°C.
Magmas with high H2O contents melt at lower temperatures.
Viscosity
Lavas vary in their ability to flow.
Influenced by silica content and temperature.
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9. Magma and Lava Figure 6.18a Viscosity of lava
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10. How, Why and Where Rocks Melt
Tectonic setting and volcanism
Lava characteristics influenced by location:
Oceanic, divergent margins.
Lava is thin with a steep geothermal gradient.
Subduction zones typically have high water content and melt at lower temperatures.
Hot spots:
Lava tends to be hot and basaltic.
Build giant shield volcanoes.
Continental divergent margins are all different:
Lava is high in silica.
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11. Lava types and tectonic settings
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12. Lava types and tectonic settings
Figure 6.19a Midocean ridge: submarine basaltic pillow lavas
Figure 6.19b Continental rift: rhyolitic and lavas with unusual composition
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13. Lava types and tectonic settings
Figure 6.19c Ocean-continent subduction zone
Figure 6.19d Shield volcano
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14. Lava types and tectonic settings
Figure 6.19e Ocean-ocean subduction zone
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15. Cooling and Crystallization
The process whereby mineral grains form and grow in a cooling magma (or lava)
Classified as:
Volcanic
Plutonic
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16. Cooling and Crystallization Rate of Cooling
Rapid cooling: Volcanic rocks and textures
Volcanic rock
An igneous rock formed from lava
Glassy
Aphanitic
Porphyritic
Pumice
Vesicular basalt
Figure 6.20a Glassy texture
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17. Cooling and Crystallization Rate of Cooling
Figure 6.20b Aphanitic texture
Figure 6.20c Porphyritic texture
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Rate of Cooling
Slow cooling: Plutonic rocks and textures
Plutonic rock
An igneous rock formed underground from magma
Phaneritic: A coarse-grained texture
Can have exceptionally large grains
Figure 6.21 Plutonic rock textures
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19. Cooling and Crystallization Chemical Composition
Igneous rocks subdivided into three categories based on silica content:
Felsic
Intermediate
Mafic
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21. Volcanoes and Volcanic Hazards
Lava
Molten rock that reaches Earth’s surface
Magma
Molten rock, which may include fragments of rock, volcanic glass and ash, or gas
Figure 6.1a Lava
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22. Volcanoes and Volcanic Hazards
Volcanic materials
Pyroclasts
Tephra
Ash
Agglomerates
Tuff
Figure 6.1b Volcanic bombs
Figure 6.1D Volcanic ash
Figure 6.1c Lapilli
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23. Volcanoes and Volcanic Hazards
A vent through which lava, solid rock debris, volcanic ash, and gasses erupt from Earth’s crust to its surface
Can be explosive or nonexplosive
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24. Volcanoes and Volcanic Hazards Eruptions, Landforms, and Materials
Strombolian eruptions
More explosive than Hawaiian
Create loose volcanic rock called spatter cones or cinder cones
Figure 6.2d Strombolian eruption
Figure 6.2e Cinder cones in Arizona
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25. Volcanoes and Volcanic Hazards Eruptions, Landforms, and Materials
Vulcanian eruptions
More explosive than Strombolian and, as a result, can generate billowing clouds of ash up to 10 km.
Produce pyroclastic flows.
Hot volcanic fragments (tephra), buoyed by heat and volcanic gases, flow very rapidly.
Figure 6.2b Mt. Mayon in Philippines
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26. Volcanoes and Volcanic Hazards Eruptions, Landforms, and Materials
Plinian eruptions
Named after Pliny the Elder, who died during eruption of Mount Vesuvius
Most violent eruptions, generating ash columns that can exceed 20 kilometers
Produce steep-sided volcanoes, called stratovolcanoes
Figure 6.2c Mount Saint Helens
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27. Volcanoes and Volcanic Hazards Eruptions, Landforms, and Materials
Stratovolcanoes
Composed of solidified lava flows interlayered with pyroclastic material.
Steep sides curve upward.
Viscosity
Degree to which a substance resists flow.
A less viscous liquid is runny, whereas a more viscous liquid is thick.
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28. Eruptions, Landforms, and Materials
Figure 6.2a Stratovolcano
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29. Volcanoes and Volcanic Hazards Eruptions, Landforms, and Materials
Shield volcanoes
Broad, flat volcanoes with gently sloping sides, built of successive lava flows
Produce flood basalts or basalt plateaus
Figure 6.2f Shield volcanoes
Figure 6.2h Flood basalts
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30. Eruptions, Landforms, and Materials
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31. Eruptions, Landforms, and Materials
Figure 6.2g Fissure eruptions
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32. Eruptions, Landforms, and Materials
Figure 6.3 Crater Lake
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33. Eruptions, Landforms, and Materials
Figure 6.4 Resurgent Dome of Mt. St. Helens
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34. Volcanoes and Volcanic Hazards Eruptions, Landforms, and Materials
Other volcanic features:
Craters
Resurgent dome
Thermal spring
Geysers
Fumaroles
Figure 6.5 The Great Geysir
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35. Volcanoes and Volcanic Hazards Volcanic Hazards
Primary effects
Pyroclastic flows
Volcanic gases
Secondary effects
Related to, but not a direct result of, volcanic activity
Fires
Flooding
Mudslides
Debris avalanche
Figure 6.6 Kalapana, Hawaii lava flow
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36. Volcanic Hazards © 2012 John Wiley & Sons, Inc. All rights reserved.
Figure 6.7 Victim of poisonous gases of eruption of Mt. Vesuvius
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37. Volcanic Hazards © 2012 John Wiley & Sons, Inc. All rights reserved.
Figure 6.8 Volcanic hazards
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38. Volcanic Hazards Figure 6.9 Deadly eruptions
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39. Volcanoes and Volcanic Hazards Volcanic Hazards
Tertiary and beneficial effects:
Change a landscape
Affect climate on regional and global scale
Renew mineral content and replenish fertility
Geothermal energy
Provide mineral deposits
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40. Volcanoes and Volcanic Hazards Volcanic Hazards
Figure 6.11 Fertile but dangerous
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41. Predicting Eruptions Figure 6.12 Volcano monitoring from the ground
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42. Volcanoes and Volcanic Hazards Predicting Eruptions
Establishing a volcano’s history
Active
Dormant
Monitoring changes and anomalies
Earthquakes
Shape or elevation
Volcanic gases
Ground temperature
Composition of water
Figure 6.13 Monitoring volcanoes from orbit
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43. Cooling and Crystallization Fractional Crystallization
Separation of crystals from liquids during crystallization
Bowen’s reaction series
Predictable melting and cooling of minerals
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44. Fractional Crystallization
Figure 6.22a Filter pressing
Figure 6.22b Crystal settling
Figure 6.22c Crystal flotation
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Plutons and Plutonism
Plutons
Any body of intrusive igneous rock, regardless of size or shape
Batholith
A large, irregularly shaped pluton that cuts across the layering of the rock into which it intrudes
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Plutons and Plutonism
Figure 6.24 How magma rises
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Plutons and Plutonism
Batholiths are so huge that map views give us the best perspective.
Stocks are smaller batholiths.
Figure 6.25 Batholiths and stocks
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48. Plutons and Plutonism Dikes and Sills
Dikes form when magma squeezes into a cross-cutting fracture and solidifies. Sills form when magma intrudes between two layers and is parallel to them.
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Plutons and Plutonism
Volcanic neck
Remnant of a volcanic pipe that once fed the magma to the volcanic vent
Figure 6.26c Devil’s Tower, Wyoming
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50. Amazing Places: Mount Saint Helens
A May 1980
C Largest debris avalanche in recorded history
B Forest flattened by blast
D Mt. St. Helens today
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Critical Thinking
What factors might prevent magma from reaching Earth’s surface?
What reasons can you think of for living near a volcano? Do you think the advantages outweigh the disadvantages?
If you were to heat up a glass beaker full of crushed rock, the beaker would melt before you could finish studying the rock-melting process. How do you think geologists study rock melting?
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