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Transcript
Chapter 6: Volcanoes and
Igneous Rocks
© 2012 John Wiley & Sons, Inc. All rights reserved.
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.
© 2012 John Wiley & Sons, Inc. All rights reserved.
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
© 2012 John Wiley & Sons, Inc. All rights reserved.
How, Why, and Where Rocks Melt
Effect of temperature and pressure on melting
© 2012 John Wiley & Sons, Inc. All rights reserved.
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
© 2012 John Wiley & Sons, Inc. All rights reserved.
How, Why, and Where Rocks Melt
© 2012 John Wiley & Sons, Inc. All rights reserved.
How, Why, and Where Rocks Melt
Magma and Lava
Magma
• 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
© 2012 John Wiley & Sons, Inc. All rights reserved.
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.
© 2012 John Wiley & Sons, Inc. All rights reserved.
Magma and Lava
Figure 6.18a Viscosity of lava
© 2012 John Wiley & Sons, Inc. All rights reserved.
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.
© 2012 John Wiley & Sons, Inc. All rights reserved.
Lava types and tectonic settings
© 2012 John Wiley & Sons, Inc. All rights reserved.
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
© 2012 John Wiley & Sons, Inc. All rights reserved.
Lava types and tectonic settings
Figure 6.19c Oceancontinent subduction
zone
Figure 6.19d
Shield volcano
© 2012 John Wiley & Sons, Inc. All rights reserved.
Lava types and tectonic settings
Figure 6.19e Ocean-ocean
subduction zone
© 2012 John Wiley & Sons, Inc. All rights reserved.
Cooling and Crystallization
Crystallization
•The process whereby mineral grains form and
grow in a cooling magma (or lava)
•Classified as:
• Volcanic
• Plutonic
© 2012 John Wiley & Sons, Inc. All rights reserved.
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
© 2012 John Wiley & Sons, Inc. All rights reserved.
Cooling and Crystallization
Rate of Cooling
Figure 6.20b Aphanitic texture
Figure 6.20c Porphyritic texture
© 2012 John Wiley & Sons, Inc. All rights reserved.
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
© 2012 John Wiley & Sons, Inc. All rights reserved.
Cooling and Crystallization
Chemical Composition
Igneous rocks subdivided into three
categories based on silica content:
•Felsic
•Intermediate
•Mafic
© 2012 John Wiley & Sons, Inc. All rights reserved.
© 2012 John Wiley & Sons, Inc. All rights reserved.
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
© 2012 John Wiley & Sons, Inc. All rights reserved.
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
© 2012 John Wiley & Sons, Inc. All rights reserved.
Volcanoes and Volcanic Hazards
Volcano
•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
© 2012 John Wiley & Sons, Inc. All rights reserved.
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
© 2012 John Wiley & Sons, Inc. All rights reserved.
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
© 2012 John Wiley & Sons, Inc. All rights reserved.
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
© 2012 John Wiley & Sons, Inc. All rights reserved.
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.
© 2012 John Wiley & Sons, Inc. All rights reserved.
Eruptions, Landforms, and Materials
Figure 6.2a Stratovolcano
© 2012 John Wiley & Sons, Inc. All rights reserved.
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
© 2012 John Wiley & Sons, Inc. All rights reserved.
Eruptions, Landforms, and Materials
© 2012 John Wiley & Sons, Inc. All rights reserved.
Eruptions, Landforms, and Materials
Figure 6.2g Fissure eruptions
© 2012 John Wiley & Sons, Inc. All rights reserved.
Eruptions, Landforms, and Materials
Figure 6.3 Crater Lake
© 2012 John Wiley & Sons, Inc. All rights reserved.
Eruptions, Landforms, and Materials
Figure 6.4 Resurgent Dome of Mt. St. Helens
© 2012 John Wiley & Sons, Inc. All rights reserved.
Volcanoes and Volcanic Hazards
Eruptions, Landforms, and Materials
Other volcanic features:
• Craters
• Resurgent dome
• Thermal spring
• Geysers
• Fumaroles
Figure 6.5 The Great Geysir
© 2012 John Wiley & Sons, Inc. All rights reserved.
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
© 2012 John Wiley & Sons, Inc. All rights reserved.
Volcanic Hazards
Figure 6.7 Victim of poisonous gases of eruption of Mt. Vesuvius
© 2012 John Wiley & Sons, Inc. All rights reserved.
Volcanic Hazards
Figure 6.8 Volcanic hazards
© 2012 John Wiley & Sons, Inc. All rights reserved.
Volcanic Hazards
Figure 6.9 Deadly eruptions
© 2012 John Wiley & Sons, Inc. All rights reserved.
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
© 2012 John Wiley & Sons, Inc. All rights reserved.
Volcanoes and Volcanic Hazards
Volcanic Hazards
Figure 6.11 Fertile but dangerous
© 2012 John Wiley & Sons, Inc. All rights reserved.
Predicting Eruptions
Figure 6.12 Volcano monitoring from the ground
© 2012 John Wiley & Sons, Inc. All rights reserved.
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
© 2012 John Wiley & Sons, Inc. All rights reserved.
Cooling and Crystallization
Fractional Crystallization
Fractional crystallization
•Separation of crystals from liquids during
crystallization
Bowen’s reaction series
•Predictable melting and cooling of minerals
© 2012 John Wiley & Sons, Inc. All rights reserved.
Fractional Crystallization
Figure 6.22a Filter
pressing
Figure 6.22b Crystal
settling
© 2012 John Wiley & Sons, Inc. All rights reserved.
Figure 6.22c Crystal
flotation
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
© 2012 John Wiley & Sons, Inc. All rights reserved.
Plutons and Plutonism
Figure 6.24 How magma rises
© 2012 John Wiley & Sons, Inc. All rights reserved.
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
© 2012 John Wiley & Sons, Inc. All rights reserved.
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.
© 2012 John Wiley & Sons, Inc. All rights reserved.
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
© 2012 John Wiley & Sons, Inc. All rights reserved.
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
© 2012 John Wiley & Sons, Inc. All rights reserved.
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?
© 2012 John Wiley & Sons, Inc. All rights reserved.