Download Volcanoes and volcanic hazards

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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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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How, Why, and Where Rocks Melt
Effect of temperature and pressure on melting
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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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How, Why, and Where Rocks Melt
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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
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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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Magma and Lava
Figure 6.18a Viscosity of lava
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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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Lava types and tectonic settings
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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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Lava types and tectonic settings
Figure 6.19c Oceancontinent subduction
zone
Figure 6.19d
Shield volcano
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Lava types and tectonic settings
Figure 6.19e Ocean-ocean
subduction zone
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Cooling and Crystallization
Crystallization
•The process whereby mineral grains form and
grow in a cooling magma (or lava)
•Classified as:
• Volcanic
• Plutonic
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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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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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Cooling and Crystallization
Chemical Composition
Igneous rocks subdivided into three
categories based on silica content:
•Felsic
•Intermediate
•Mafic
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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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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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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
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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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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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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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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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Eruptions, Landforms, and Materials
Figure 6.2a Stratovolcano
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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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Eruptions, Landforms, and Materials
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Eruptions, Landforms, and Materials
Figure 6.2g Fissure eruptions
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Eruptions, Landforms, and Materials
Figure 6.3 Crater Lake
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Eruptions, Landforms, and Materials
Figure 6.4 Resurgent Dome of Mt. St. Helens
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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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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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Volcanic Hazards
Figure 6.7 Victim of poisonous gases of eruption of Mt. Vesuvius
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Volcanic Hazards
Figure 6.8 Volcanic hazards
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Volcanic Hazards
Figure 6.9 Deadly eruptions
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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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Volcanoes and Volcanic Hazards
Volcanic Hazards
Figure 6.11 Fertile but dangerous
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Predicting Eruptions
Figure 6.12 Volcano monitoring from the ground
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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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Cooling and Crystallization
Fractional Crystallization
Fractional crystallization
•Separation of crystals from liquids during
crystallization
Bowen’s reaction series
•Predictable melting and cooling of minerals
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Fractional Crystallization
Figure 6.22a Filter
pressing
Figure 6.22b Crystal
settling
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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
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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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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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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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