Post on 03-Jun-2020
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Section 18.1: Volcanoes
Objectives:
Describe how plate tectonics
influences the formation of
volcanoes.
Locate major zones of volcanism
Identify the parts of a volcano
Differentiate between volcanic
landforms.
Section 18.1: Volcanoes
Main Idea:
The locations of
volcanoes are mostly
determined by plate
tectonics.
Types of Volcanoes
Shield volcanoes
Cinder cones
Composite volcanoes
Volcanoes The locations of volcanoes are
mostly determined by plate tectonics.
Volcanoes are fueled by magma.
Magma is a slushy mixture of molten
rock, mineral crystals, and gases.
Magma rises toward Earth’s surface
because it is less dense than the
surrounding mantle and crust.
Volcanoes Volcanism describes all the
processes associated with the
discharge of magma, hot fluids, and
gases.
There are approximately 20
volcanoes currently erupting
somewhere on Earth.
The distribution of volcanoes on
Earth’s surface is not random.
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Volcanoes
Most volcanoes form at plate
boundaries.
The majority form at
convergent and divergent
boundaries.
Only about 5% of magma
erupts far from plate
boundaries.
Active Volcanoes
Active Volcanoes Zones of Volcanism We know that tectonic plate collide at
convergent boundaries.
As tectonic plate collide between
oceanic and continental plate, a
subduction zone forms.
Around subduction zones, magma
moves upward because it is less
dense than the surrounding solid
material.
Zones of Volcanism Zones of Volcanism
Most volcanoes located on
land result from oceanic-
continental subduction.
These volcanoes are
characterized by explosive
eruptions.
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Two major belts
The volcanoes associated with
convergent plate boundaries
forms a major belt.
The Circum-Pacific belt is also
called the Pacific Ring of Fire.
The outline of this belt
corresponds to the outline of the
Pacific Plate.
Active Volcanoes
Mount. St. Helens, WA Mount. St. Helens, WA
Two major belts
A smaller belt is called the
Mediterranean Belt.
It includes Mount Etna and Mount
Vesuvius, two volcanoes in Italy.
Its outline corresponds to the
boundary between the Eurasian,
African, and Arabian plates.
Mediterranean Belt
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Mount Etna Mount Etna
Divergent Volcanism
Unlike Explosive volcanoes,
volcanism at divergent
boundaries tends to be non
explosive.
About two thirds of Earth’s
volcanism occurs underwater
along divergent boundaries at
ocean ridges.
Pillow Lava
Pillow Lava Hot Spots
Some volcanoes form far from
plate boundaries over hot spots.
Scientists hypothesize that hot
spots are unusually hot regions
of the Earth’s mantle where high-
temperature plumes of magma
rise to the surface.
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Hot Spot Volcanoes The Hawaiian islands are located
over a plume of magma.
As the rising magma melted through
the crust, it formed volcanoes.
The world’s most active volcano,
Kilauea, on the Big Island is currently
located over the hot spot.
Flood basalts
When hot spots occur beneath
continental crust, they can lead
to the formation of flood basalts.
Flood basalts form when lava
flows out of long cracks in
Earth’s crust, called fissures.
Flood Basalts Huge amounts of lava erupting from
fissures accumulate on the surface,
forming layers 1 km thick. Over time,
streams can erode the layers, leaving
plateaus.
Palouse Canyon,
WA
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Anatomy of a Volcano
Lava reaches the surface of a
volcano by traveling through a
tubelike structure called a conduit,
and emerges through an opening
called a vent.
As lava flows through the vent and
out onto the surface, it cools and
solidifies around the vent.
Anatomy of a Volcano
At the top of a volcano,
around the vent, is a bowl-
shaped depression called a
crater.
The crater is connected to the
magma chamber by the
conduit.
Anatomy of a Volcano
Volcanic craters are usually less
than 1km in diameter.
Larger depressions, called calderas,
can be up to 50km in diameter.
Calderas often form after the magma
chamber beneath a volcano empties
from a major eruption.
Mount Aniakchak, Alaska
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Crater Lake, Oregon Yellowstone Caldera
Types of Volcanoes
There are three major types of
volcanoes:
Shield Volcanoes
Cinder Cones
Composite Volcanoes
Each type of volcano differs in
size, shape, and composition
Shield Volcanoes
A shield volcano is a mountain
with a broad, gently sloping side
and nearly circular base.
They form when layers of lava
accumulate from nonexplosive
eruptions.
Largest type of volcano.
Mauna Loa, Hawaii
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Cinder Cones
Cinder Cones form when
eruptions eject small pieces of
magma into the air which falls
back to Earth and piles up
around the vent.
Cinder cones have Steep sides
and are generally small
volcanoes.
Composite Volcanoes
Composite volcanoes are formed of
layers of hardened chunks of lava
from violent eruptions alternating
with layers of lava that oozed
downslop before solidifying.
Larger than cinder cones, and
explosive in nature.
Mount Augustine, Alaska
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Mount Nyiragongo
Location: Democratic Republic of
the Congo
Lava Type: Basaltic =
low silica content
Speed: lava flows at 60mph
Section 18.2: Eruptions
Objectives:
Explain how magma type influences
volcanic activity
Describe the role of pressure and
dissolved gases in eruptions.
Recognize classifications of material
ejected by eruptions
Making Magma
Main Ideas:
The composition of magma
determines the characteristics of
a volcanic eruption.
The activity of a volcano depends
on the composition of the
magma.
Temperature
Depending on their composition,
most rocks melt between 800°C
and 1200°C (1472°F - 2192°F)
These temperatures are found in
the crust and upper mantle.
Temperature increases with
depth beneath the Earth’s
surface.
Pressure
In addition to temperature,
pressure and the presence of
water also affect the formation of
magma.
Pressure increases with depth
because of the weight of
overlying rocks.
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Pressure
As pressure increases, the
temperature at which substances
melt also increases.
The effect of pressure explains
why most of the rocks in Earth’s
lower crust and upper mantle do
not melt.
Composition of Magma
The composition of magma
determines a volcano’s
explosivity, which is how it
erupts and how its lava flows.
Factors that determine magma’s
composition are overlying crust,
temperature, pressure, dissolved
gas, and amount of silica.
Dissolved Gases
As the amount of gases in
magma increases, the
magma’s explosivity also
increases.
Important gases in magma
are H2O CO2 SO2 and H2S.
Dissolved Gases
Water is the most common
dissolved gas in magma.
The presence of dissolved water
vapor lowers the melting
temperature of minerals, causing
mantle material to melt into
magma.
Viscosity
The physical property that
describes a material’s resistance
to flow is called viscosity.
Cooler magma has a higher
viscosity than hot magma.
Cool magma resists flowing.
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Types of Magma
The silica content of magma
determines not only its
explosivity and viscosity, but
also which type of volcanic rock
it forms as lava cools.
There are three types of magma:
Basaltic, Andesitic, and Rhyolitic
Basaltic Magma
Basaltic magma forms when
rocks in the upper mantle melt.
Basaltic magma has less than
50% silica.
Its low silica content produces
low-viscosity magma.
Erupts non explosively.
Basaltic Magma Andesitic Magma
Source material of andesitic
magma is oceanic crust and
sediments.
Andesitic magma has around 50-
60% silica content and erupts
explosively.
Intermediate viscosity.
Andesitic Magma Rhyolitic Magma
Source material of Rhyolitic
magma is continental crust
Rhyolitic magma has more than
60% silica content and erupts
explosively.
High viscosity.
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Rhyolitic Magma Explosive Eruptions
When lava is too viscous to flow
freely from the vent, pressure
builds up in the lava until the
volcano explodes, throwing lava
and rock into the air.
The erupted materials are called
tephra.
Explosive Eruptions
The smallest type of tephra is
ash (<2mm)
The largest tephra thrown from a
volcano are called blocks (some
as large as cars)
Pyroclastic Flows Rapidly moving clouds of tephra
mixed with hot, suffocating gases are
called pyroclastic flows.
Pyroclastic flows can have internal
temperatures of more than 700°C
(1300°F).
They can reach speeds up to 700 km/h
(450 mph)
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Section 18.3: Intrusive Activity
Main Idea:
Magma that solidifies below
ground forms geologic features
different from those formed by
magma that cools at the
surface.
Plutons
A pluton is a body of intrusive
igneous rock that is crystallized
from magma slowly cooling
below the surface of the Earth.
Plutons include batholiths,
stocks, dikes, sills, and laccoliths
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Plutons Batholiths and Stocks
Batholiths are irregularly shaped
masses of coarse-grained
igneous rocks that cover at least
100 km2 and take millions of
years to form.
Plutons Coast Range Batholith, BC
Batholiths and Stocks
Irregularly shaped plutons
that are similar to batholiths
but smaller in size are called
stocks.
Both batholiths and stocks
form 5-30km beneath Earth’s
surface.
Laccoliths
A laccolith is a lens shaped
pluton with a round top and
flat bottom.
When they form, rock layers
bow upward as a result of the
intense pressure of the
magma body.
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Plutons Laccolith, Montana
Sills
A sill forms when magma
intrudes parallel to layers of
rock.
A sill can range from a few
centimeters to hundreds of
meters in thickness.
Sills
Dikes
Unlike a sill, which is parallel
to the rocks it intrudes, a dike
is a pluton that cuts across
preexisting rock layers.
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Dikes
Volcanoes Volcanism describes all the
processes associated with the
discharge of magma, hot fluids, and
gases.
There are approximately 20
volcanoes currently erupting
somewhere on Earth.
The distribution of volcanoes on
Earth’s surface is not random.
Two major belts
The volcanoes associated with
convergent plate boundaries
forms a major belt.
The Circum-Pacific belt is also
called the Pacific Ring of Fire.
The outline of this belt
corresponds to the outline of the
Pacific Plate.
Two major belts
A smaller belt is called the
Mediterranean Belt.
It includes Mount Etna and Mount
Vesuvius, two volcanoes in Italy.
Its outline corresponds to the
boundary between the Eurasian,
African, and Arabian plates.
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Active Volcanoes Mediterranean Belt
Two major belts
The volcanoes associated with
convergent plate boundaries
forms a major belt.
The Circum-Pacific belt is also
called the Pacific Ring of Fire.
The outline of this belt
corresponds to the outline of the
Pacific Plate.
Two major belts
A smaller belt is called the
Mediterranean Belt.
It includes Mount Etna and Mount
Vesuvius, two volcanoes in Italy.
Its outline corresponds to the
boundary between the Eurasian,
African, and Arabian plates.
Divergent Volcanism
Unlike Explosive volcanoes,
volcanism at divergent
boundaries tends to be non
explosive.
About two thirds of Earth’s
volcanism occurs underwater
along divergent boundaries at
ocean ridges.
Pillow Lava
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Hot Spots
Some volcanoes form far from
plate boundaries over hot spots.
Scientists hypothesize that hot
spots are unusually hot regions
of the Earth’s mantle where high-
temperature plumes of magma
rise to the surface.
Temperature
Depending on their composition,
most rocks melt between 800°C
and 1200°C (1472°F - 2192°F)
These temperatures are found in
the crust and upper mantle.
Temperature increases with
depth beneath the Earth’s
surface.
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Pressure
In addition to temperature,
pressure and the presence of
water also affect the formation of
magma.
Pressure increases with depth
because of the weight of
overlying rocks.
Dissolved Gases
As the amount of gases in
magma increases, the
magma’s explosivity also
increases.
Important gases in magma
are H2O CO2 SO2 and H2S.
Dissolved Gases
Water is the most common
dissolved gas in magma.
The presence of dissolved water
vapor lowers the melting
temperature of minerals, causing
mantle material to melt into
magma.
Viscosity
The physical property that
describes a material’s resistance
to flow is called viscosity.
Cooler magma has a higher
viscosity than hot magma.
Cool magma resists flowing.
Basaltic Magma Andesitic Magma
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Rhyolitic Magma Mount Fuji
Explosive Eruptions
When lava is too viscous to flow
freely from the vent, pressure
builds up in the lava until the
volcano explodes, throwing lava
and rock into the air.
The erupted materials are called
tephra.
Pyroclastic Flows Rapidly moving clouds of tephra
mixed with hot, suffocating gases are
called pyroclastic flows.
Pyroclastic flows can have internal
temperatures of more than 700°C
(1300°F).
They can reach speeds up to 700 km/h
(450 mph)
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Plutons
A pluton is a body of intrusive
igneous rock that is crystallized
from magma slowly cooling
below the surface of the Earth.
Plutons include batholiths,
stocks, dikes, sills, and laccoliths
Plutons
Sills Alive, Extinct, or Fiction?
The Red-Lipped Batfish
CHAPTER 18 REVIEW:
Volcanoes
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Chapter 18 Review Questions
A(n) ____________ is a bowl-shaped
depression that surrounds the vent at
a volcano’s summit.
1. Fissure
2. Sill
3. Conduit
4. Crater
Chapter 18 Review Questions
A(n) ____________ is a bowl-shaped
depression that surrounds the vent at
a volcano’s summit.
1. Fissure
2. Sill
3. Conduit
4. Crater
Chapter 18 Review Questions
A(n) ___________ forms in the
depression left when an empty
magma chamber collapses.
1. Caldera
2. Fissure
3. Conduit
4. Tephra
Chapter 18 Review Questions
A(n) ___________ forms in the
depression left when an empty
magma chamber collapses.
1. Caldera 2. Fissure
3. Conduit
4. Tephra
Chapter 18 Review Questions
The type of volcano that is the
smallest and has the steepest slopes
is called a(n)______________.
1. Shield volcano
2. Cinder cone
3. Composite volcano
4. Pluton
Chapter 18 Review Questions
The type of volcano that is the
smallest and has the steepest slopes
is called a(n)______________.
1. Shield volcano
2. Cinder cone 3. Composite volcano
4. Pluton
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Chapter 18 Review Questions
Any intrusive igneous rock body that
has formed at great depths
underground.
1. Batholith
2. Laccolith
3. Pluton
4. Sill
Chapter 18 Review Questions
Any intrusive igneous rock body that
has formed at great depths
underground.
1. Batholith
2. Laccolith
3. Pluton 4. Sill
Chapter 18 Review Questions
Flowing cloud of tephra and lava
mixed with hot, suffocating gases.
1. Plutonic flow
2. Pyroclastic flow
3. Lava flow
4. Basaltic flow
Chapter 18 Review Questions
Flowing cloud of tephra and lava
mixed with hot, suffocating gases.
1. Plutonic flow
2. Pyroclastic flow 3. Lava flow
4. Basaltic flow
Chapter 18 Review Questions
Which area is surrounded by the Ring
of Fire?
1. The Atlantic Ocean
2. The United States
3. The Mediterranean Sea
4. The Pacific Ocean
Chapter 18 Review Questions
Which area is surrounded by the Ring
of Fire?
1. The Atlantic Ocean
2. The United States
3. The Mediterranean Sea
4. The Pacific Ocean
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Chapter 18 Review Questions
Which of the following is NOT true?
1. An increase in silica increases the
viscosity of magma.
2. Andesitic magma has both intermediate
gas content and explosiveness.
3. An increase in temperature increases
magma’s viscosity.
4. Basaltic magma has a low viscosity and
retains little gas.
Chapter 18 Review Questions
Which of the following is NOT true?
1. An increase in silica increases the viscosity
of magma.
2. Andesitic magma has both intermediate gas
content and explosiveness.
3. An increase in temperature
increases magma’s viscosity. 4. Basaltic magma has a low viscosity and
retains little gas.
Chapter 18 Review Questions
What causes magma to rise upward in a mantle plume?
1. The magma is less dense than the surrounding material.
2. The magma is denser than the surrounding material.
3. The magma is pulled upward by the air pressure.
4. The magma is pushed upward by the surrounding rock.
Chapter 18 Review Questions
What causes magma to rise upward in a mantle plume?
1. The magma is less dense than the surrounding material.
2. The magma is denser than the surrounding material.
3. The magma is pulled upward by the air pressure.
4. The magma is pushed upward by the surrounding rock.