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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.