Inside Earth
Chapter 3: Volcanoes
Section 3-1
Volcanoes and Plate Tectonics
Sec 3-1
 Volcano—a weak spot in the crust where magma has
come to the surface
 Magma—the molten mixture of rock-forming
substances, gases, and water from the mantle
 Lava—liquid magma that reaches the surface; also the
rock formed when liquid lava hardens
 Ring of Fire—a major belt of volcanoes that rims the
Pacific Ocean
 Island arc—a string of islands formed by the volcanoes
along a deep ocean trench
 Hot spot—an area where magma from deep within the
mantle melts through the crust above it
Sec 3-1: Volcanoes and Plate Tectonics
What is a Volcano?
Lava
Volcano
Magma
A weak spot
in the crust
where molten
material, or
magma,
comes to the
surface.
A molten
mixture of
rock-forming
substances,
gases, and
water from
the mantle.
What magma
that reaches
the surface is
called.
Forms solid rock.
Builds up Earth’s
surface.
Forms new islands
and adds new
rock.
3-1…cont
About 600 active
volcanoes on
land
Location of
Volcanoes
Along diverging
plate boundaries
(mid-ocean
ridge), or in
subduction zones
Major volcanic belt that is
formed by the many volcanoes
that rim the Pacific Ocean
Ring of Fire
Volcanic belts form along the
boundaries of Earth’s plates.
Some volcanoes form at “hot
spots” away from boundaries.
3-1…cont
Volcanoes at
Diverging Plate
Boundaries
Form along the mid-ocean ridge
and in a few places (Iceland and
the Azores Islands in the Atlantic
Ocean). Volcanoes rise above
the surface.
3-1…cont
Volcanoes at
Converging
Boundaries
Happens when oceanic
and continental plates
collide and form deepocean trenches. Crust
sinks, forms magma and
erupts as lava to form
volcanoes.
String of islands caused
by volcanoes. Curve of
an island arc echoes the
curve of its deep-ocean
trench.
Island arc
3-1…cont
An area where magma from deep
within the mantle melts through the
crust.
Hot Spot
Volcanoes
A hot spot in the oceanic floor can
gradually form a series of volcanic
mountains.
Hot spots can also form under
continents.
Yellowstone
Section 3-2
Volcanic Activity
Section 3-2: Volcanic Activity
Objectives
 Students will be able to describe what happens when a
volcano erupts.
 Students will be able to explain how the two types of
volcanic eruptions differ.
 Students will be able to identify some hazards of
volcanoes.
 Students will be able to identify types of volcanic
activity other than eruptions.
Section 3-2: Volcanic Activity
 Magma chamber—the pocket beneath a volcano where
magma collects
 Pipe—a long tube through which magma moves from
the magma chamber to Earth’s surface
 Vent—the opening through which molten rock and gas
leave a volcano
 Lava flow—the area covered by lava as it pours out of
a volcano’s vent
 Crater—a bowl-shaped area that forms around a
volcano’s central opening
Section 3-2: Volcanic Activity
 Silica—a material that is formed from the elements
oxygen and silicon; silica is found in magma
 Pahoehoe—a hot, fast-moving type of lava that
hardens to form smooth ropelike coils
 Aa—a slow-moving type of lava that hardens to form
rough chunks; cooler than pahoehoe
 Pyroclastic flow—the explusion of ash, cinders, bombs,
and gases during an explosive volcanic eruption
 Active—said of a volcano that is erupting or has shown
signs of erupting in the near future
Section 3-2: Volcanic Activity
 Dormant—said of a volcano that does not show signs
of erupting in the near future
 Extinct—said of a volcano that is unlikely to erupt again
 Hot spring—a pool formed by groundwater that has
risen to the surface after being heated by a nearby
body of magma
 Geyser—a fountain of water and steam that builds up
pressure underground and erupts at regular intervals
 Geothermal energy—energy from water or steam that
has been heated by magma
3-2: Volcanic Activity
Magma Rises
Magma rises until it
reaches the surface
or a solid layer of
rock.
Liquid magma is less
dense than the
surrounding
materials.
How Magma Reaches
Earth’s Surface
A Volcano
Erupts
The dissolved gases
trapped in magma are
under great pressure.
As magma rises to
the surface, the
pressure decreases
and the gases form
bubbles.
During a volcanic eruption, the gases
dissolved in magma rush out, carrying the
magma with them.
3-2…continued
Inside a Volcano
Magma chamber-a pocket of magma that collects
underneath a volcano.
Pipe—a long tube in the ground that connects the
magma chamber to Earth’s surface.
Vent—an opening in a volcano where molten rock
and gas escape
Lava flow—area covered by lava as it pours out of
a vent
Crater—a bowl-shaped area that may form at the
top of a volcano around the central vent.
3-2…continued
The forces of a
volcanic eruption
depends partly on the
amount of gas
dissolved in the
magma.
Thick or thin,
temperature, and
silica content are
important factors.
Silica
Characteristics of
Magma
Rocks
formed
High—rhyolite, pumice,
obsidian
Obsidian—lava cools quickly
Pumice—gas bubbles are
trapped in lava
Low--basalt
A material that’s
formed from the
elements of oxygen
and silicon.
One of the most
abundant materials in
Earth’s crust and
mantle.
More silica = thicker.
Silica
levels
High—light-colored lava
that’s too sticky to flow very
far.
Low—flows readily and
produces dark-colored lava
3-2…continued
The silica content of
magma helps to
determine whether
the volcanic eruption
is quiet or explosive.
Types of Volcanic
Eruptions
Quiet Eruptions
Magma flows easily.
The lava can flow
many kilometers
from the volcano’s
vent.
Aa: cooler and
slower-moving
Forms a rough
surface consisting of
jagged lava chunks.
Pahoehoe: fast-moving, hot
lava.
Forms a solid mass of
wrinkles, billows, and ropelike
coils.
Explosive Eruptions
Thick magma builds up in the
pipe and ends up exploding
out.
Breaks the lava into pieces—
ash, cinders, and bombs.
Pyroclastic flow
Occurs when an
explosive eruption hurls
out ash, cinders, and
bombs as well as
gases.
3-2…continued
Stages of a Volcano
Active
A volcano that is
erupting or has
shown signs that it
may erupt in the near
future.
Dormant
Sleeping or
hibernating.
Expected to wake up
in the future.
Extinct
Dead: volcano that is
unlikely to erupt
again.
3-2…continued
Does not
involve the
eruption of lava.
Hot springs
Forms when
groundwater
heated by a
nearby body of
magma rises to
the surface and
collects in a
natural pool.
Other Types of Volcanic
Activity
Geyser
A fountain of
water and
steam that
erupts from the
ground.
Geothermal energy
Water heated by
magma that
provides a clean,
reliable energy
source.
Used as a source of
electricity and to
heat homes.
3-2…continued
Monitoring Volcanoes
Methods
Tiltmeter,
laser-ranging
devices, and
other
instruments.
Monitor
Magma moves
underground and
causes elevation
and tilt to change.
The local magnetic field,
water level in a volcano’s
crater lake, and any gases
escaping from a volcano.
Temperature of underground
water.
Many small earthquakes
caused by movement of
magma into the magma
chamber and pipe.
3-2…continued
Volcano Hazards
Although quiet eruptions and explosive eruptions involve different volcano
hazards, both types can cause damage far from the crater’s rim.
Quiet Eruption
Lava flows pour
from vents,
setting fire to and
then burying
everything in
their path.
Explosive Eruption
A volcano can
belch out hot,
burning clouds of
volcanic gases
as well as
cinders and
bombs.
Volcanic Ash
Can bury entire
towns, damage
crops, and clog
car engines.
Ex.—Pompeii
Section 3-3
Volcanic Landforms
Section 3-3 Objectives
 Students will be able to identify the landforms
that lava creates on Earth’s surface.
 Students will be able to explain how magma
hardens beneath the surface creates
landforms.
Section 3-3: Volcanic Landforms
 Shield volcano—a wide, gently-sloping mountain
gradually built by lava flows and formed by quiet
eruptions
 Cinder cone—a steep, cone-shaped hill or mountain
made of volcanic ash, cinders, and bombs piled up
around a volcano’s opening
 Composite volcano—a tall, cone-shaped mountain I
which layers of lava alternate with layers of ash and
other volcanic materials
 Caldera—the large hole at the top of a volcano formed
when the roof of a volcano’s magma chamber
collapses
Section 3-3: Volcanic Landforms
 Volcanic neck—a deposit of hardened magma in a
volcano’s pipe
 Dike—a slab of volcanic rock formed when magma
forces itself across rock layers
 Sill—a slab of volcanic rock formed when magma
squeezes between layers of rock
 Batholith—a mass of rock formed when a large body of
magma cooled inside the crust
3-3 Volcanic Landforms
Shield
Volcanos
Cinder Cone
Volcanoes
Landforms
from Lava
and Ash
Thin layers of lava pour out of a vent
and harden on top of previous layers;
builds a wide, gently sloping mountain
Hawaiian
Islands
A steep, cone-shaped hill or
mountain; if lava is thick and stiff, it
may produce ash, cinders, and bombs
that pile up around the vent.
Paricutin,
Mexico
400 m high
Composite
Volcanoes
Lava
Plateaus
Calderas
Tall, cone-shaped mountains in
which layers of lava alternate
with layers of ash
Flows of lava that spread out on top of
other layers; lava flows out of several
long cracks and spreads out
Mountain is empty and top
collapses inward
Mt. St.
Helens,
Washington
Columbia
Plateau;
Washington,
Oregon, & Idaho
Crater Lake,
Oregon
3-3…continued
Soils from Lava and Ash
Originally
barren
Lava, ash,
and cinders
Hard surface
breaks down
to form soil
Breaks down
into soil
Richest soils in
the world
Releases
potassium,
phosphorus, and
other materials
that plants need
Dome Mountains
3-3…continued
Batholiths
Mass of rock
formed when a
large body of
magma cools
inside the crust
Sierra Nevada
Mts., California
Landforms from Magma
Volcanic Necks, Dikes, and Sills
Volcanic Necks
Magma
hardens in
a
volcano’s
pipe;
softer rock
wears
away
Dikes
Hardened
magma tha
forced itself
across rock
layers
Sills
When
magma
squeezes
between
layers of
rock
Formed when
rising magma is
blocked by
horizontal
layers of rock;
magma forces
the layers of
rock to bend
upward into a
dome shape
Black Hills,
South Dakota
Section 3-4
Volcanoes in the Solar System
Section 3-4 Objectives
 Students will be able to explain how volcanoes on Mars
and Venus compare with volcanoes on Earth.
 Students will be able to describe the volcanic activity
found on the moons of Jupiter and Neptune.
3-4: Volcanoes in the Solar System
Darker Area
Unusually smooth; lava
flows; more than 3 billion
years ago
Earth’s
Moon
Lightcolored
Crater
Mark where meteorites have
smashed into the moon over
billions of years
3-4…continued
Signs of widespread
volcanic activity
Largest volcano on
Venus
Thousands of
volcanoes
Volcanoes on
Venus
Made of thin,
runny lava
Theia Mons
800 km
across and 4
km high
150 large volcanoes
measuring between 100
and 600 km across and
about ½ a km high
Gently sloping shield
volcanoes with broad
bases
Long, riverlike lava
flows
More than 6,800 km
long
3-4…continued
Long history of
volcanic activity
Volcanoes are
found in only a
few regions on
Mars
Large shield
volcanoes
Olympus Mons
Volcanoes on
Mars
Variety of volcanic
features
Lava
flows
Cone-shaped
volcanoes
Biggest
volcano on
Mars is the
largest
mountain in
the solar
system
Shield
volcano
similar
to
Mauna
Loa;
covers
an area
as large
as Ohio
Over 8
times
taller
than
Theia
Mons
on
Venus
3-4…continued
Volcanic features are very different from those on Earth,
Mars, and Venus
Volcanoes on
Distant
Moons
Io
Volcanic eruptions have
been observed
A moon of
Jupiter
Sulfur
volcanoes
Erupt like fountains or spread out
like umbrellas above the colorful
surface
A moon of
Neptune
Triton
Nitrogen
Most is frozen solid; some is melted
by absorbing heat from the sun;
expands and erupts through the icy
crust