Download section 12.2

The Changing Earth
Chapter Twelve: Earthquakes
• 12.1 Earthquakes
• 12.2 Volcanoes
Investigation 12B
Volcanoes
• How are volcanoes and plate boundaries related?
12.2 Looking inside a volcano
• A volcano is a site
where melted rock and
other materials from
Earth’s mantle are
released.
• Mount St. Helens is a
type of volcano called a
composite volcano
(also known as a
stratovolcano).
12.2 Looking inside a volcano
• Melted rock within Earth’s crust called
magma leaves the magma chamber (where
magma is stored) and moves up the conduit
during an eruption.
• The magma leaves at an opening known as
a vent either gently or with violent force.
• Magma may move upward in a sheet, which
is called a dike.
• Magma is called lava after it leaves the vent.
12.2 Volcano features after an
eruption
• The bowl-shaped
depression formed
by the collapse of a
volcano after it has
erupted is known
as a caldera.
12.2 Volcano features after an
eruption
• If magma flows
back up the
conduit, a mound
called a
resurgent dome
may form on the
caldera floor.
12.2 Volcano features after an
eruption
• If water fills the
caldera, or
magma doesn’t
drain
completely, a
lava lake
remains.
12.2 The life of a volcano
• Active volcanoes are erupting or have
erupted recently, and are expected to
erupt again in the near future.
• Dormant volcanoes are not active now,
but may become active again in the
future.
• Extinct volcanoes are at the end of their
life and are no longer able to erupt.
12.2 The life of a volcano
• Devil’s Tower and
Ship Rock are
examples of extinct
volcanic “necks”.
As the volcano erodes once
it becomes extinct, a core
of solid magma gets
exposed.
12.2 Where does magma come
from?
• There are two ways to
make rock melt.
• One way is to reduce
the pressure.
• Rock under low
pressure melts at a
lower temperature.
12.2 Where does magma come
from?
• The other way is to
mix water with the
hot rock.
• Rock that
contains water
melts at a lower
temperature.
12.2 Types of magma
•
•
•
Magma with low
amounts of silica makes
runny magma.
Magma with a lot of silica
makes thick, sticky
magma.
Volcanoes near
subduction zones tend
to form magma high in
silica.
12.2 Types of magma
• Another important property of magma is
how much gas is dissolved in it.
• High gas content in magma results in more
explosive eruptions.
12.2 Volcanoes with low silica
magma
• Low silica magma can’t build up tall
sides and produces a shield volcano.
12.2 Volcanoes with low silica
magma
• Because low-silica
magma is runny, it
can’t build up a tall,
cone-shaped
volcano.
12.2 Volcanoes with low silica
magma
•
•
When low silica magma has
high levels of dissolved gas,
gas bubbles out as it reaches
the volcano vent. A fire
fountain is produced.
Imagine shaking a soda bottle
to produce a shower of soda.
12.2 Cinder cones
•
•
The lava cinders
form a cone
around the vent
called a cinder
cone.
Cinder cones are
a common form
of volcano.
Cinders are loose clumps of
particles of lava.
12.2 Volcanoes with high silica
magma
•
A tall cone, or
composite volcano
forms from thick,
sticky magma that
piles up near the vent
because it is silicarich.
12.2 Volcanoes with high
silica magma
• Before volcano
eruption, the magma
may be under so
much gas pressure
that the volcano
cone bulges.
12.2 Volcanoes with high
silica magma
• The cone may explode near the vent,
throwing a column of gas and lava bits high
into the atmosphere.
• The lava bits puff up and rip apart as the
dissolved gas expands inside each bit.
12.2 Explosive eruptions
• When a column of exploding material collapses, it
races down the side of a composite volcano as
a pyroclastic flow.
• The speed force, and heat of the flow make it
extremely destructive.
12.2 Explosive eruptions
• If water is present in the
ground, mudflows called
lahars may accompany a
composite volcano
eruption.
• The mudflows, called
lahars can destroy
forests and property and
add to the death toll.
12.2 Volcanoes divergent
boundaries
What causes lava to melt?
• Mid-ocean ridges occur
underwater at diverging
plate boundaries.
• When lava oozes out at
a mid-ocean ridge, it
immediately hits cold
seawater, forming a
crust.
12.2 Volcanoes at mid-ocean
ridges
• Basalt magma is
runny because of its
low silica content.
• The seawater cools the lava,
forming a crust. The crust
cracks and another blob of
basalt magma oozes out.
When this cycle repeats over
and over, the lava looks like
a pile of pillows.
12.2 Volcanoes at convergent
boundaries
• About half of the
active surface
volcanoes on Earth
occur along the
shores of the
Pacific Ocean. This
region is called the
Ring of Fire.
What is the Ring of Fire?
12.2 Volcanoes at convergent
boundaries
• The Ring of Fire is found
where the oceanic crust of the
Pacific Plate is subducting
under nearby plates.
• Most volcanoes are located
along plate boundaries.
12.2 Volcanoes on continents
• The famous granite domes of Yosemite
National Park in California were formed as
silica-rich magma rose through the edge of
the North American Plate.
• The surrounding land later eroded away,
exposing granite domes.
12.2 Volcanic islands away from
plate boundaries
• Volcanic island
chains are formed
as lithospheric plates
move over the top of
plumes that are
known as hot spots.
(such as Hawaii)
12.2 Volcanic chains
1. As the plate moves, it carries the volcanic
island away from the active hot spot.
2. Without the hot spot to supply magma, the
volcano becomes extinct.
3. The hot spot begins to form a new volcano
beside the old one.
4. The result is a volcanic island chain.
12.2 Volcanic chains
12.2 Volcanic chains
• The island of Hawaii sits
on top of a hot spot.
• The hot spot has formed
the Mauna Loa and
Kilauea volcanoes on
the island.
• Currently, the hot spot is
making the undersea
volcano Loihi to the
A hot spot is making Loihi
southeast of the island.
bigger, so Hawaii is growing.
12.2 Measuring motion of a plate
• By studying a
volcanic island
chain, scientists
can determine the
direction and
speed that a plate
is moving.
Geology Connection
Indian Ocean Earthquake and Tsunami
• Tilly Smith probably never imagined what she
learned in geography class would help save lives.
Activity
The Dragon and Toad Mystery
• In this activity, you will
learn something about
the history of using
seismographs.