Transcript Chapter 4 Lesson 2

Students will be
able to explain
why a volcano
erupts and
describe how
volcanoes build
land.
Chapter 4
Lesson 2
Volcanoes
Kilauea Volcano, Hawaii
How can
scientists
predict
whether a
volcano
will erupt?
Where are volcanoes found?
 Volcanoes
form on land and on the ocean floor.
A volcano is an opening in Earth’s crust. They are
only located in certain places on Earth’s surface.
Most volcanoes are found where plates meet.
 The Ring of Fire – surrounds the Pacific Ocean. It
follows the boundaries of the plates that meet
around the Pacific Ocean. Volcanoes are more
likely to erupt at plate boundaries than anywhere
else on Earth. An eruption is an outpouring of
melted rock, ash, gases, or a combination of
these.
Brain pop https://www.brainpop.com/sc
ience/earthsystem/volcanoes/
Where are volcanoes found?
 Volcanoes
do not erupt at all plate boundaries.
Scientists conclude that volcanoes tend to erupt
where one plate is pushed under another plate.
When rocks in the plate that is being pushed
down reach the heat and pressure in the mantle,
they melt. Magma forms and pools in a chamber
underneath the crust. Sometimes a crack forms
above the chamber or the pressure in the
chamber grows too great and the magma rushes
upward toward the surface.
 All
volcanoes have at least one vent, or opening.
Once magma reaches Earth’s surface, it is called
lava. Over time, a cup-shaped depression may
form around a vent. This depression is called a
crater. Most craters are found at the top of a
volcanic mountain. Sometimes the magma
chamber beneath a volcano is emptied. The
volcano may then collapse inside itself. The hole
that forms is called a caldera.
Galapagos Islands - caldera
How does magma form
geological features?
 Dike
– when magma hardens in vertical or
nearly vertical cracks
How does magma form
geological features?
 Sill
– forms when magma hardens
between horizontal layers of rock. Dikes
and sills can be large or small.
 Laccolith
– When the magma pushed into
a sill does not spread horizontally. Instead
it pushes upward and forms a dome
shape. When a laccolith forms, it may
raise the rock layers above it.
 Batholith
– the largest and deepest of all
underground magma formations – huge
and irregularly shaped. It reaches deep
into the crust.
 When
lava comes out of a vent, it is liquid.
The lava cools to form a solid layer of rock
as it hardens. Over thousands of years,
layers of lava may increase the height of
a volcano and form a volcanic mountain.
The mountain is new land that the
volcano has built.
Volcanoes






Active volcano – currently erupting
Dormant volcano – has not erupted for some time
Extinct volcano – a volcano that does not erupt
anymore
Shield volcano – built by thinner fluid lava that spreads
over a large area. These mountains have a broad base
and gently sloping sides.
Cinder-cone volcanoes – built by thick lava that is
thrown high into the air and falls as chunks or cinders.
These mountains form as a cone shape with a narrow
base and steep sides.
Composite volcanoes – built by layers of ash and
cinders sandwiched between layers of hardened lava.
The shape on one side of a cone formed by a
composite volcano usually looks the same as the shape
on the opposite side.
Shield volcano
Cinder-cone volcano
Composite volcano
Review
 How
might a cone-shaped mountain form?
 It can form from rock fragments (a cinder-cone
volcano) or from alternating layers of rock
fragments and lava(a composite volcano).
 Explain
an extinct, dormant, and active
volcano.
 Extinct
– no longer erupting, dormant – is not
erupting at this time, active – erupting at the
present or recently
How do volcanoes build islands?
 The
Hawaiian Islands are an island chain, or a line of
volcanic mountains. Scientists know that the islands
rest on a slowly moving tectonic plate. As it moves,
the plate passes over a stationary pool of magma
called a hot spot. Over years the lava erupting
formed a mountain. Eventually the mountain grew
taller than the ocean’s surface and became a
volcanic island. As the plate moved, that island
moved away from the hot spot and a new island
began to form. In areas where an ocean-floor plate
is pushed under another ocean-floor plate, an
island arc forms. As it is pushed down, it melts.
Magma forms, rises upward, and erupts through the
ocean floor. This forms a series of volcanic islands.
Review

What information would you need to figure
out if islands formed from a hot spot?

You would need to know whether all the
islands formed form volcanoes, if any of the
islands had an active volcano, and the ages
of the islands to determine if the extinct
volcanoes where moving away from the
active volcano, which is where the hot spot
would be.
Review - games
 http://discoverykids.com/gamesvolcano-
explorer/
 http://www.neok12.com/Volcanoes.htm
 http://www.scholastic.com/play/prevolc
ano.htm