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Volcanoes
Sections 7.1, 7.2
Section 7.1
Volcanoes and Plate Tectonics
The asthenosphere has very high
temperatures, but because it has so much
pressure around, it remains rock.
 There are some areas where it melts to
form Magma or liquid rock.
 Any activity where magma moves toward
the surface is called volcanism.

Section 7.1
Volcanoes and Plate Tectonics
Magma slowly moves to the crust because
it is less dense than solid rock
 Most magma forms near the edges of
plates
 Magma that erupts to the earth’s surface
is called lava

Section 7.1
Volcanoes and Plate Tectonics
The opening where molten rock flows to
the surface is called a vent.
 The structure that forms and builds up
around a vent is called a volcano.

Section 7.1
Volcanoes and Plate Tectonics
Like earthquakes, most volcanoes occur
near both convergent and divergent
boundaries
 When there is a subduction of one plate
under another, the plate that goes
underneath will melt and create a volcano
about 100 miles inland

Section 7.1
Volcanoes and Plate Tectonics
The greatest amounts of magma comes to
the surface where the plates are moving
apart.
 Large volcanoes are formed here
 One of those is the country of Iceland
 Half of Iceland is moving toward Europe
and the other half is moving toward North
America

Section 7.1
Volcanoes and Plate Tectonics
Not all volcanoes form on plate boundaries
 Some form over Hot Spots.
 Hot spots are just pockets of magma in
the middle of a plate
 The Hawaiian Islands were formed this
way

Section 7.2
Volcanic Eruptions
There are 2 different types of lava.
 The first type is dark colored when it
hardens and is rich in magnesium and iron
 It is called mafic lava.
 This forms much of the oceanic crust.

Section 7.2
Volcanic Eruptions
The second kind of lava is lighter in color
and is made of silica and lesser amounts
of iron and magnesium
 It is called felsic lava
 Felsic lava found in continental crust

Section 7.2
Volcanic Eruptions
When mafic lava cools quickly, a crust
forms on top, but the bottom still flows.
The rock wrinkles forming pahoehoe
 If the rocks break into jagged chunks, it is
called aa
 Rounded blobs of hardened lava on the
ocean floor are called pillow lava

Section 7.2
Volcanic Eruptions

Lava that has a lot of trapped gases will
produce large violent eruptions out of
continental volcanoes.
(like Mt. St. Helens)

Oceanic Volcanoes produced less violent
volcanoes with very hot and thin mafic
lava (like Hawaii)
Section 7.2
Volcanic Eruptions
Felsic lava explodes out of a volcano
throwing Pyroclastic material
 Volcanic ash is small particles, they are
less than 2mm in diameter
 Volcanic dust is the smallest particles,
they are less than 0.25mm in diameter

Section 7.2
Volcanic Eruptions
Dust and Ash can be carried all around the
world by wind
 Large pyroclastic material up to 64mm in
diameter is called lapilli
 Sometimes lava gets thrown into the air
while it is still hot, that’s called a volcanic
bomb
 If it is solid when it is blasted, it’s called a
volcanic block

Section 7.2
Volcanic Eruptions
There are 3 kinds of volcanoes.
 Shield volcanoes are very wide
volcanoes that are made from layers of
lava
 Cinder Cones are taller but are made
from ejected pyroclastic material
 Composite Volcanoes are the tallest and
look like mountains. They are made with
layers of lava and pyroclastic material.

Section 7.2
Volcanic Eruptions
Craters are formed on volcanoes when a
large explosion blows the top off.
 If the magma chamber below a volcano is
empty, the volcano can fall in. The hole
that is left in the ground is called a
caldera.
