How Magma Reaches the
Surface
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Magma forms in the (asthenosphere) mantle.
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Magma is less dense than solid material so it
rises toward the surface.
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When a volcano erupts, dissolved gases form
bubbles that rush out, taking the magma with
them (like opening a can of soda pop)
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the magma with them. Like When a volcano
Reaches Earths Surface
Inside a Volcano
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Pipe: a long tube in the
ground that connects the
chamber to the Earth’s
surface.
Vent: where magma, and gas
leave the volcano.
Lava flow: the river of lava
that comes out and over the
land.
Lava: Magma that reaches
the surface is called lava.
Crater: a bowl-shaped area
that forms around the vent.
Magma chamber: collects in
pockets beneath the
volcano.
Characteristics of Magma
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The force of an eruption depends on four things……
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Silica also determines how easily the magma flows.
More silica = thicker and lighter colored lava = sticky
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Amount of gas in magma
How thick or thin the magma is
Its temperature
Silica content
Makes obsidian and Rhyolite and pumice
Less Silica = darker colored lava + flows easily

Makes basalt
Types of Volcanic Eruptions

Quiet Eruptions
 Magma is thin and flows easily

Pahoehoe: is a fast-moving
hot lava, mass of wrinkles,
billows and ropelike coils
when it hardens.

AA: is a slow moving cool
lava, rough and jagged when it
hardens
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Explosive Eruptions- thick
magma, pipe gets plugged and
explodes when pressure builds
up.

Causes pyroclastic flow
explosive eruption hurls out
gas, ash (sand-like), cinders
(pebble sized), and bombs
(larger pieces, ranging from
baseball to car sized).
Stages of a Volcano
1.
2.
3.
Active or live volcano that is erupting or
shows signs that it erupt in the near
future.
Dormant or sleeping – scientists expect
the volcano to erupt in the future and
become active.
Extinct or dead is unlikely to erupt again.
Volcanic Landforms
Landforms from Lava & Ash
A. Shield Volcanoes
B. Cinder Cone Volcanoes
C. Composite Volcanoes
D. Lava Plateaus
E. Calderas
Shield Volcanoes

Thin layers of lava
pour out of a vent and
harden on top of
previous layers.
 Build a wide, gently
sloping mountain.
 Hawaiian Islands are
shield volcanoes over
a hot spot.
Mauna Loa, Hawaii
Cinder Cone Volcanoes

A steep, cone-shaped
hill or mountain.
 Ash, cinders, and
bombs pile up around
the vent.
 Cinders erupt
explosively.
 Sunset crater is an
example.
Composite Volcanoes
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A steep, cone-shaped
volcano built up of
layers and of rock
fragments.
Steep at the top,
flatens at the bottom
Magma- high in silica,
making it pasty.
Erupt explosively.
Mount St. Helens and
Mount Rainier are
examples.
 St.
Augustine volcano, Alaska.
Lava Plateaus
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Continued eruptions
form high, level
areas.
Lava flows out of
several long cracks,
and after millions of
years, these layers
form high plateaus.
Example: Columbia
Plateau
Columbia Plateau
Calderas
A
huge eruption may empty the
main vent and the magma chamber
beneath the volcano.
 Mountain becomes a hollow shell
and the top collapses in.
 Huge hole left is called a caldera.
 Example: Crater Lake, Oregon
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Poas Volcano,
Costa Rica
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Crater Lake,
Oregon
Volcano Hazards

Mount St. Helens erupted in 1980 after 123
years of being dormant. Before that people
viewed it as a peaceful mountain.
 Quiet eruption hazards- lava flows from vents,
burning everything in their path.
 Explosive eruption hazards- volcano will send
out burning clouds of volcanic gases, cinders
and bombs.
 Volcanic ash can bury entire towns, damage
crops and clog car engines. Wet ash can cause
roofs to collapse and plane engines to stall.