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Volcanoes: The Fire Within
Chapter 9: Volcanoes
What is a Volcano?
Kilauea
• A vent that lets out heat
from inside the Earth ,
spewing out lava and
eventually forming a
mountain.
• 3 classifications of volcanic
activity: extinct (does not
erupt), dormant (sleeping),
and active (currently
erupting).
• The most active volcano on
the Earth is Kilauea on the
big island of Hawaii because
it has been erupting almost
daily since 1983!
Where Are They Found?
• Volcanoes are found wherever
plates are being subducted or
spreading apart.
• At these locations, pressure
and heat build up and melt the
surrounding rock.
• Since magma is less dense than
solid rock, it will find any little
crack in the crust to squeeze
into to relieve the pressure.
• Any place there are lots of
earthquakes, you will usually
find active volcanoes.
• You will not find volcanoes at
transform boundaries!
Converging and Diverging
• Converging: where
subduction is happening, like
around the Ring of Fire. The
more dense oceanic plate
gets re-melted and creates
volcanoes along coastlines
(Andes Mountains)
• Diverging: where plates are
spreading apart. Along the
mid-ocean ridges, lava pours
out and gets immediately
cooled by the ocean water.
(Mid-Atlantic Ridge exposed
in Iceland)
Hawaii: A Tropical Paradise
• Sometimes, very hot parts of the
mantle actually burn holes through
the crust. These are called hot
spots.
• Hot spots are located in the middle
of tectonic plates.
• Over time, as the plate moves, the
“hot spot” will burn new holes
through the crust, creating new
places of volcanic activity along the
way.
• Two examples: Hawaii (in the middle
of the Pacific plate) and
Yellowstone National Park (home of
Old Faithful, and located in the
middle of the North American
plate)
Old
Faithful
Where Are They Most Violent?
• Subduction zones produce
the most violent volcanic
eruptions.
• Why? The oceanic plate is
water-logged. Therefore,
when it is subducted, the
water turns to steam (which
is a gas). This steam gets
trapped in the magma, and
then explodes out of the
volcano!
• Just think about what you
feel like when you have gas.
Doesn’t it feel better when
you let it go? Well, so does
the volcano!
Types of Lava
• Basaltic: flows very easily and has
a low silica content. Usually, this
type is associated with nonexplosive eruptions. Hot spots
and rift zones have this type of
lava.
• Hawaiians call basaltic lava “aa”
which has sharp edges when it
cools, or “pahoehoe” which they
also call “grandma lava” because it
is wrinkly when it cools.
• Granitic: very thick, high silica
content, associated with very
violent eruptions, like around the
Ring of Fire
• Examples are Mt. St. Helens, Mt.
Rainier, Mt. Fuji, Mt. Pinatubo
Pahoehoe
Aa
Pyroclastic flow during the
eruption of Mt. Pinatubo in
the Philippines
Types of Volcanoes
• Shield: gently sloping sides,
basaltic lava, quiet eruptions
(but destructive!), example is
Hawaii.
Mt. Fuji in Japan
• Cinder Cone: cone-shaped,
granitic lava, violent eruptions,
throw pyroclastics into the air
(ash, bombs, tephra), example
is Paricutin in Mexico.
• Composite: tall, steep sides,
grantitic and basaltic lava,
violent and quiet eruptions,
examples are ALL volcanoes
along the Ring of Fire. Mt. St.
Helens, Pinatubo, Fuji and
some in Europe like Vesuvius,
Etna, Stromboli.
Mauna Loa in Hawaii
Paricutin in Mexico
Mt. St. Helens in Washington
Intrusive Features
• Sometimes magma doesn’t make it out of a volcano
and gets trapped below ground.
• We can only see these features due to weathering
and erosion.
• 5 main features: sill, neck, dike, laccolith, batholith