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Volcanoes
• Volcano
• -weak spot in crust
where magma comes
up
• Magma
• -molten mixture of
rock-forming substance,
gases, water from the
mantle
• -called lava on surface
• -lava forms solid rock
when cooled
• Volcanic belt
• -forms along plate
boundaries
• -magma can reach
surface when plates
diverge/converge and
fracture crust
• Ring of Fire
• -major volcanic belt
• -rims (circles) Pacific
Ocean
• Island Arc
•
• -string of volcanic
islands
• -formed from
subduction at
converging oceanic
plates
• -ex. Japan, Caribbean
Islands
• Hot Spots
• -where magma melts
thru crust
• -found in middle of
plates and along
boundaries
• -volcanoes can form
above hot spot
• -ex. Hawaiian Islands
Properties of Magma
• Viscosity
• -resistance of a liquid to
flow
• -greater viscosity,
slower it flows
• Viscosity of Magma
• -depends on silica
content and
temperature
• Silica Content
• -made of oxygen and
silicon
• -one of the most
abundant materials in
crust
• -more silica = higher
viscosity (sticky, flows
slow)
• -less silica = low
viscosity (flows quickly)
• Temperature
• -viscosity increases as
temp decreases
• -hotter magma flows
faster
• -cooler flows slower
Volcanic Eruptions
• Magma reaches the
surface
• -materials in
asthenosphere under
great pressure
• -liquid magma less
dense than solid
material around it
• -flows upward until
opening in rock allows
to magma to reach
surface
• Inside a Volcano
• Magma Chamber
• -pocket that collects
magma
• Pipe
• -tube that connects
chamber to surface
• Vent
• -opening thru which
molten rock and gas
leave
• -central vent at top
• -sometimes, side vents
on sides
• Lava Flow
• -area covered by lava as
pours out of vent
• Crater
• -bowl-shaped area that
may form at top around
central vent
• Eruption
• -gases in magma under
lots of pressure
• -gases expand and form
bubbles, as magma
rises to surface
• -when erupts, force of
gas pushes magma from
chamber thru pipe until
it flows or explodes out
of vent
• Quiet Eruptions
• -low silica; low viscosity;
flows easily
• -oozes out quietly and
flows for many km
• -sets fire to and buries
everything in path
• -produces both
pahoehoe & aa
• -ex. Hawaiian islands
• Explosive Eruptions
• -high in silica; high
viscosity; thick/sticky
• -builds up in pipe; plugs
it like a cork
• -trapped gases build up
pressure until they
explode
• -breaks lava into fast
cooling pieces
• -volcanic ash: smallest;
fine, rocky particles
• -cinders: pebble-sized
• -bombs: large pieces;
baseball-sized to size of
car
• -ex. Mount St. Helens
• Pyroclastic Flow
• Life Cycle
• -explosive eruption
hurls out mixture of hot
gases, ash, cinders,
bombs
• -active: live; erupting or
showing signs of
erupting in near future
• -dormant: sleeping;
expected to awaken in
future and become
active
• -extinct: dead; unlikely
to erupt again
• Shield Volcanoes
• -wide, gently sloping
mountain made of
layers of lava from quiet
eruptions
• -ex. Hawaiian islands
• Cinder Cone Volcano
• -steep, cone-shaped hill
or small mountain
made of volcanic ash,
cinders, bombs piled up
• -ex. Paricutin
• Composite Volcanoes
• -tall, cone-shaped
mountain in which
layers of lava alternate
with layers of ash/other
materials
• -ex. Mount St. Helens
• Calderas
• -huge hole left by a
collapsing volcano
• -enormous eruptions
empty the vent and
chamber, leaving it
hollow with no support
• -sometimes fill with rain
water
• -ex. Crater Lake in
Oregon