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Chapter 14
BFRB Pages 220-222
VOLCANIC
ERUPTIONS
• Rocks melt underground where
TEMPERATURE & PRESSURE are great
• Since the melted rock is slightly less
dense then the surrounding rock, it
begins to rise to the surface (it may
travel through cracks in the rock, or
melt its way upward)
• When the magma reaches the surface, it
may erupt out of the ground through an
opening called a volcano
Volcanoes take on
the characteristic
cone shape as lava
and ash pile up
around the opening
in the ground over a
long period of time
FELSIC MAGMA
• Contains a lot of silica
• Lighter in color, slow moving and very
thick
• Moves slowly out of the volcano
• This allows pressure from gases to build
up (think of shaking up a bottle of soda),
causing very violent explosions – very
little lava flowing, mostly ash, dust and
hot gases
Mt. St.
Helens was
a FELSIC
magma
eruption VERY
VIOLENT
and VERY
EXPLOSIVE!
Animation of a
FELSIC volcano
erupting
MAFIC MAGMA
• contains very little silica
• magma/lava darker in color, faster
moving, and very thin
• These volcanoes tend to have less
violent eruptions, since the lava
easily flows out of the volcano
• The eruptions result mostly in fast
moving lava flows
Mt. Etna – Sicily, Italy
Stromboli – see the lights of the city in
the background of the “river of lava”?
Kilauea – Pahoehoe Lava Flow
“Toe” like lava
“Rope” like lava
Types of
Eruptions
I Rift Eruptions
• occur at narrow cracks (fractures) in the
Earth’s crust
• can occur on land, or on the ocean floor
(at spreading centers – ex. mid-Atlantic
Ridge)
• contain mafic magma that flows out
smoothly and quickly
• resulting rock (when the magma/lava
cools) is usually the igneous rock basalt
Icelandic Rift Eruption
Underwater Rift Zones
(Mid-Atlantic Ridge to
be exact!)
Ancient Rift Zone – note the BASALT on either side!
II Subduction Boundary
Eruptions
• occur when magma is formed by the
melting rock at subduction plate
boundaries
• magma tends to felsic, which makes it
very thick – causing violent eruptions
• Most of the world’s active volcanoes
occur along subduction plate
boundaries
Chain of Subduction
Boundary Volcanoes
along the Andes
Mountains in South
America (as viewed
from space – note
that the white areas
are snow peaks)
The Cascade
“Mountain”
Range – in
reality a range of
“VOLCANOES”!
III Hot Spot Eruptions
• occur in the middle of tectonic plates
where there is a concentration of heat
• magma melts its way through the plate
and pours out onto the surface
• magma is usually mafic, so it easily
flows out of the ground
• the hot spot remains in one location, as
the plate moves above it
• results in a chain of volcanoes
• the only active volcano will be the one
above the hot spot
• the direction of the chain indicates the
direction the plate is moving
• if the hot spot is on the ocean floor, a
chain of volcanic islands will form (ex.
Hawaiin Islands chain)
The next Hawaiian island that will come into being - already
named Loihi - is building on the seafloor southeast of Kilauea. It’s
top is 1000 m (3000 feet) below the water surface, and it will
break the surface in the next 10,000 to 100,000 years.
The shoreline is the boundary between yellow and green and is
labeled. Purples are the deepest water. Light reds are the highest
peaks on the island.
Direction of Plate
movement as
indicated by the
Hawaiian Hot Spot
Note that the elevation of “Hawaii” is greater because
of the hot spot supplying new rock. The older islands
are “sinking” into the ocean because the rate of
erosion supercedes the rate of formation (which is
zero because they are off the hot spot)
Galapagos Islands off the coast of western South America are an
another example of Hot Spot island formation.
See the volcanic “mountain peaks”?
CALDERAS
• an erupting volcano empties a shallowlevel magma chamber & the edifice of
the volcano collapses into the voided
reservoir, thus forming a steep, bowlshaped depression called a caldera
Ancient
volcanic
opening
Crater Lake, Oregon is one of the USA’s
most famous Calderas. It is part of the
Cascade volcanic mountain range. With
a water depth of 600 m, Crater Lake is
the deepest fresh-water lake in North
America. This large depression formed
from the violent eruption and collapse
of the ancestral volcano Mt. Mazama
about 6850 years ago!
Yellowstone Caldera
• Yellowstone National Park sits on a high
plateau supported by a hot mantle
plume. This hot-spot has been the site
of three caldera-forming eruptions in
the last two million years. The last
eruption was ~630,000 years ago and
the tephra (ash)fell as far away as
Louisiana!
• This hot mantle plume is what fuels all
of the hot springs and geysers in the
park – including Old Faithful.
Krakatoa,Indonesia
(1883)
• In 1883, after over 200 years of calm,
the massive volcano under the island of
Krakatoa exploded in one of the worst
eruptions in modern history.
• 2/3 of the island exploded and sank into
the caldera formed
• Over 36,000 people died and over 150
coastal cities were completed destroyed
by the Tsunami that was created by the
violent eruption
Devil’s Tower Revisited
The Legend of Devil's Tower
Devil's Tower, Wyoming, is a
volcanic plug or neck. In Kiowa
Indian mythology it was said
that:
Once upon a time seven little
girls were playing in the woods
far from home and they came
upon some bears that chased
after them. They found refuge
on a great rock, that rose into
sky with them on it, making
them into stars. The bears tried
to pursue them, but all their
efforts were in vain. You can
still see the struggle and their
claw marks in the rock that
makes up Devil's Tower.