Transcript Earth Science 10.1 Volcanoes and Plate Tectonics

Earth Science 10.1 Volcanoes and Plate Tectonics
Volcanoes
and
Plate
Tectonics
Origin of Magma
Starting in 1999, the 5000 meter high
Tungurahua volcano in Ecuador began to
erupt. It spewed chunks of lava,
volcanic ash, and gases into the sky.
Than on August 2006, streams of hot gas
and rock exploded down the volcanoes
sides. Villagers fled as ash buried their
homes and fields.
Tungurahua volcano is one of the 800
active volcanoes on average that occur
on Earth each year.
Origin of Magma
Recall that magma is molten rock from
beneath Earth’s surface.
Magma is a complex mixture that contains
partly melted mineral crystals,
dissolved gases, and water.
Magma forms in the crust and upper
mantle when solid rock partially melts.
The formation of magma depends on
several factors; including heat,
pressure and water content.
Origin of Magma: Heat
What source of heat is sufficient to melt
rock?
At a depth of 100 kilometers, the
temperatures of the mantle ranges
between 1400 degrees centigrade and
1600 degrees centigrade.
At these temperatures, the solid rock of
the lower crust and upper mantle is
close to it’s melting point.
Origin of Magma: Heat
The additional heat needed to produce
magma comes from three sources
 Friction generated by slabs of
lithosphere sliding past each other
 The mantle itself heats the
subducting slabs
 Hot mantle rock rising into the
cooler lithosphere
Origin of Magma: Pressure
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We have learned that pressure
increases with increased depth inside
the Earth.
Increasing pressure raises the melting
point of rock deep inside the Earth.
Decreasing pressure, decreases rock’s
melting point.
When pressure drops enough;
decompression melting occurs.
Example: As hot yet solid mantle rock
rises, the pressure on the rock
decreases. As the decreasing pressure
lowers the rock’s melting point, pockets
of hot magma form.
Origin of Magma: Water Content
Water Content:
 The water content of rock also lowers
the rock’s melting point.
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Because of the, “wet rock” deep
beneath the surface melts at a much
lower temperature than does “dry rock”
of the same composition and under the
same pressure.
Lab studies have shown that the
melting point of basalt can be lowered
by up to 100 C by adding only 0.1
percent water.
Volcanoes and Plate Boundaries
Volcanoes and Plate Boundaries:
 Fortunately for us, hot magma only reaches the surface in certain
areas. What determines where volcanoes form?
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Most volcanoes form along divergent and convergent plate boundaries.
Some volcanoes form far from plate boundaries above “hot spots” in
the crust.
Volcanoes and Plate Boundaries
Convergent Boundary Volcanism:
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Volcanoes form at convergent
plate boundaries where slabs of
oceanic crust are pushed down
into the mantle.
As a slab sinks deeper into the
mantle, the increase in
temperature and pressure drives
water from the oceanic crust.
Volcanoes and Plate Boundaries
Convergent Boundary Volcanism:
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Once the sinking slab reaches a
depth of about 100 to 150
kilometers, this water reduces
the melting point of mantle rock
low enough for melting to begin.
The magma formed slowly
migrates upward forming
volcanoes.
Volcanoes and Plate Boundaries
Convergent Boundary Volcanism:
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Volcanoes form at convergent
boundaries where two oceanic
plates meet and oceanic
lithosphere is subducted beneath
another oceanic plate.
Volcanoes and Plate Boundaries
Convergent Boundary Volcanism:
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This process results in a chain of
volcanoes being formed on the
ocean floor. Eventually these
volcanic mountains grow large
enough to rise above the surface
and are called volcanic islands.
Volcanoes and Plate Boundaries
Convergent Boundary
Volcanism:
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Several volcanic island
arcs, such as the Tonga
Islands and the Mariana
island arcs, lie on the
eastern side of the Pacific
ocean
Volcanoes and Plate Boundaries
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Together with other volcanoes bordering the pacific, they form the
Ring of Fire.
The Ring of Fire is the long belt of volcanoes that circles much of the
Pacific Ocean.
Volcanoes and Plate Boundaries
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Volcanism may also occur at
convergent plate boundaries where
a continental plate meets an
oceanic plate and slabs of oceanic
lithosphere are subducted under
continental lithosphere.
The result is a continental volcanic
arc.
The process is basically the same
as for an island arc.
Intraplate Volcanism
Kilauea volcano in Hawaii is earth’s
most active volcano. But Kilauea
is in the middle of the pacific
plate, thousands of kilometers
away from any plate boundary.
Kilauea is an example of intraplate
volcanism; volcanic activity that
occurs within a plate.
Intraplate Volcanism
Most intraplate volcanism occurs
where a mass of hotter-thannormal mantle material, called a
mantle plume, rises toward the
surface.
Once the plume nears the top of the
mantle, decompression melting
forms magma.
Intraplate Volcanism
The result may be a small volcanic
region a few hundred kilometers
across called a hot spot.
More than 40 hot spots are known.
Most of these hot spots have
lasted for millions of years.
Intraplate Volcanism
The volcanic mountains that make up
the Hawaiian Islands have
formed as the Pacific plate
moves over a hot spot.
The age of each volcano indicates
the time when it was over the
hot spot. Kauai is the oldest
while the most recently formed
island has two volcanoes; Mauna
Loa and Kilauea.