Transcript Seafloor Spreading

Seafloor Spreading
Plates on the Move
Introduction
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Most scientists believe that Earth’s crust is
broken into about 20 pieces called plates.
Each plate is the thickness of the crust and
the rigid upper mantle.
The plates move slowly, floating on the
mushy, flowing mantle below them.
Some of the plates move up to two inches
(five cent.) a year.
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The surface of some plates is mostly
ocean, while that of others is made up
of entire continents and parts of
oceans.
Where two plates meet--– They can spread apart
– Come together
– Or slide past each other
Interactions Between
Plates
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These interactions between plates and
the intense pressure, friction, and
crust melting…are responsible for
much of our planet’s volcanic and
earthquake activity.
They also build mountains and recycle
Earth’s crust.
Basic Steps:
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Color each piece
-Letters A, E, & H -color red (Magma)
-Letters B & C –color yellow (Mid-ocean
Ridges)
-Letters D & G-color brown (subduction
zones) arrows any color but brown.
-Letter F – color blue & brown (Volcanic
Arcs & ocean)
-Letter I – color blue & brown ( Ex. Of
Mount St. Helens)
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Students are looking at parts of four
plates.
Each plate is numbered with arrows
showing its directions of movement.
Most of each plate’s surface is ocean.
Look at the edges of plates 2 & 3.
The plates are spreading apart.
This opens giant cracks called rifts in
the seafloor.
DIRECTIONS:
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Paste piece A on the model/it shows
that hot magma rises and slowly oozes
out of the rifts as lava. As the lava
cools, it builds underwater mountains.
Label piece A MAGMA
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Paste piece B & C in their places. As
plates 2 & 3 move apart, they act like
giant conveyor belts moving the
mountains away from the cracks.
More mountains form in their place.
In this way, underwater mountain
ranges called ridges build up. Label B
& C Ridges.
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Plates 1 & 2 are moving toward each
other. Where they meet, the edge of
2 bends and dives under 1. This
forms a trench. Label D TRENCH.
As the edge of plate 2 dives into
Earth’s crust, it grinds and scrapes
against plate 1. If the edges lock
together, pressure can build, causing
an earthquake.
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High heat inside the crust melts the
edge of plate 2 into magma. Some of
the magma may rise and break
through the seafloor. Paste piece E in
its place. Label MAGMA.
Magma that erupts from the seafloor
can build up a line of island volcanoes
called a volcanic arc. Paste piece F in
place. Label F VOLCANIC ARC.
Japan, Philippines, and the Aleutian
Islands off Alaska are parts of volcanic
arcs.
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Look at plates 3 & 4---notice how they
are moving toward each other. At the
edge of 3 there is an ocean. At the
edge of 4 there is a continent. Paste
piece G in place. This is a trench that
forms when 3 dives under 4. Label it
TRENCH.
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Paste I in place. This is a line of
volcanoes on land built from lava
during eruptions. Label VOLCANOES.
Mount St. Helens formed this way.
Also, many earthquakes on land occur
where the ocean edge of one plate
dives under the continent edge of
another.
Paste piece H in place. This is magma
rising from the melting edge of 3.
Piece H is pasted on top on piece I.
Label
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Trenches
Subduction zones
Convergent boundaries
Divergent boundaries
Lithosphere
Asthenosphere
Convection current
Sea Floor Spreading
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Plate tectonics is a theory that the Earth’s Crust
is broken into sections (plates) that move around
the Asthenosphere. The Lithosphere is made up
of the crust and part of the upper mantle. Plates
can move together as Convergent Boundaries.
Convection Currents are the energy in the mantle
that forces magma up to the crust. Volcanoes
and earthquakes are a result of movement of
these currents. Magma rises up and pushes old
magma to the side, causing Sea-floor spreading.
When one plate moves under another, the plate
creates a Subduction Zone.