Download Chapter 33

Our Restless Planet
Dynamic Earth Processes Standards
3a,b,f
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Believed that the cooling of the planet
resulted in its contraction.
In 1910 Alfred Wegener begins to wonder….
What’s the relationship?
Perhaps all these pieces used to be
connected.
Continental drift=slow movement
over Earth’s surface
It all started 300 million years
ago………..
Scientist saw the Earth as a dynamic planet with
continents in slow, but constant motion. He said:
 the continents were one supercontinent: Pangaea
 geological boundary of each continent lay not at its
shore, but at the edge of its continental shelf.
 Also continents share similar fossils.

Is the gently sloping platform between the
shoreline and the steeper slope that leads to
the deep ocean floor.
Pangea –(Greek)
all lands
Tens of Millions of years!
Earth is a huge magnet; magnetism generated
from the outer core.
 Magnetic north and south pole are near the
geographic poles during regular polarity (like
now).
 Paleomagnetism: In the past, the magnetic
poles were reversed.
When rock is formed, certain magnetic
minerals will align with the direction of the
magnetic poles, leaving many rocks with a
record of pole reversal. We see the best
example at the Mid-Atlantic Ridge.
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As new basalt is extruded at a mid-ocean
ridge, it is magnetized according to the
existing magnetic field.
Magnetic patterns of the spreading floor tell
us both AGE of the seafloor and RATE at
which it spreads.
Oceanic crust found to be thin and young
near central ridge and progressively thicker
and older away from the ridge.
N, N
N, S
Earth
Normal
Polarity
(NOW);
magnetic
north and
south are
matched with
geographic N
and S
S, S
Earth
Reverse Polarity
(THEN); magnetic
north and south
opposite of
geographic N and
S
S, N
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Hess believed the seafloor was not
permanent
Evidence of seafloor spreading can be seen at
the Mid-Atlantic Ridge.
It is the result of the upwelling of magma
New lithosphere is formed at the ridge; while
old is destroyed near the edges of the
continents.
Some of the deepest parts of the ocean are actually
near some continents, and out in the middle of the
oceans the water is relatively shallow because of the
underwater mountains.
 Seafloor spreading is a mechanism for continental
drift.
 Major interactions occur along plate boundaries.
 The youngest rock is found at the middle of the
ridge; the rock gets older further away from the
spreading center.
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The Big Picture
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Wegener’s Idea
Describes the forces within the Earth that
create the continents, ocean basins,
mountain ranges, earthquake belts, and
large- scale features of the Earth’s surface.
The Earth’s outer shell, the lithosphere, is
divided into eight relatively large plates and a
number of small ones.
Divergent Plate Boundaries
 2 plates move apart = tension stretches lithosphere.
 spreading center result = hot, molten rock from
asthenosphere upwells and creates new lithosphere.
 Examples: Mid-Atlantic Ridge
 Continents on either side of the ridge grow apart.
 Spreading centers can also develop on land.
 rising magma up lifts continental crust (in lithosphere.)
 crust is pulled apart causing rift valley
 can be the beginning of a new ocean basin
•
Convergent Plate Boundaries
• Occur where plates come together
• Motion from convection cells pushes plates together
• Compression
• Results in mountains building
• 3 types of plate collisions: defined by type of crust involved.
Oceanic-Oceanic Convergence: two oceanic plates collide, one plate descends beneath the
other-subduction, forms deep ocean trench then volcanoes island.
Oceanic-Continental Convergence: ocean and continental plates come together, the
denser oceanic (basalt) subducts under the less dense continental plate, forms deep
ocean trenches, earthquakes are a characteristic of the area, mountains rise from the
convergence.
Continental-Continental Convergence: collision of two land masses, always proceeded by
oceanic-continental convergence, compression causes crust to fold/break thinner
crust, no volcanic activity, many earthquakes.
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Transform Fault Boundaries
Horizontally slipping past each other,
pressure builds up causing earthquakes.
A
Divergent
•plates are moving
apart
•new crust is
created
•Magma is coming
to the surface
B
Convergent
•plates are coming
together
•crust is returning to
the mantle
C
Transform
•plates are slipping
past each other
•crust is not created
or destroyed
A
Divergent
Continental crust
 rift valley
B
Convergent
2 continental plates 
mountain range
C
Transform
Plates move
against each
other
Stress builds up
Oceanic crust  midocean ridge
2 oceanic plates or
oceanic + continental
subduction
Stress is released
earthquake