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Chapter 10
Plate Tectonics
Faustina Academy
Earth Science
Section 1: Continental Drift
Abraham Ortelius first noted more than 400 years ago that Africa and
South America's coastlines fit together like puzzle pieces
Alfred Wegener thought the fit of the continents wasn't just a
coincidence, but that all continents were joined together at some time
in the past
Pangea
1912, Wegener proposed continental drift hypothesis
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Continents once connected as one large landmass that broke
apart 200 million years ago
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He called landmass “Pangea” , meaning “all land”
His ideas were controversial, and it wasn't until after his death (1930)
that his hypothesis was accepted
Fossil Clues
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Fossils
 Provide support for continental drift
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Fossils of the same animals and plants have been found on
both Africa and South America
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Reptile Mesosaurus lived in freshwater and land
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How could it have swam across the ocean?
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Pg. 277
•Glossopteris
•Plant fossil found in Africa, Australia, India, South America,
Antarctica
•Evidence that all continents were connected and at one time had
similar climates
Climate Clues
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Wegener
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Found fossils of warm-weather plants on island in Arctic Ocean
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called Spitsbergen
Believed the island drifted from tropical to arctic region
Found glacial deposits in places like Africa & India, but how
could glaciers be on these continents?
Thought they had once been near South Pole
Rock Clues
 If continents were connected, then rocks that make up continents
should be same in locations where they were joined
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Parts of Appalachian Mountains are similar to those found in
Greenland and Europe
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Rocks in western Africa and eastern South America are also
similar
 Wegener's evidence for continental drift
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Rocks
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Fossils
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Climate
How Could Continents Drift?
 Wegener couldn't explain how, when, or why changes occurred
 His idea that low density continental material could plow through the
high density ocean-floor through the spinning of earth was rejected by
scientists
Quick Question
Why is the fact that similar fossils have been found on different continents
considered evidence for continental drift?
Answer
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Section 2: Seafloor Spreading
Before WWI, ocean depth measured by lowering rope to seafloor
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This was repeated to get map of ocean floor
Now, sound waves echo off ocean bottom
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Longer the wave takes to return to ship, deeper the water is
Researchers discovered underwater system of ridges (mountains)
and valleys like those on continents
The Seafloor Moves
Harry Hess formulated seafloor spreading theory (1960’s)
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Proposed that hot, dense material below Earth's crust rises
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toward surface at mid-ocean ridges
Then flows sideways, carrying seafloor away from ridge in both
directions
As it spreads apart, magma moves upward and flows from cracks
Magma becomes solid as cools and forms new seafloor
As new seafloor moves away from the mid-ocean ridge, it cools,
contracts and becomes denser
This denser, colder seafloor sinks, helps to form the ridge
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Evidence For Spreading
Scientists aboard research ship Glomar Challenger gathered
information about rocks on sea floor (1968)
Found youngest rocks located at mid-ocean ridges and ages of rocks
become increasingly older farther from ridge
Deep Sea Life
•Lava brings heat and chemicals to support exotic life forms
•E.g., giant clams, mussels, tube worms
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Magnetic Clues
Earth's magnetic field has north and south pole
Magnetic lines leave Earth near south pole and enter earth near north
pole
Earth's magnetic field has reversed itself many times in past
Reversals are recorded in rock along the mid-ocean ridges
Magnetic Time Scale
 Iron-containing minerals, such as magnetite, found in seafloor can
record Earth's magnetic field direction when they form
 Using a magnetometer to detect magnetic fields, scientists have
mapped regions of alternating magnetic direction parallel to the
mid-ocean ridges
 Magnetic reversals show that new rock is being formed
Quick Question
How could seafloor spreading be related to continental drift?
Answer
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The Theory of Plate Tectonics
Scientists combined continental drift and seafloor spreading into new
theory called plate tectonics (1960's)
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Sections of sea floor and continents move in relation to one
another
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Earth's crust and part of the upper mantle are broken into
sections called plates
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Plates move on plasticlike layer of the mantle
Lithosphere
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Crust and part of upper mantle (62 mi thick)
Asthenosphere
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Plasticlike layer below lithosphere on which plates of
lithosphere float (pg. 284)
Plate Boundaries
When plates move, they can move toward each other and converge
They also pull apart or slide alongside one another
When the plates interact, result of movement is seen at plate
boundaries
Divergent Boundary
 Boundary between two plates that are moving apart
 E.g., Mid-Atlantic Ridge
 E.g., Great Rift Valley in eastern Africa
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Valley has formed where a continental plate is being pulled
apart
(pg. 287)
Subduction
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Convergent boundary
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Two plates move together
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Denser oceanic plate sinks under continental plate creating
subduction zone (pg. 287) (Andes)
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High temperatures cause rock to melt, and newly formed magma
is forced upward, forming volcanoes
Where Plates Collide
A subduction zone can also form where two oceanic plates converge
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Colder, older, denser oceanic plate bends and sinks into the
mantle
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Earthquakes are common, but no volcanoes (no magma)
When two continental plates collide they crumple, forming mountains
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E.g., Himalaya
Transform Boundary
Where two plates slide past one another
When one plate slips past another suddenly, earthquakes occur
E.g, The San Andreas Fault
Causes of Plate Tectonics
Convection currents
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Caused by hot, less dense liquid forced upward by surrounding,
cooler liquid
As liquid reaches surface, it cools and sinks back down
Scientists think convection currents in mantle are force behind plate
tectonics
It is transfer of heat inside Earth that provides energy to move plates
(pg. 289)
Features Caused by Plate Tectonics
Mountains, volcanoes, ocean basins, rift valleys, faults
When rocks in Earth's crust break and move, energy is
released in the form of seismic waves (earthquakes)
Tension forces (forces that pull apart) can stretch Earth's crust,
causing large blocks of crust to break, creating a fault
Mountain ranges can be made in the process, called fault-block
mountains
Rift valleys and mid-ocean ridges can form where Earth's crust
separates
Mountains and Volcanoes
Where continental plates come together, forces cause massive
folding and faulting of rock layers into mountain ranges
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E.g., Himalaya, Appalachian
Type of faulting produced is reverse faulting
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Rock layers above fault surface move up (pg. 291)
Island Arcs
When two oceanic plates converge, curved chains of volcanic islands
called island arcs form above the sinking plate
When an oceanic plate converges with a continental plate, the
oceanic plate slides underneath
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Can produce mountains or volcanoes
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Features Cause by Plate Tectonics
Strike-Slip Faults
At transform boundaries, two plates slide past one another without
converging or diverging
The plates stick and then slide along strike-slip faults
Rocks on opposite sides of the faults move in opposite directions, or
in the same direction at different rates
Testing for Plate Tectonics
Until recently, the only tests scientists could used to check for plate
movement were indirect
They could study the magnetic characteristics of rocks on the seafloor
They could study volcanoes
and earthquakes
However, these didn't provide proof
Using lasers and satellites, scientists can measure the movement of
the continents (pg. 293)
Plate Movement
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Hawaii is moving
toward Japan at a
rate of 8.3 cm/year
Maryland is moving
from England at a
rate of 1.7 cm/year
Plates move at rates
of about
1-12 cm/year
Quick Question
What will happen if a continental plate collides with an oceanic plate?
What will happen if two continental plates collide?
Answer