1) The Earth's surface is made up of rigid tectonic plates that slowly move over time. 2) These plates interact with one another at plate boundaries, where new crust is created at mid-ocean ridges and old crust is destroyed through subduction. 3) The movement of tectonic plates is driven by convection currents in the underlying mantle and forces at plate boundaries, and explains phenomena like earthquakes and volcanic activity.

1. The Theory of Plate Tectonics
Chapter 14 Lesson 3
p510-519

2. Vocabulary
• Plate tectonics (511) – Earth’s surface is made of rigid slabs of rock, or
plates, that move with respect to each other
• Lithosphere (512) – the cold and rigid outermost rock layer
• Divergent plate boundary (513) – forms where two plates separate
• Transform plate boundary (513) – Forms where two plates slide past each
other
• Convergent Plate Boundary (513) – form where two plates collide
• Subduction (513) – A process in which the denser plate sinks below the
more buoyant plate
• Convection (516) – the circulation of material caused by differences in
temperature
• Ridge Push (517) – the forces that causes rising mantle material at mid-
ocean ridges that creates the potential for plates to move away from the
ridges
• Slab Pull (517) – As a slab sinks, it pulls on the rest of the plate with this
force

3. The Plate Tectonic Theory
• Earth’s crust is constantly being created and
destroyed
• The theory of plate tectonics states that Earth’s
surface is made of rigid slabs of rock, or plates,
that move with respect to each other
• Each plate moves over Earth’s hot and semi-
plastic mantle
– The term tectonic describes the forces that shape
Earth’s surface and the resulting rock structures
– Plate tectonics is used to explain earthquakes and
volcanic eruptions

4. Tectonic Plates

5. Tectonic Plates
• The Pacific Plate is the largest plate
• The Juan de Fuca is one of the smallest
• The boundaries that run through oceans mark the positions
of the mid-ocean ridges
• Earth’s outermost layers are cold and rigid compared to the
layers in Earth’s interior. It is called the lithosphere.
– It is made up of the crust and the solid, uppermost mantle
– It is thin below ocean ridges and thick below continents
– The tectonic plates are just large pieces of the lithosphere
• Just below the lithosphere is called the asthenosphere.
– This layer is so hot that it behaves like a plastic material
– This enables to lithosphere to move

7. Plate Boundaries
Convergent
Convergent

8. Divergent Plate Boundaries
• Mid-ocean ridges are located
along divergent plate boundaries.
• A divergent plate boundary forms
where two plates separate
• When the seafloor spreads at a
mid-ocean ridge, lava erupts,
cools, and forms new oceanic
crust.
• Divergent plate boundaries can
also exist in the middle of a
continent
– They pull continents apart and
form rift valleys
– The East African Rift is an
example.

9. Transform Plate Boundaries
• A transform plate boundary
forms where two plates slide
past each other.
– The San Andrews Fault in
California is a well known
example.
– As the plates move past each
other, they can get stuck and
stop moving,
– Stress builds up where the
plates are stuck.
– Eventually the stress is too
great and the rocks break
– The resulting rapid energy
release is an earthquake.

10. San Andreas Fault

11. Convergent Plant Boundaries
• Convergent plate
boundaries form where
two plates collide
• The denser plate sinks
below the more buoyant
plate in a process called
subduction
– The area where a denser
plate descends into Earth
is called a subduction
zone.

12. • When an oceanic plate and
continental plate collide the
denser oceanic plate subducts
under the edge of the
continent
• This creates a deep ocean
trench
• A line of volcanoes forms
above the subducting plate
on the edge of the continent
• This process can also happen
with two oceanic plates.
– Typically the older, denser
plate will subduct beneath the
younger plate.
– This creates a deep ocean
trench and a line of volcanoes
called an island arc

13. • When two
continental plates
collide, neither
plate is subducted.
• Instead rock is
uplifted and create
mountains like the
Himalayas

14. Evidence for Plate Tectonics
• Continents move apart or come together at speeds of a few
centimeters per years
• Today scientists use a network of satellites called the Global
Positioning System (GPS) to measure the movement of the plates
• The theory of plate tectonics explains why earthquakes and
volcanoes occur in certain places

15. • All types of plate boundaries can result in
earthquakes due to the rapid release of energy
• Diverging and converging plates result in
volcanoes
• Mountains form when two continental plates
converge

16. Plate Motion
• Convection Currents
– Convection is the circulation of material caused by
differences in temperature and density
• For example, the upstairs floors of most houses often
warmer than the lower floor
• This is because warm air rises while denser, cold air
sinks.

17. • Tectonic plate activity is related to convection in the mantle.
• Radio active elements heat Earth’s interior.
• When materials such as solid rock are heated, they expand and
become less dense
• Hot mantle material rises upward and comes in contact with Earth’s
crust
• Thermal energy is transferred to the surface
• As the mantle cools, it becomes denser and then sinks, forming a
convection current
• These currents in the asthenosphere act like a conveyor belt moving
the lithosphere

18. FORCES CAUSING PLATE MOTION
Scientists are still uncertain about which force has the
greatest influence

19. Forces Causing Plate Motion
• Basal Drag
– This is when convection currents in the
asthenosphere move or drag the lithosphere
much like how walking sidewalks at the airport
move people.

20. Ridge Push
• Recall that mid-ocean ridges have greater elevation than the
surrounding seafloor.
• Because they are higher, gravity pulls the surrounding rock
down and away from the ridge
• Rising mantle material at mid-ocean ridges creates the
potential for plates to move away from the ridge with a force
called ridge push.
– This moves the lithosphere away from the mid-ocean ridge.

21. Slab Pull
• When plates converge, the denser plate will sink into
the mantel along a subduction zone
• This plate is called a slab. It is usually old and cold
which makes it denser.
• As a slab sinks, it pulls on the rest of the plate with a
force called slab pull.

22. A Theory in Progress
• Plate tectonics has become the unifying
theory of geology
• It explains the connection between
continental drift and the formation and
destruction of crust along plate boundaries.
• It also helps to explain the occurrence of
earthquakes, volcanoes, and mountains

23. Unanswered Questions
• Several questions remain unanswered:
– Why is Earth the only planet in the solar system
that has plate tectonic activity?
– Why do some earthquakes and volcanoes occur
far away from plate boundaries?
• Part of the answer has to do with plate thickness. The
other part is the scientists are still learning how active
the mantle really is.
– What forces dominate plate motion?