The theory of plate tectonics describes Earth's surface as consisting of rigid slabs, or tectonic plates, that shift over the hot and semi-plastic mantle, resulting in the creation and destruction of the crust. It explains various geological phenomena, including earthquakes, volcanic eruptions, and mountain formation through different types of plate boundaries: divergent, convergent, and transform. Despite its success as a unifying theory in geology, several questions regarding the forces driving plate motion and unique aspects of Earth's tectonics remain unanswered.

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?