THE THEORY OF PLATE TECTIONICS AND ITS EVIDENCES.

continental drift

1. Plate TectonicsPlate Tectonics
The theory that the Earth’s outermost layer is The theory that the Earth’s outermost layer is
fragmented into a dozen or more large and fragmented into a dozen or more large and
small plates that move relative to one another small plates that move relative to one another
as they ride on top of hotter, more mobile as they ride on top of hotter, more mobile
material.material.

2. •A plate is a large, rigid slab of
solid rock.
–Plates are formed from the
lithosphere: the crust and
the upper part of the
mantle.
•The plates “float” on the slowly
flowing asthenosphere: the
lower part of the mantle.
•The plates include both the
land and ocean floor.
•The Mohoriovicic discontinuity
or Moho is the boundary
between the crust and the
mantle.
What is a Tectonic Plate?What is a Tectonic Plate?

3. What Drives Plate What Drives Plate
Tectonics?Tectonics?
•The slow movement of hot,
softened mantle lies below
rigid plates.
•The hot, softened rock in the
mantle moves in a circular
manner in a convection flow
– the heated, molten rock
rises to the surface,
spreads, and begins to cool,
and then sinks back down to
be reheated and rises again.

4. Earth’s PlatesEarth’s Plates

5. Movement of the Plates Over Movement of the Plates Over
TimeTime
Permian
248 million years ago
Triassic
206 million years ago
Jurassic
206 million years ago
Cretaceous
65 million years ago
Present Day
•Continental Drift:
continents have occupied
different locations on
Earth’s surface in the
geologic past
−240 million years ago the
continents were all
together in a
“supercontinent”, Pangaea
−Continents “drifted”
across surface of Earth to
their present locations

6. •Progression of Progression of
Indian Land Indian Land
MassMass

7. •Matching fossils in Matching fossils in
different continentsdifferent continents

8. •A continuous mountain belt A continuous mountain belt
can be formed when Pangaea can be formed when Pangaea
is reassembledis reassembled

9. ••Opposing edges of Opposing edges of
continents fit together along continents fit together along
the shallow continental shelfthe shallow continental shelf

10. ••Evidence of a thick ice Evidence of a thick ice
sheet throughout the sheet throughout the
southern continentssouthern continents

11. Plate BoundariesPlate Boundaries

12. Different Types of Different Types of
BoundariesBoundaries
•Convergent boundaries come together
–Places where crust is destroyed as one
plate dives under another
•Divergent boundaries spread apart
–Places where new crust is generated as
the plates pull away from each other
–New crust is created from magma
pushing up from the mantle
•Transform boundaries slide against each
other
–Places where crust is neither produced
nor destroyed as the plates slide
horizontally past each other

13. Oceanic-ContinentalOceanic-Continental
ConvergenceConvergence
•The oceanic plate subducts under the continental plate because it has lower
density.
 
•The oceanic Nazca Plate is being subducted under the continental part of the
South American Plate.
•The South American Plate is being lifted up, creating the Andes mountains.
•Strong, destructive earthquakes and rapid uplift of mountain ranges are
common in this region.
•These earthquakes are often accompanied by uplift of the land by as much as a
few meters.
•Mount Saint Helens is along the subduction zone of the Juan de Fuca plate (an
oceanic plate) and the North American plate (a continental plate).
Oceanic – Continental Convergence

14. Oceanic-Oceanic-
Oceanic Oceanic
ConvergenceConvergence
•When two oceanic plates converge, one
is usually subducted under the other.
•An older oceanic plate is colder,
therefore more dense and less buoyant,
and will subduct under a younger,
hotter, less dense, and more buoyant
oceanic plate.
•In the process, a trench is formed.
–The deepest trenches in the oceans
are along oceanic-oceanic
subduction zones (i.e., the Marianas
Trench in the Pacific, which is
deeper than Mt. Everest is high).
•Subduction in oceanic-oceanic plate
convergence can result in the formation
of volcanoes.
•Examples of oceanic-oceanic
convergence are the arcuate chains of
islands in the southwest Pacific, Japan,
and the Aleutian Islands.
Oceanic – Oceanic Convergence

15. Continental-Continental Continental-Continental
ConvergenceConvergence
Continental – Continental Convergence
• When two continents meet head-on,
neither is subducted because the
continental rocks are relatively light
and, like two colliding icebergs, resist
downward motion.
• Instead, the crust tends to buckle and
be pushed upward or sideways.
The collision between the Indian and
Eurasian plates has pushed up the
Himalayas and the Tibetan Plateau.

16. Continental-Continental Continental-Continental
ConvergenceConvergence
•50 million years ago, the
Indian Plate collided into the
Eurasian Plate.
•After the collision, the slow
continuous convergence of the
two plates over millions of
years pushed up the Himalaya
and the Tibetan Plateau to
their present heights.
•The Himalaya form the highest
continental mountains in the
world.

17. DivergenceDivergence
• Divergent boundaries occur
along spreading centers
where plates are moving apart
and new crust is created by
magma pushing up from the
mantle.
• The Mid-Atlantic Ridge is a
divergent boundary.
• Sea-floor spreading over the
past 100 to 200 million years
has caused the Atlantic Ocean
to grow from a tiny inlet of
water between the continents
of Europe, Africa, and the
Americas into the ocean that
exists today.

18. •Iceland is splitting along the Mid-Atlantic Ridge between the North
American and Eurasian Plates, as North America moves westward
relative to Eurasia.
•In East Africa, spreading processes have already torn Saudi Arabia
away from the rest of the African continent, forming the Red Sea.
•The actively splitting African Plate and the Arabian Plate meet in
what geologists call a triple junction, where the Red Sea meets the
Gulf of Aden.
DivergenceDivergence

19. TransformTransform
• The zone between two plates that
slide
past one another is called a
transform-fault boundary, or
transform boundary.
• These large faults connect two
spreading centers or connect
trenches.
• Most transform faults are found on
the ocean floor.

20. •The San Andreas Fault is
one of the few transform
faults exposed on land.
– It connects the East
Pacific Rise, a divergent
boundary to the south, with
the Juan de Fuca Ridge, a
divergent boundary to the
north.
– Most earthquakes in
California are caused by
the accumulation and
release of strain as the two
plates slide past each
other.
TransformTransform

21. Consequences of Plate Consequences of Plate
TectonicsTectonics
•Earthquakes and volcanic activity are linked to plate tectonic
processes.
•The Ring of Fire is the most seismically and volcanically active zone
in the world.

22. Consequences of Plate Consequences of Plate
TectonicsTectonics
The San Andreas Fault – a
transform fault
Aerial view of the area around Thingvellir, Iceland,
showing a fissure zone (in shadow) that is an on-land
exposure of the Mid-Atlantic Ridge. Right of the fissure,
the North American Plate is pulling westward away from
the Eurasian Plate (left of fissure).

23. Consequences of Plate Consequences of Plate
TectonicsTectonics
The Aleutian Islands, an island arc
The 1980 eruption of Mount
Saint Helens

24. Consequences of Plate Consequences of Plate
TectonicsTectonics
The convergence of the Nazca and South American Plates
has deformed and pushed up limestone strata to form
towering peaks of the Andes, as seen here in the
Pachapaqui mining area in Peru.
Helicopter view (in February 1994) of the active
lava lake within the summit crater of 'Erta 'Ale
(Ethiopia), one of the active volcanoes in the
East African Rift Zone.