Tectonic plates are large, rigid slabs of solid rock that float on the asthenosphere and include both land and ocean floors. Their movement is driven by convection currents in the mantle, resulting in three types of boundaries: convergent, divergent, and transform, each playing a role in geological activity such as earthquakes and the formation of mountains and trenches. Notable examples of tectonic interactions include the collision that formed the Himalayas and the divergent movement at the Mid-Atlantic Ridge.

1. What Are Tectonic Plates?
What Are Tectonic Plates?
• 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.

2. 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.

3. Earth’s Plates
Earth’s Plates

4. Movement
Movement
of the
of the
Plates Over
Plates Over
Time
Time
Permian
248 million years ago
Triassic
206 million years ago
Jurassic
206 million years ago
Cretaceous
65 million years ago
Present Day

5. Progression
Progression
of Indian
of Indian
Land Mass
Land Mass

6. Plate Boundaries
Plate Boundaries

7. Different Types of
Different Types of
Boundaries
Boundaries

8. Different Types of
Different Types of
Boundaries
Boundaries
• 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

9. Oceanic-Continental
Oceanic-Continental
Convergence
Convergence
• 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

10. Oceanic-Oceanic
Oceanic-Oceanic
Convergence
Convergence
• 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

11. Continental-Continental
Continental-Continental
Convergence
Convergence
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.

12. Continental-Continental
Continental-Continental
Convergence
Convergence
• 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.

13. Divergence
Divergence
• 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.

14. Divergence
Divergence
• 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.

15. Transform
Transform
• 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.

16. Transform
Transform
• 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.

17. Consequences of Plate
Consequences of Plate
Tectonics
Tectonics
• 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.

18. Consequences of Plate
Consequences of Plate
Tectonics
Tectonics
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).

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

20. Consequences of Plate
Consequences of Plate
Tectonics
Tectonics
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.

21. Factors that Cause
Factors that Cause
Tectonic Plate Movement
Tectonic Plate Movement
1.
1. Density and Plate Movements
Density and Plate Movements
2.
2. Gravitational Pressure
Gravitational Pressure
3.
3. Earth’s Spin
Earth’s Spin

22. References
References
• Understanding Plate Motions. USGS.
http://pubs.usgs.gov/publications/text/unde
rstanding.html
• Plate Movement. OptIPuter Outreach.
http://education.sdsc.edu/optiputer/teacher
s/platemovement.html
• Plate Tectonics. The Way the Earth
Works. LHS GEMS, 2002.