1. Plate Tectonics
The theory that the Earth’s outermost layer is
fragmented into a dozen or more large and
small plates that move relative to one another
as they ride on top of hotter, more mobile
material.
2. 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.
3. What Drives Plate 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.
5. Movement of the Plates
Over Time
Permian
248 million years ago
Triassic
206 million years ago
Jurassic
206 million years ago
Cretaceous
65 million years ago
Present Day
9. Different Types of 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
10. Oceanic-Continental 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
11. Oceanic-Oceanic 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
12. Continental-Continental 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.
13. Continental-Continental 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.
14. 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.
15. 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.
16. 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.
17. 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.
18. Consequences of Plate 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.
19. Consequences of Plate 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).
20. Consequences of Plate Tectonics
The Aleutian Islands, an island arc The 1980 eruption of Mount
Saint Helens
21. Consequences of Plate 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.
22. 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.
