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Theory of Plate Tectonics
Theory of
Continental Drift
The Theory of Plate
Tectonics starts with
another idea… Continental
Drift. The Earth once had
a single land mass called
Pangaea that broke apart
into pieces that drifted
away to become the major
continents of today.
Pangaea
Evidence that supports
Continental Drift
• Identical types of
fossils found in Africa
and South America
• Continents can
“piece”together with
the same rock
formations
• Glacial deposits left by
same sheet of ice
Africa-South America
The continents are moving
because of sea floor spreading.
• Running down the middle of the oceans are long
mountain chains with valleys in the middle called mid
ocean ridges. Lava erupts from the center of the valley
and pushes the ocean floor away on the other side. The
Mid-Atlantic Ridge is separating at approximately 4 cms
per year.
More evidence…
• Rocks are younger by
the mid ocean ridge
than farther away
• Magnetic stripes
match up on either
side of the ridge as the
poles reverse
themselves
Magnetic Stripes
Why doesn’t the Earth keep getting
bigger?…Subduction Zones
• The ocean floor is also being destroyed
• Parts of the ocean floor are going back down into
the mantle at subduction zones. These trenches
are the deepest part of the ocean.(Ex: Marianas
Trench)
Subduction Zone
The Theory of Plate Tectonics explains the
formation, movements, collisions, and destruction of
the Earth’s crust.
Areas of Sea Floor
Spreading
Key Subduction Zones
How do the plates move?
• One hypothesis is that
large convection
currents within the
mantle move the
plates.
• Movement is caused
by differences in
temperature causing a
rising and sinking
cycle.
Convection Currents
The lithosphere is divided into 7 major and minor
plates. Plates move at different speeds in different
directions. Plate boundaries are where two plates meet.
Major Plates
Three Types of Plate Boundaries:
• Divergent—Boundary where plates move apart
(ex: Great Rift Valley—Africa; Mid ocean
Ridges)
• Convergent—Boundary where plates come
together (ex: Himalayan mountains, Japan, and
Philippines)
• Transform—Boundary where two plates slide
along side each other (ex: San Andreas Fault—
San Francisco)
Divergent Boundaries occur where
plates move apart. There are two types
of divergent or constructive boundaries:
**Mid-Ocean Ridge
**Rift Valley
Divergent Boundary
Mid- Ocean Ridge- New ocean floor, mountains,
earthquakes and volcanic action occur when an
ocean plate spreads apart (Mid-Atlantic Ridge:
largest mountain range in world.)
Mid-Ocean Ridge
Mid-Atlantic Ridge
Rift Valley- Continent starts to split apart forming a
rift valley and eventually a seaway and then an
ocean. Volcanic activity and earthquakes occur in
these areas. (Ex: African Rift Valley)
Rift Valley
The Great African Rift Valley
Three Types of Plate Boundaries:
• Divergent—Boundary where plates move apart
(ex: Great Rift Valley—Africa; Mid ocean
Ridges)
• Convergent—Boundary where plates come
together (ex: Himalayan mountains, Alps)
• Transform—Boundary where two plates slide
along side each other (ex: San Andreas Fault—
San Francisco)
Convergent Boundaries form when two
tectonic plates come towards each other.
There are three types of convergent boundaries
or destructive boundaries.
**continent - continent
**ocean – ocean
**continent – ocean
Continent -Continent: Mountains form and
earthquakes occur when continental plates run into
each other and fold upwards. (Ex: Himalayas and
Alps)
Continent to continent
The summit of Mt. Everest
is made of marine limestone.
Ocean Plate-Ocean Plate: Volcanic island arcs,
trenches & earthquakes occur when older ocean
plate subducts under a younger ocean plate.
(Ex: Japan, Aleutian Islands-off Alaska,
Philippines, Tonga Islands, Marianas Trench).
Ocean to Ocean Boundary
Earth Observatory Center: Japan
Ocean Plate-Continent Plate: Mountains,
volcanoes and earthquakes occur as an ocean plate
subducts under a continental plate. The Oceanic
plate melts; less dense-magma rises to form
volcanoes. (Ex: Andes, Cascade Range, Sierra
Nevada)
Ocean to Continent Boundary
Goddard Space Flight Center
Ocean to Ocean and Ocean to continent are destructive boundaries
Three Types of Plate
Boundaries:
• Divergent—Boundary where plates move
apart (ex: Great Rift Valley—Africa; Mid
ocean Ridges)
• Convergent—Boundary where plates come
together (ex: Himalayan mountains,
• Transform—Boundary where two plates
slide along side each other (ex: San
Andreas Fault—San Francisco)
Strike-slip or transform boundaries occur
where two plates slide along side each other.
Earthquakes occur in these areas (Ex: San
Andreas Fault in California).
Transform Boundary
San Andreas
Plate Movements
Hot Spots The Hawaiian Islands do not occur
by a plate boundary. These islands form as the
Pacific plate moves over a hot spot in the
mantle.
Hot spot or Mantle Plume
Earth Observatory
What will the Earth look like in
the future?
Present Day Earth