CHAPTER 9
PLATE TECTONICS
CONTINENTAL DRIFT
In 1915 Alfred Wegener proposed the Theory
of Continental Drift
 = continents are not fixed and in the past all
continents had been joined

PANGAEA
The “Supercontinent”
WEGENER’S EVIDENCE
EVIDENCE #1
Better maps showed outline of
continents fit together like a
puzzle

EX: South America
& Africa
EVIDENCE #2
Identical fossils were found on
different continents
EX: Mesosaurus
EVIDENCE #3
Similar rocks & mountains were
found on different continents
EX: Appalachian Mts.
EVIDENCE #4
Climate
change
 Wegener
was a meteorologist and
studied climate

He found out that there were glacier
deposits that showed that between
220-300 million years ago ICE once
covered the southern hemisphere
PROBLEM…

How can you move a continent?
(what mechanism did this)
 Answer:
In 1968 the ocean floor was
being extensively mapped & earthquakes
were being discovered on the ocean floor
= gave scientists a “mechanism”
9.2 & 9.3
PLATE TECTONICS
& Actions at Plate
Boundaries
THEORY OF PLATE TECTONICS
 Earth
is divided into 8 major
lithospheric plates that move and
interact on the Earth’s surface
because of unequal heating within
Earth’s interior
…SO HOW DO THEY MOVE YOU ASK?????
 hot
molten material from outer core
slowly rises upward, cooler material
descends to take its place = circular
pattern of magma

rigid lithosphere slides on top of
molten asthenosphere depending
on convection currents

- Interaction
of plates causes
earthquakes, mountains, and
volcanoes
3 MAIN TYPES OF PLATE BOUNDARIES
1.
Divergent = 2 plates are moving
apart, allowing magma to upwell,
and form new crust
EX: SA & African plates
- at the boundary an oceanic ridge
(mts on the ocean floor)
forms on either side of boundary
DIVERGENT
DIVERGENT…CONT’D
- exactly at center of boundary is a rift valley
where magma reaches surface
- can happen on continents too
EX: East African Rift
EAST AFRICAN RIFT VALLEY
TRANSFORM
2. Transform = 2 plates are sliding
past each other
- many found on ocean floor but some
on land = Faults
EX: San Andreas Fault
CONVERGENT
3. Convergent = 2 plates are moving
closer together (colliding), very
complicated because of oceans vs
continents
OCEAN-CONTINENT COLLISION
A. Ocean-Continent Collision = heavier
ocean crust is forced below continent
creating a Subduction Zone
- as ocean crust goes deeper, it melts,
new magma rises to create volcanoes
on continent EX: Andes Mts of SA
- subducting ocean creates deep
trenches at plate boundary
OCEAN-CONTINENT COLLISION
B. Ocean-Ocean Collision = one slightly
heavier slab of ocean crust is subducted
beneath the other one
- creates volcanic chains in ocean =
Volcanic Island Arc
EX: Pacific Plate and Eurasian Plate forming the
Japanese Islands
OCEAN-OCEAN COLLISION
C. Continent-Continent Collision
= no subduction, continents
collide and fold together to create
tall & complex mountains
EX: Indian Plate colliding with Eurasian
Plate forming …..
The Himalayan Mountains
CONTINENT-CONTINENT COLLISION
“ MODERN” EVIDENCE

1. Continental Drift = continents fit together,
fossils, rocks, climates
2. Maps of the sea floor showed that close to plate
boundaries there was young crust and the
farther away from the boundary, the older the
crust
- ages matched up on either side of the
plate boundary
 Theory of Sea-Floor Spreading
3. When magma cools it records the direction
of North (like a compass), scientists found
that N & S pole flip periodically
- The sea floor rocks record this flip of
poles and match up on either side of
the plate boundary
 Paleomagnetism
 4.
Earthquakes are much more
frequent at plate boundaries than
anywhere else
WHY???
Plates are moving
5. Ocean drilling showed that ages of
sea-floor rock changed, youngest near
plate boundary, oldest near the
continent coasts
- Oceans are much younger than
continents
(180 million years compared to 4 billion years)
6. Ocean mapping around Hawaii showed
it was part of a large chain of volcanoes
extending throughout the Pacific
The Big Island (Hawaii) is the only one
erupting, farther away from this is
progressively older

 BUT in the middle of
Pacific Plate ???
 There
is a large plume (ball) of molten
magma near surface, melting the crust,
and forming a volcano = Hot Spot

 Pacific
plate is moving over the hot spot
and creating a chain of volcanoes =
indicates direction of movement
