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• Why does Earth have mountains?
–Where do the come from?
• Why are there ocean fossils at the top
of the Himalayas?
• Why does California have
Earthquakes?
• Where do volcanoes come from?
Plate Tectonics
Chapter 8
Alfred Wegener
• Continental Drift
– Wegener thought
whole sections of the
crust moved
• Evidence
– Shape of Continents
– Rock Evidence
• Same mountains on
different continents
– Fossil Evidence
• Same fossils on
different continents
Continental Drift
Africa and South America
look like they fit
together
– Click on the picture to
see the evidence
1. Ancient Mountain Belts
2. Ancient Sand Dunes
3. Fossil Evidence
Continental Drift
Fossil Evidence
- Same fossils found on
different continents
1.
2.
3.
4.
Mesosaurus
Lystrosaurus
Glossopteris
Cynognathus
Continental Drift
• What the hypothesis was missing was the
HOW?
– Alfred Wegener proposed that the continents
were moving on a more fluid layer under the
crust that could possibly be moving due to
internal heat . . .
• He had no way to prove it!
He was right!. . .well mostly
• He died before his hypothesis was
accepted as a theory.
• New technology allowed scientists to
examine the structure of Earth
– They found that the lithosphere was moving
due to Earth’s internal heat.
– This developed into the theory of Plate
Tectonics
Plate Tectonics
• Evidence
– Continental Drift (continents fit, fossils, rocks)
– New Evidence
• Location of Earthquakes and Volcanoes (p173)
• Magnetism of the ocean floor (p174)
• Age of the ocean floor (p175)
Plate Tectonics Evidence
Earthquakes and Volcanoes:
– Most earthquake and volcano activity happen
along plate boundaries.
• Plates are moving apart, moving together, or
sliding past one another.
– This creates earthquakes
• Where plates move apart and come together
magma is brought to the surface
– This creates volcanoes
Plate Tectonics
World Earthquakes
See page 712 to view direction of
plate movement
Plate Tectonics Evidence
Magnetism of the Ocean Floor (see page 174)
– Mid-ocean Ridges are places where NEW
rock is forming.
– These rocks contain magnetic minerals
(minerals with iron) they point to the north
pole
– The north pole can flip with the south pole
– These minerals flip too. This is called a
magnetic reversal.
Plate Tectonics
Seafloor Magnetism
Plate Tectonics Evidence
• Age of the Ocean Floor
– New rock is formed at a mid-ocean ridge or
spreading center
• These are formed in the middle of the ocean
– The rock gets older as you move away from
the ridge
• The ocean floor is not that old because it subducts
under continents
• The oldest ocean floor is ~180 million years old
oldest
Plate Tectonics
Age of the ocean floor
youngest
Earth’s Structure
• Crust + Upper Mantle =
Lithosphere (solid)
• Asthenosphere “fluid”
portion of the mantle
• Mantle - Solid
• Outer Core – Liquid
• Inner Core – Solid
– Heat comes from
radioactive material in the
core
Mantle Convection
Composition
Thin
High Density
Low Silica
Sinks
Thick
Low Density
High Silica
Floats
3 Types of Plate Boundaries:
1. Divergent
2. Convergent
3. Transform
Divergent Boundary
Sea floor spreading
• Large continents
begin to crack and
split apart
• The gaps fill with
water
• Small seas become
oceans
• The mid ocean ridge
continues to produce
new crust
Divergent Boundary
Characteristics
• 2 plates are moving apart
• Shallow Earthquakes
• Magma comes to the surface and cools
– Basalt rock forms
• Dense and dark in color
• Creates a Mid-Ocean Ridge
– Rift valleys form in the center
• Examples: Mid Atlantic Ridge and East
Pacific Rise
Using the Map on page 712-713 Highlight the
Divergent Boundaries
Divergent Boundary
How?
• The plates are pulled
apart by convection
currents in the mantle
below
• Caused by heat released
from natural radioactive
processes
• At mid-ocean ridges
molten rock from below
rises up to fill the gap with
new basaltic rock
Let’s Draw a Divergent Boundary
3 Types of Plate Boundaries:
1. Divergent
2. Convergent
3. Transform
Convergent Boundaries
• 2 plates are moving together
– Ocean-Ocean
– Ocean-Continent
**Deep Earthquakes
– Continent-Continent
**Earthquakes
Subduction Zone
Volcanoes Form
Creates Mountain Belts
NO VOLCANOES
Convergent Plate Boundaries
• Where lithospheric plates are moving towards one another at
their boundary, lithospheric area must be consumed. This is
accomplished by subduction or thickening and delamination.
32
Convergent Boundaries
-Subduction• Ocean-Ocean or Ocean-Continent
• The denser plate always subducts
Volcano Forms
•Subducting Plate
• more dense
Plate Melts
Ocean – Continent Subduction
Ocean – Continent Subduction
• The ocean plate always subducts because
it is more dense.
• Continental volcanic arcs or a mountain
chain of volcanoes form on the continental
plate.
• Examples: Cascades of N. America
Andes of S. America
Ocean – Ocean Subduction
Ocean – Ocean Subduction
• The denser ocean plate always subducts
because it is more dense.
• Volcanic island arcs or chains of volcanic
islands form on the OTHER OCEAN plate.
• Examples: Aleutian Islands, Alaska
Mt Pinatubo, Philippines
Mt Fuji, Japan
Lets Draw a Subduction
Boundary
Collision Boundary
Collision Boundary
Convergence of India
Collision Boundary
• There is NO SUBDUCTION because both
plates are continental and have low
density.
• They buckle up forming mountains (not
volcanoes)
• Examples: Himalayas, India/Asia
Highlight and Label the Major
Convergent Boundaries
3 Types of Plate Boundaries:
1. Divergent
condom
3. Transform
Transform Boundaries
• Occur when 2 plates are sliding past one
another
** Earthquakes!
Ocean – Ocean
Ocean – Continent
Continent - Continent
• No Volcanic Activity
• Examples
– San Andreas Fault
– Along Ocean Floor
Highlight and Label the Major
Transform Boundaries
Let’s Draw a Transform
Boundary