Transcript Theory of plate tectonics

Theory of plate tectonics
Continental drift hypothesis
Before the theory of plate tectonics in the
1960’s, there was Alfred Wegener’s
hypothesis of continental drift.
 He thought all the continents were once
connected to make a single
supercontinent called Pangea.

Pangea 300 million years ago
Evidence that supported Pangea
Why do the continents move?
The theory of plate tectonics states that the
Earth’s lithosphere is broken up into plates.
These plates continually move and change
shape and size.
 When plates move, they cause earthquakes and
volcanoes along the plate boundaries.
 Plates move slow-on average 5 cm a year!
 Plate motion is caused by convective currents in
the asthenosphere

Types of plate boundaries
Divergent- plates pull apart
 Real life example: atlantic ocean and east
african rift valley
 Causes: earthquakes, sea floor spreading,
rift valleys, and mid ocean ridges
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divergent

Mid ocean ridge

East African rift valley
Convergent continental continental
2 continents collide-push together
 Example: how Appalachian mountains
formed and the Himalayan mountains
 Causes earthquakes and mountains


Appalachian
mountains form

Himalayan mountains
Convergent oceanic oceanic
2 ocean plates collide-push together
 Example: islands of japan, indonesia
 Causes: earthquakes, subduction zones,
tsunamis, volcanic islands

Convergent oceanic continental
An ocean plate and continental plate
collide
 Examples: Cascade volcanic mountains in
northwestern US (Mt st Helens)
 Causes: earthquakes, volcanic eruptions,
tsunamis

Transform fault
2 plates slide past each other
 Example: San andreas fault in California
 Causes only earthquakes

What evidence supports the theory
of plate tectonics?

Paleomagnetism- alternating layers of rock
at the bottom of ocean floors show normal
and reverse polarity
Earthquake patterns
Hot spots