1. Plate Tectonics The theory of plate tectonics was first proposed by
Alfred Wegener in Wegener noticed that the
shorelines of the continents seemed to ‘fit together’
like the pieces of a giant jig saw puzzle.

2. Plate Tectonics Wegener’s theory stated:
The continents were once all together in one place forming a supercontinent, Pangea.
The continents broke apart and drifted to their present locations.

3. Plate Tectonics Wegener’s theory was not taken seriously
because no one could believe that things as
large as continents could move and because
Wegener could not propose a mechanism
which could explain such motion.

4. Today plate tectonics and continental drift are accepted as facts.
The shorelines of the continents do fit together like the pieces of a puzzle.

5. Sea Floor Spreading
If the continents are moving apart then the seas between them must be
getting wider.
This is called sea floor spreading.
A series of under-water mountains called mid-ocean ridges can be found throughout the world.

6. Because the sea floor is spreading away from the center, rocks which are equidistant but on opposite sides of the ridge are the same age. Rock B is the same age as rock D. Rock A is the same age as rock E. The oldest rocks are found at the edges of the continents.

7. Continental drift seems to be a proven theory…
Wegener was right!
But why......?

8. Convection currents in the mantle.
Rock near
the core is heated and
becomes less dense. It
slowly rises while
cooler rock nearer the
surface sinks.
This forms
convection currents
As these convection currents in the mantle circulate, they cause the
continents above them to move.
Convection currents in the mantle.

9. Let's sum up everything we've learned:
1) The earth's crust is divided into many pieces called tectonic
plates which move around over long periods of time.
2) Evidence for crustal movement includes:
a) The 'fit' of the continents.
b) Correlation of rock types from opposite shores.
c) Correlation of fossils from opposite shores.
d) Age of the sea floor (youngest near mid-ocean ridges
and oldest near the continents).
3) The force driving the plates are convection currents (density
differences) in the earth's mantle.