Unit 1: Internal
Geological Processes
Chapter 1: Plate
Tectonics
I- Continental drift
In 1912, a German scientist
called
Alfred
Wegener
proposed a hypothesis called
continental drift. According to
this hypothesis, the continents
were united together in one
supercontinent 200 million
years ago, Wegener named this
super continent Pangea.
Continental drift
 What were Wegener’s proofs of continental drift?
To support his hypothesis,
Wegener provided a number
of proofs:
1- The complimentary shapes
of the continents: the
continents look as if they were
pieces of a giant jigsaw puzzle
the fit together to make one
piece.
2- The fossil
distribution: reptile
fossils aged more
than 200 million
years old are found in
separate continents
Example: the
Mesosaurus
3- distinctive rock
distribution: rock
structures found in
West Africa and East
Brazil are identical.
The rock formations
match in both age
and nature.
 What were Wegener’s proofs of continental drift?
 Conclusion
- These 3 proofs along with other ones are valid
evidence of continental drift.
- Sadly, Alfred Wegener died during an excursion
looking for more evidence to support his
hypothesis.
- Wegener could not explain continental drift, it
was not until the 1960s and 1970s that other
scientists made more discoveries.
II- Seafloor spreading
In the 1960s, more evidence of
continental drift has been
discovered. Harry Hess proposed
the seafloor spreading theory.
Using the next figure, let’s try to
explain what is Hess’s theory.
Atlantic Seafloor basalt ages
0 – 10 million years old
8 – 55 million years old
56 – 95 million years old
96 – 135 million years old
The Mid-oceanic ridge activity
Explanation
• Nearing the seafloor center (mid-oceanic ridge) the
basalt ages become younger. This suggests that the
mid-oceanic ridge is where new basalt is created.
• The mid-oceanic ridge is a volcanic mountain range.
Molten basalt (magma) comes out of the ridge which
creates new seafloor crust that takes the place of the
old one.
III- Tectonic Plates
1- Boundaries of tectonic plates
Earthquake and Volcanic Activity
Tectonic Plates
Earthquakes and volcanoes are not scattered
randomly, instead they are confined to continuous
bands.
These bands of earthquakes and volcanoes outline the
boundaries of tectonic plates.
Definition: a tectonic plate (lithospheric plate) is a
fragment of the lithosphere (first 2 layers of the earth),
limited by areas where there is great seismic and
volcanic activity.
Tectonic plates are mobile, their motion is measured
using Global Positioning System.
Measuring plate motion using GPS
IV- The energy source for plate tectonics
Experiment
Separation of the wood
pieces
Convection currents
Explanation
• When the water gets hotter, it becomes less dense
(lighter) and moves upwards.
• The colder water on top is more dense (heavier) so it
moves down.
• This creates convection currents that cause the wood
pieces to separate.
Comparison of convection in a boiling pot to the motions in the mantle.
•The heat coming from the core causes
convection currents to form in the
mantle.
•Convection currents are the engine
behind the motion of the plates: Plate
Tectonics.
Converging and diverging plates
•In places where convection currents rise up,
tectonic plates move away from each other:
diverging plates.
•In places where convection currents fall
down, tectonic plates move closer to each
other: converging plates.