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Continental Drift & Plate
Tectonics
 The theory of Continental Drift was
proposed by Alfred Wegener (18801930) a German scientist.
Smoking
eventually killed
him!!!!
 Wegener proposed his theory in
1915 and (like many Scientists) was
ridiculed for his theory.
One Critic of Wegener
 Dr. Rollin T. Chamberlin of the
University of Chicago said, "Wegener's
hypothesis in general is of the footloose
type, in that it takes considerable
liberty with our globe, and is less bound
by restrictions or tied down by
awkward, ugly facts than most of its
rival theories."
 By the 1960’s, however, with the
exploration of the ocean floors and the
development of the Theory of Plate
Tectonics, almost all geologists
accepted Wegener’s Theory of
Continental Drift…. Too bad he was
long dead by then 
Continental Drift
 Alfred Wegener’s continental
drift hypothesis stated that the
continents had once been joined to
form a single supercontinent.
 Wegener proposed that the
supercontinent, Pangaea, began
to break apart 200 million years
ago and form the present
landmasses.
Evidence: The Continental Puzzel
Why might have Wegener thought the continents were once joined together?
What happens to shorelines over time?
Which continents appear to have separated first?
Which ocean began to form when this happened?
Evidence: Matching Fossils
 Fossil evidence for continental drift includes several
fossil organisms found on different
landmasses.
 Example: Mesosaurus fossils (freshwater animal) found
on Eastern South America and Southern Africa.
Evidence: Rock Types & Structures
 Rock evidence for continental exists in the
form of several mountain belts that end at one coastline,
only to reappear on a landmass across the ocean.
 Example: Appalachian Mnts. in the USA are similar to
Mnts. in the British Isles & Scandinavia.
Evidence: Ancient Climates
 Glacial deposits show
that ice sheets covered
large areas of the
southern hemisphere.
 In what type of climate
do these past ice sheets
now belong in?
Rejecting Wegener
 Wegener could not provide an explanation of exactly
what made the continents move.

He proposed a tidal influence from the Moon.
 New technology lead to findings which then lead to a
new theory called plate tectonics by 1968.
 Plate tectonics provides the framework for
understanding most geological processes.
Plate Tectonics
A Review
 The lithosphere is made of the upper mantle (above
the asthenosphere) plus the crust (both continental
and oceanic).
 The lithosphere is solid and makes up the plates.
 The asthenosphere is the part of the mantle that is
liquid or plastic or molten.
 The plates (lithosphere) float or are balanced on the
asthenosphere. The interactions between plates are
called plate tectonics.
 The lithospheric plates move about
as fast as your fingernails grow
(between 2.5 cm -5.0 cm per year).
This is between 1-2 inches per year.
 The movement of the plates is by convection
currents in the asthenosphere. Warm/hot
magna rises and cool magna falls.
Major Plates of the Earth
 Plates interact with each other.
There are three types of plate
interactions.
 Divergent Plate Boundaries
 Convergent Plate Boundaries
 Transform Fault Boundaries
Divergent: Plates pull apart from
each other. When the plates pull apart,
new magma seeps up.
 Forms: rifts and new seafloors (seafloor
spreading)
Rift from Divergent Plate Boundary
Great Rift Valley in Africa
Arabian Plate
Rift
African Plate
Rift Valley and Seafloor Spreading
Transform Fault Boundary: the two
plates grind past each other without generating new
lithosphere. Mostly occurs on seafloors.
 Forms: Earthquakes
Our Local Fault is a Transform Fault!!!
The San Andreas Fault
Convergent Plate Boundary: the
plates move toward each other. The two
plates hit and deform the crust. Tends
to cause volcanoes and earthquakes
 There are three different types of
convergent plate boundaries.
Oceanic-Oceanic
Continental-Continental
Oceanic-Continental
Oceanic-Oceanic
 When Oceanic crust hits Oceanic crust,
one of the oceanic crusts descends
(subducts) under the other which
creates underwater volcanoes and
trenches.
 Volcanic island arcs form such as the
Aleutian, Mariana and Tonga Islands.
Tonga
Continental-Continental
 When Continental Crust hits
Continental Crust, mountain ranges
such as the Himalayas (Indian and
Eurasian Plates) are formed.
Oceanic-Continental
 When Oceanic Crust hits Continental Crust,
the Oceanic Crust (which is lighter and
denser) descends or subducts under the
Continental Crust. This is called subduction.
 This forms earthquakes, trenches, mountain
ranges and volcanoes.
Subduction
Zone
 Off the coast of South America, the
Nazca Plate (oceanic) is subducting
under the South American Plate
(continental).
 This creates the Andes Mountains. It
also causes earthquakes and volcanoes
in this region.
Andes Mountains
 Subduction also is the cause of the Ring
of Fire.
Why does Subduction cause Volcanoes?
Let’s Review!!!!!
Hot Spots!
 A hot spot is a concentration of
heat in the mantle capable of
producing magma, which rises to
Earth’s surface; The Pacific plate
moves over a hot spot, producing
the Hawaiian Islands.
Hawaii
How do we know that Plate Tectonics is
happening?
 Paleomagnetism is the natural
magnetism found in rocks; this
permanent magnetization can be used
to determine the location of the
magnetic poles at the time the rock
became magnetized.
Two Types of Polarity
 Normal polarity—when rocks show
the same magnetism as the present
magnetism field
 Reverse polarity—when rocks show
the opposite magnetism as the present
magnetism field
Paleomagnetism Found in Lava Flow
 The discovery of strips of
alternating polarity, which lie as
mirror images across the ocean
ridges, is strong evidence of
seafloor spreading.
More Evidence for Plate Tectonics
 Scientists found a close link between
deep-focus earthquakes and ocean
trenches.
 The lack of deep earthquakes along
oceanic ridges is also consistent with
the Theory of Plate Tectonics.