LG: To understand how scientific evidence
supports the ideas of continental drift and
plate tectonics
What do we know about
Plate Tectonics?
Plate Tectonics

Plate tectonics was originally considered before
the 20th century. A theory was proposed
but scientists were quick to say it was wrong.
It was not until the 1950s that evidence began
to partially support the theory and continuing
examination of evidence developed further
understanding of the processes involved.
Plate Tectonics

The theory states that Earth's outermost layer,
the lithosphere, is broken into 7 large, rigid
pieces called plates: the African, North
American, South American, Eurasian,
Australian, Antarctic, and Pacific plates. Several
minor plates also exist, including the Arabian,
Nazca, and Philippines plates.
Major Tectonic Plates of
the World

Alfred Wegener

A German scientist
In 1915 suggested that Africa, South America,
Australia, Antarctica and India were joined
200 million years ago to form the
supercontinent Pangaea
His hypothesis was called continental drift
Wegener’s Theory

 Video time (4mins)
Questions

What was the ‘supercontinent’ called and
what did it break up into?
Alfred Wegener’s idea of continental drift
was not taken very seriously at the time. Why
not?
What would you say to Wegener about his
theory if he was still alive today?
Pangaea

 From Greek:
pan meaning “all, whole”
Gaia meaning “Earth, land”
Wegener used 3 types of evidence
to prove his theory!
1.Evidence from landforms
2.Evidence from fossils
3.Evidence from climate
Evidence from landforms
The shapes of the
continents seemed to
fit together.
Mountain ranges and
other features also
lined up.
Evidencefrom landforms
The ‘jig-saw’ puzzle
Evidence from landforms
The ‘jig-saw’ puzzle
Evidence from landforms
The ‘jig-saw’ puzzle
Evidence from Fossils

Glossopteris: a fern
 Fossils from one continent
matched fossils from other
continents.
 Wegener used both plant
and animal fossils.
found on the southern
continents
Mesosaurus: a
freshwater swimming
reptile found in Africa
and South America
Evidence from Fossils
fossil matches
Evidence from Fossils
fossil matches
Evidence from Climate
Wegener looked at
certain areas on Earth
and their climates
He noted that the
fossils he found on
certain sections of
Earth did not match
the current climate
Glacier scratches in S.
Africa
Evidence from Climate
Antarctic ice
Evidence from Climate
Ice could not have extended this far.
So...continents MUST have moved..
Continental Drift Theory
At first
nobody
believed
Alfred
Wegener’s
theory…..
WHY YOU
ASK!

He could
not prove
how or
why the
continents
moved.
Drift away
Scientists still do not appear to understand sufficiently that
all earth sciences must contribute evidence toward unveiling
the state of our planet in earlier times, and that the truth of
the matter can only be reached by combing all this evidence.
. . It is only by combing the information furnished by all the
earth sciences that we can hope to determine 'truth' here,
that is to say, to find the picture that sets out all the known
facts in the best arrangement and that therefore has the
highest degree of probability. Further, we have to be
prepared always for the possibility that each new discovery,
no matter what science furnishes it, may modify the
conclusions we draw."
Alfred Wegener.
The Origins of Continents and Oceans (4th edition)

Activity Time

1. Follow the instructions on the information page for colouring the
right areas in the right colours.
2. Cut out each of the continents along the edge of the continental
shelf (the outermost dark line). Alfred Wegener's evidence for
continental drift is shown on the cut-outs. Wegener used this
evidence to reconstruct the positions of the continents relative to
each other in the distant past.
3. Try to logically piece the continents together so that they form a
giant supercontinent.
4. When you are satisfied with the fit of the continents, glue them
into your book (along with the information page). Discuss with
your partner your choices in placement of each piece based on
the evidence and decide if the evidence is compelling or not.
Explain your decision and reasoning on the evidence.


Newer Evidence

 Wegener’s continental drift idea was not well accepted
because few scientist were convinced there was enough
evidence to support it
 In the 1950s two American scientists, Maurice Ewing and
Marie Tharp, used new technology called echo-sounding
to map the depths of parts of the Atlantic Ocean
 In the middle of the ocean they found a mountain range
(called the Mid-Atlantic Ridge), in the middle of the ridge
Tharp discovered a deep trench
Mid-Atlantic Ridge

Newer Evidence

 Rock samples were taken from the trench and found to
be much younger than those further away from the
trench
 They also found that the temperature of the water was
eight times higher along the ridge than anywhere else
 From these two observations Ewing inferred that at the
Mid-Atlantic Ridge new rocks are being made as hot
material rises from the Earth’s mantle and pushes the
older rocks away from the ridge line
Newer Evidence

 The idea of new rocks created a dilemma because the
Earth is not getting any bigger
 Just as the crust under the oceans is created along
oceanic ridges, somewhere else it must be destroyed
 If one plate rides over the top of another plate, the
bottom plate sinks into the semi-solid material
beneath it and is destroyed. This process is called
subduction
Newer Evidence

Further Evidence

If the crust is being created and destroyed,
then the crust should not be as old as the
Earth itself.
This was found to be the case. The oldest
rocks on the ocean floor are only 180 million
years old, less than the estimated age of the
Earth (4.6 billion years)

Questions

 How do mid-ocean ridges provide evidence to
support the theory of plate tectonics?
 The rocks on the Earth are much younger than the
Earth itself. How does this observation support the
theory of plate tectonics?
 If the Earth’s crust isn’t getting bigger, how is crust
disappearing?
Tectonic Plates

 http://www.youtube.com/watch?v=HrKTuCDierM
(27mins)