Transcript History of Plate Tectonics PPT

History of Plate Tectonics
starring Alfred Wegener’s Theory of
Continental Drift… & the Theory of
Plate Tectonics
How to start your timeline
• Get out a sheet of blank
notebook paper
• Turn it so the holes are
at the bottom
• Add the title “Plate
Tectonics Timeline”
(Note the red—when
you see it, write those
words down.)
• Draw a line along the
long side of the paper,
from one end to the
other
Plate Tectonics Timeline
Still starting Timeline
• Add the 9 hatch marks where shown (they do
not have to be exactly
Plate Tectonics Timeline
as shown—try to space
them this way)
• You will add the years
marked in red as we
go along.
• To write the info in red,
you can write above or below the date—or
you can number them #1-9 and write the info
on back of your sheet
Alfred Wegener
• 1) In 1912 a
German
meteorologist,
Alfred Wegener, had
an idea about
continental movement.
• He noticed that some
continents fit together
like puzzle pieces.
• His hypothesis was
called Continental
Drift.
• His hypothesis
proposed that the
continents had
once been joined
as a single
landmass, called
PANGAEA.
• ‘Pangaea’ is a
Greek word
meaning ‘All the
Earth’
PANGAEA
• Wegener believed
that Pangaea began
to break apart 250
million years ago.
• The continents have
been continuously
moving since then to
where they are now.
Wegener’s Evidence
1. Rocks – on different continents were
the same type!
2. Fossils – cases of the same animal
and plant fossils being found on
different continents!
3. Climate – evidences of climates in
the past that were very different from
today’s climate at some locations
(We will go over this evidence in
greater detail on another day.)
Was he believed? NO!
• Other scientists rejected Alfred Wegener’s
hypothesis because he
couldn’t explain what was
causing the continents to
move (a.k.a. HOW?)
• He was a laughingstock in the science
community at that time. He returned to
working as a meteorologist but
continued to believe in his theory.
Link to funny YouTube video about Alfred Wegener:
https://biodeluna.wordpress.com/2014/02/20/wegener-song-by-theamoeba-people/
Note that this is #3, not
#2. That is on the next
slide.
Arthur Holmes
• 3) 1929, a
geoscientist, Holmes
proposed that
convection occurred
in the mantle.
• This convection
could push and pull
plates (continents)
apart or together.
• But, again, NO
evidence!!!
FINDING EVIDENCE
The first way to use technology to map the ocean floor
was done in the 2) 1920’s & used an early type of Sonar.
This way stunk because it didn’t accurately map the floor.
Then the 4) Magnetometer was invented in
1955. This could detect small changes in the
magnetic field in rocks and map them.
So what?
• Sailors had known for a long time that the
ocean floor was not smooth. They used
ropes tied with knots to get a rough estimate
of the ocean floor near the coasts.
• All of a sudden, with this new technology,
an accurate map of mountains in the middle
of the Atlantic Ocean floor were revealed…
scientists called them a mid-ocean ridge!
Seafloor Spreading
• Mid-Atlantic Ridge
Seafloor Spreading – new ocean crust is formed
at ocean ridges and destroyed at deep sea
trenches.
5) Robert Dietz, a geophysicist
and oceanographer, proposed
in 1961 a hypothesis he
called “seafloor spreading”.
He is the one who came up
with that phrase.
Harry Hess
• 6) In 1962, Harry Hess, a professor of
geology and former Navy officer who had mapped
ocean floors during WWII while his ship hunted
submarines, formally published a similar
hypothesis originally in a 1959 paper
– again, seafloor
spreading!
Both men are
credited with
this amazing
discovery.
Drum Matthews & Fred Vine
• In Cambridge, England 7) in1963, British Drum
Matthews and his research assistant Fred Vine
published a paper based
on magnetic surveys
of the ocean floor that
Matthews & and others
had done.
• Their findings about magnetic variations in
ocean crust at mid-ocean ridges supported Hess’
& Dietz’ theory!
These new maps showed the different magnetism
in different layers of rock.
As new crust is created the iron bearing minerals
point to where the magnetic pole is at the time of
creation.
This is called Paleomagnetism
Mid-Ocean Ridge
• Isochron – line of same aged rock
Magnetic Pole Reversals
What these results showed
• Random reversal of Earth’s Polarity
– Average irregular 300,000 year cycles
– Last one was over 780,000 years ago = overdue
• Unknown as to why
– Maybe external, maybe internal…
This proved that new sea floor was being created at
mid-ocean ridges.
Identical magnetism on opposite sides of ridges showed
the plates where moving away from each other…
Dan McKenzie
• In 8) 1966, British scientist Dan McKenzie
published a paper that provided a
mathematical model for convection
currents in the mantle
• Now, finally, an explanation for why and
how the tectonic plates move!
Plate Boundaries and Convection Currents
Another Driving Force for Plate
Movement?
• Recently, scientists have been looking
at the subduction end of the seafloor
spreading model
• In 9) 1994, Professor Seiya Uyeda of Japan, an
expert in plate tectonics, concluded in a speech
that the process of “Slab-Pull” may even be a
bigger driving force to move tectonic plates
than ridge pushing… but what is slab-pull?
Slab-Pull, a new theory about what
causes tectonic plates move
- As the denser ocean
plate subducts under
the other plate, the
weight of the cold,
dense slab “pulls” the
plate
behind it into the
mantle.
- The pacific plate, with a subduction zone, is
destroyed at a faster rate than is the north american
plate—which has no subduction zones!
Is slab-pull accepted?
• This is a relatively new idea, so it is not yet widely
accepted by all of the scientific community even
though it helps explain things like why the Pacific
Ocean is shrinking while the Atlantic Ocean is
growing.
• There is a lot of controversy among geophysicists
about whether the mechanism of slab-pull is strong
enough to move enormous tectonic plates.
• We will have to wait and see how the debate goes.
Some of you may find out in college whether this
becomes an accepted theory or is discredited!
The End…for now
• OK, that’s enough for now.
• Next, we’ll further examine the evidence
that Alfred Wegener found that supported
his theory of continental drift.
• Let’s hope we can do it in class.
• If not, look for another wonderful flipped
video!