Download Plate Tectonic Theory

A. Three stages to get
there

1. Continental Drift
2. Seafloor Spreading
3. Plate Tectonic Theory
B. What did people believe about
the earth before Plate Tectonics?

1. The continents were in fixed
positions
2. The map of the earth was
unchanging
a) Same past, present, and
future
locations of continents
II. Continental Drift

A. Alfred Wegener (1880-1930)
German
Astonomer/Meteorologist/clim
atologist/geologist
First to use weather balloons
Wrote a textbook on Geology
that Germans LOVED
Just wanted to be a professor
somewhere!
Proposed that continents have slowly
drifted to current locations

All land masses were once joined
together in a single large mass called
“Pangea”-Greek for “All Land”
Pangea split apart and the continents
gradually moved to their current
positions= Continental Drift

5. Most of the evidence was already known.
Wegener just assembled it and proposed
the theory
EPIC.
"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." The Origins of Continents and Oceans (4th
edition)
Evidence for
Continental Drift

Continents fit together
like a puzzle
Atlantic Coastlines
of South America
and Africa
Evidence for
Continental Drift

The same fossils are found on separate continents
Plants (Glossopteris) and Animals (Mesosaurus,
Cynognathus, Lystrosaurus)
These plants and animals couldn’t fly or swim
that far
Evidence for
Continental Drift

Similar rock sequences in
separate areas
In South America,
Africa, India, Antarctica,
Australia
Basalt lava on top, then
sandstone/shale, then
glacial till
Evidence for
Continental Drift

Evidence of different
climates in the fossil
record
Antarctica has
fossilized
plants…somewhere
warmer
Africa has glacial
till….it was somewhere
colder
REJECTED
The continents didn’t have a perfect fit, and
most didn’t fit at all
Land bridges…big ones… could have
allowed crossings
Coincidence or widespread events
Polar wandering- the earth’s polar locations
may have changed
Most importantly…wouldn’t we feel them
move?
Couldn’t measure it
Couldn’t explain the cause of movement.
Alfred was ridiculed, but finally got a
professorship
Soon after, he reluctantly went on his 4th
trip to Greenland
Died collecting Ice samples
Seafloor Spreading

Harry Hess
Naval Rear Admiral. Served in WWII
Post-WWII, helped map the seafloor
Worried about the Soviets and subs
Discoveries by others of the undersea
ridges and mountains
He reviewed the data again and
made a hypothesis
Seafloor Spreading

Sonar allowed mapping of sea floor
“Scar” encircles the whole globe. Mid-ocean
ridge
A mirrored profile emerged
Undersea mountains with a rift in the middle
Mountains led to low abyssal plains on both sides
Ended with continental slope and shelf on each side
Not perfectly symmetrical though
Proposed that hot, less dense mantle
material rises toward the mid-ocean ridges
Some material solidifies. Some flows
sideways and drags the seafloor along with
it in both directions
As the seafloor spreads apart, new
seafloor is created
Older seafloor moves away in opposite
directions
This could explain how the crust moved!
Supports Continental Drift Theory.
Evidence for Seafloor
Spreading

Mid-Ocean Ridge System encircles globe
Less sediments and fossils around the midocean ridges (Younger)
Evidence for Seafloor
Spreading

1968- Glomar Challenger Expedition
Collected rock samples to see ages
Younger rocks near mid-ocean ridges.
Older near continents.
Ocean floor age mirrored on each side
of mid-ocean ridge
Evidence for Seafloor
Spreading

Paleomagnetism
When rocks form, they take on the
magnetic profile of the earth at that
time
Magnetic field switches every so often
though.
Pattern mirrors on ocean floor as well
Seafloor spreading
explains how
continental drift can
occur.
IV. Plate Tectonic
Theory

It’s a combination of continental drift
and seafloor spreading
The lithosphere is broken into tectonic
plates.
They move across the upper mantle
(Asthenosphere)
They collide and divide to create the
major landforms on Earth today
Plate Tectonic Theory

Caused by convection currents
Heated Asthenosphere flows upwards (less
dense)
When it hits the crust it spreads outwards
Flowing Asthenosphere drags lithosphere
with it
Asthenosphere material cools, sinks.
5. Cycle repeats
C. Tectonic Plates
1. Explains
a) Mountains
b)Earthquakes
c) Volcanoes
Other geologic patterns and
features
Can use these features to find the
plates