Plate Tectonics
Continental Drift and Seafloor Spreading
What is the problem?
 The idea of continental movement was not even proposed
until the late 1800’s, why?
 In the average human’s lifetime there are little or no changes
seen in the Earth’s surface.
 The processes that we can see in our lifetime are:
 Earthquakes
 Volcanic Eruptions
 Landslides/ mud slides, etc.
 However, if you measure time in millions of years, there is
tremendous amounts of movement.
Early Ideas
 Eduard Suess (pronounced Zys) was the first person to
hypothesize that the southern continents were once joined.
 Suess’s evidence was the presence of a fossilized land plant,
the Glossopteris fern, which was found in South America,
Africa and India.
 Right!
 He was concluded that the southern continents were connected,
called the super-continent “Gondwanaland”
 He proposed that the rest of the planet was a massive ocean he
called the “Tethys Ocean” which covered most of the globe.
 Wrong!
 He thought the continents were connected by a series of land
bridges that have since been covered by water.
Alfred Wegener
 Eduard Suess only had one real piece of evidence and was
not taken seriously.
 In 1912 Alfred Wegener (vegener) presented his theory of
continental movement and was finally taken seriously.
 Theory of Continental Drift
 All of Earth’s land masses were once joined, which he called,
“Pangea”, which is Greek for “all the Earth.”
 He believed that about 200 million years ago the continents
began to move apart and have done so in a slow and constant
fashion ever since.
Evidence: Puzzle Like Fit
 Looking at the edges of some of the continents it is easy to
see how they could have fit together.
Evidence: Split Geologic Structures
 Wegener thought that when Pangea broke apart it would split
large geologic structure. Ex. Mountain ranges, rock layers, etc.
 Similar mountain range structures are found from Alaska to
Russia.
 The Appalachian Mt. range extends through Greenland and
across to northern Europe.
Evidence: Land Fossils
 Wegener hypothesized that since living things would have
once lived all across Pangea that there would be fossil
evidence on continents now separated by thousands of miles!
 Wegener used the Glossopteris fern as evidence
 The Lystosaurus, a small pig like reptile found in Africa, India
and Antarctica that lived about 200 million years ago..
Evidence: Coal Deposits
 Coal is formed when water logged plant material in tropical
areas is buried and the rich organic material is compacted.
 Wegener noticed that there are large deposits of coal near
the north pole.
 Meaning that those land masses had to have been closer to
the equator when the coal was forming.
Evidence: Glacial Deposits
 Massive glacial deposits located in southern Africa, India,
Australia, and South America about 300 million years ago.
 These continents had to have been close to the south pole.
 Grooves from the growing glaciers further show how the
continents fit together.
Five Pieces of Evidence
 What were Wegener’s five piece of evidence to support
Continental Drift?





1
2
3
4
5
 Don’t say that he didn’t have any evidence!
 BUT…
Rejected!
 After decades of gathering impressive data, his theory was
rejected.
 He could not explain what force could move the massive
continents.
 He thought it was the rotation of the earth, if true all the continents
would be piled up around the equator.
 He could not explain how the continents moved without shattering.
 He thought that the continents plowed through the ocean floor , but
geophysicists proved him wrong.
 Wegener froze to death in 1930 in Greenland on an expedition to
collect more data to complete his theory.
 With Wegener gone, his theory was completely dismissed.
Seafloor Spreading
 During Wegener’s day, it was thought that the ocean floors were
thick, flat and very old compared to the continents. They were
WRONG!
 As it turns out, all the evidence Wegener needed to complete his
theory was at the bottom of the ocean.
Ocean Floor Topography
 With the invention of sonar, scientists were able to map the
depth of the ocean. Here is what they discovered.
 Ridges – underwater mountain chains that were the longest
on Earth.
 Trenches – narrow, elongated depressions in the sea floor
with steep sides. Mariana trench is over 6 miles deep.
Ocean Rocks and Sediments
 Samples were taken of the rocks on the ocean floor and two
patterns were discovered.
 1 – The ages of the rocks made a pattern. The youngest rocks
were located near the ocean ridges and the oldest rocks were
located near the trenches.
 The ocean floor was also very young, some of the oldest is around 180
million years old. (Bed rock in Nicholasville is about 270 million years
old.
 2 – Sediments on the ocean crust were much thinner than
expected. Thickness increased as you got farther from the ridges.
Magnetism
 Basalt – is an iron rich rock
produced from lava.
 The iron in the lava aligns
to Earth’s magnetic field
like a compass needle
 When the lava cools the
iron rich minerals (compass
needles) are locked in
place.
 Earth’s magnetic poles flip
about 4 or 5 times every
million years.
Ocean Floor Magnetism
 When scientist studied the
magnetism of the ocean floor,
another pattern emerged.
 In places where the minerals were
pointing the SAME direction as
earth’s current magnetic field the
reading was STRONGER than
normal.
 In places where the minerals were
pointing the OPPOSITE direction
as earth’s current magnetic field
the reading was WEAKER than
normal.
Ocean Floor Magnetism
 The pattern of
magnetism was perfectly
mirrored on either side of
mid ocean ridges.
 We know that due to
volcanism new rock is
being made at the ridges.
 This allows us to count
the magnetic stripes like
tree rings to determine
rough relative age.
Putting It All Together
 Harry Hess, a young American
scientist came up with the theory
of seafloor spreading.
 New rock is created at the ridges,
when it cools acts like a wedge,
pushing the continents apart. (5%
- 10% of motion)
 Old rock is destroyed at the
trenches, as it is forced under it
pulls the continent towards the
trench, called slab pull. (90% 95% of motion)
Putting It All Together
 Seafloor Spreading was
the missing part to
Wegener’s theory.
 Continents move due to
convection currents in the
mantle.
 Continents do not plow
through the oceanic crust,
but are passengers on it as
it travels from ridge to
trench.