GEOLOGY 112 (GEO12) Continental Drift - Plate Tectonics
Name: ______________________________________
Two complimentary theories on what shapes the Earth’s surface:


1st Theory – Continental Drift - Alfred Wegner
The 2nd Theory - Plate Tectonics - suggests forces that would cause Continental
Drift.
youtube: Alfred Wegener: Great Minds scishow
https://www.youtube.com/watch?v=nbU809Cyrao
In 1915, the German geologist and meteorologist Alfred Wegener first proposed the
theory of continental drift, which states that parts of the Earth's crust slowly drift
atop a liquid core. The fossil record supports and gives credence to the theories of
continental drift and plate tectonics.
Wegener hypothesized that there was a
gigantic supercontinent 200 million years
ago, which he
named Pangaea
, meaning "Allearth".
Pangaea started to break up into two smaller supercontinents, called Laurasia and
Gondwanaland, during the Jurassic period. By the end of the Cretaceous period, the
continents were separating into land masses that look like our modern-day
continents.
Wegener published this theory in his 1915 book, On the Origin of Continents and
Oceans. In it he also proposed the existence of the supercontinent , and named it
(Pangaea means "all the land" in Greek).
Everyone laughed at Wegener. Obviously the continents couldn’t
move! What force could be big enough to move a continent? Besides
Wegner was just a weatherman.
Plate Tectonics
Before WWII the ocean bottom was thought to be mostly flat and featureless.
During World War II a Princeton professor named Harry Hess was a captain of a
transport in the Pacific. He kept records of the fathometer (depth sounder) reading
everywhere he went. After the war he mapped the Pacific Ocean bottom revealing
mountains, trenches, plains, etc. He devised the theory of sea-floor spreading, and
suggested that convection cells in the earth’s mantle caused the movement.
A Canadian geophysicist named John Tuzo Wilson put the whole thing together – the
plate tectonic theory with all its details, especially the theory of “Wilson Cycles.”
In Wilson Cycles ocean basins open and close over hundreds of millions of years. The
ocean crust is continually recycled, and older continental crust rides on top.
The theory of plate tectonics (meaning "plate structure") was developed in the 1960's.
This theory explains the movement of the Earth's plates (which has since been
documented scientifically) and also explains the cause of earthquakes, volcanoes,
oceanic trenches, mountain range formation, and other geologic phenomenon.
The plates are moving at a speed that
has been estimated at 1 to 10 cm per
year. Most of the Earth's seismic
activity (volcanos and earthquakes)
occurs at the plate boundaries as they
interact.
Type of
Average Average
Major
Crust
Thickness
Age
Component
Continental 20-80
3 billion
Granite
Crust
kilometers years
Hundreds
Oceanic
10
of
Basalt
Crust
kilometers millions
of years
The top layers of the plates are called
the crust. Oceanic crust (the crust
under the oceans) is thinner and denser than continental crust. Crust is constantly
being created and destroyed; oceanic crust is more active than continental crust.
TYPES OF PLATE MOVEMENT: Divergence, Convergence, and Lateral Slipping
At the boundaries of the plates, various deformations occur as the plates interact;
they separate from one another (seafloor spreading), collide (forming mountain
ranges), slip past one another (subduction zones, in which plates undergo destruction
and re-melting), and slip laterally.
Read section 9.1; Pages 248 to 253
When was the hypothesis of continental drift first proposed?
Who made the proposal?
What did Wegner’s continental drift hypothesis state?
What did he call the last supercontinent?
What does Pangea mean?
Briefly (no more than two sentences) summarize the evidences that Wegner used in
his proposal

Evidence – The Continental Puzzle

Evidence – Matching Fossils

Evidence – Rocks and Structures

Evidence – Ancient Climates
Why did most people reject Wegner’s hypothesis? What was missing?