Alfred Wegener
1912
Proposed idea of
“CONTINENTAL DRIFT”
Continental Drift
The continents were all connected at one
time
Supercontinent was called Pangea
Pangea broke apart up into two
continents call Laurasia and
Gondwanaland
Continents are still moving today
Evidence for Wegner’s Idea
Continents fit together like puzzle
pieces
Fossils found on different continents
were identical…how’dey do dat?
Patterns from movement of ice
indicate continents were once
together
Fossil Evidence
Marks from ice
movement
striations
No one believed Wegner…..
1930
Wegener leaves for Greenland
to find evidence for his
hypothesis.
He is never seen again
The last photo him
His theory is hotly debated
for the next 30 years
Plate Tectonics
Continental drift meets
seafloor spreading
(animations from USGS)
Plate Tectonics
Earth’s crust is divided into about 22
pieces called plates
Plates ‘float’ on the mantle
Convection currents in the mantle move
the plates around
Movement causes earthquakes and
volcanoes
Evidence for plate
tectonics
1. Location of earthquakes
and volcanoes
Map of earthquakes for January 2005
Pacific Ocean Floor:
The Plates
Evidence cont’d
Age of seafloor
–Oceanic igneous rock is much
younger than continental igrox
–Oceanic rock gets younger as you
approach the mid-ocean ridge
(mountain range which runs
through all world’s oceans)
Map showing
Ages of
oceanic crust
Magnetic pattern in rocks
– Earth’s magnetic pole reverses
– Magnetic crystals line up with earth’s poles
when the magma hardens
– Patterns in ocean rocks show parallel strips
on either side of the ridge that match in width,
age, distance from the ridge, and polarity
What pulls the lithosphere apart?
Heat produced by
radioactive decay
creates convection
currents in mantle
Flow of mantle
pulls lithosphere
above it apart
Plates move
Types of Plate Boundaries
Divergent / Rift
Convergent/ Subduction
Transform
Divergent/ Rift
Plates move apart from one
another
New basaltic crust is formed
Mid-ocean ridge: volcanic
mountain range which circles
the earth beneath the oceans
Mid-ocean ridge
Rift valley
Black
Smoker,
East Pacific
Rise
Marie Tharp & Bruce Heezen used echo soundings to build up a
picture of the ocean floor
Continent
Mid-ocean
ridge
Continent
Sea mount
Central rift zone
trench
Mid-ocean ridge
seamounts
Convergent Boundary
Plates collide
Continental/ oceanic plates
Oceanic/ oceanic plates
Continental/ continental
plates
Crust is destroyed
Ocean- Continent collision with subduction
Ocean- Continent collision with subduction
Andes Mountains
Ocean-ocean collision with subduction
Ocean-ocean collision with subduction
Hugo Benioff suggested that this Earthquake pattern show a plate
subducting (sinking) into the mantle
Continent-continent collision (No Subduction)
Continent-continent collision (No Subduction)
1963 Tuzo Wilson
Developed the hypothesis of hot spots to explain islands like Hawaii
& a third kind of plate boundary called a transform plate boundary
Transform
Plates slide past one another
–Ex. San Andreas fault
Crust is neither created nor
destroyed
Hot Spots
Places where magma
reaches earth’s
surface
Not associated with
plate boundary
As plate moves over
hot spot, island
chains form
Example: Hawaii
So by the mid-60’s all the evidence came together to form the
Theory of Plate tectonics
This theory states that the Earth crust is broken into a series of
plates which are constantly on the move.