Transcript Theory of Plate tectonics

Just as the theory of evolution underpins Biology
The theory of Plate tectonics is the basis of Earth Science
So how did the theory of Plate tectonics develop?
Early ideas: Continents seem to be able to fit together!!
1858 Geographer Antonio Snider-Pellegrini made these maps
The Development of a theory
What is a theory?
In science it is a grand scheme that relates and explains many
observations and is supported by a great deal of evidence.
In everyday language it means a guess, a hypothesis, a prediction.
Oh “it’s just a theory” But this is not the meaning in science
Theories in science
It may be a currently accepted theory, widely supported theory, a
discarded theory, and a new theory.
Alfred Wegener 1912
Proponent of the continental
drift theory/hypothesis
German meteorologist
The Hypothesis of Continental Drift
•Continents were once together in a super continent he
called Pangea
•Then broke up into two super continents call Laurasia
and Gondwanaland
•Continents can move by plowing through the crust like
ships in the sea
•Continents are still moving today
Wegener’s evidence for Continental Drift
Scientific community ridicule Wegener’s ideas
His theory is rejected
•Continents & Oceans didn’t move!
•Not enough evidence
•Does not make sense in terms of physics, what forces
could move whole continents over large distances?
•Evidence in the southern hemisphere only!
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
1947 Ocean floor is mapped
•Before the 19th century most people thought that the ocean floor
was relatively flat & featureless.
•19th century measurements improved, discovery of underwater
mountains in the mid Atlantic Ocean.
•After World War I early sonar showed the ocean floor to be much
more rugged than was previously thought.
•In 1947 the survey ship Atlantis found that the sediment layer on
the floor of the ocean was very thin.
•In the 1950’s a great mountain range on the ocean floor was
discovered that virtually encircled the Earth. (Global mid-ocean
ridge) It zigzags between the continents with an average height of
4,500 m
Mid Ocean Ridges:
Marie Tharp & Bruce Heezen used echo soundings to build up a
picture of the ocean floor
Continent
Mid-ocean
ridge
Abyssal plain
Sea mount
Continental
shelf
Central rift zone
Oceanic
trench
Ocean floor features:
Trenches
Sea mounts
Mountain chains
Fracture zones
Sediments on the Ocean Floor:
1950’s More evidence is discovered:
Magnetic strips & Polar reversals
After World War II magnetometers that were used to find
submarines began finding odd magnetic patterns across the ocean
floor. The ocean floor is made up of Basalt, which is rich in
magnetite. Basalt had already been found that had different magnet
properties.
Some has normal polarity in which the magnetic crystals have the
same polarity as the Earth’s present magnetic field. (The north end
of the rock’s crystal points toward the magnetic pole). Other rocks
have reversed polarity.
How do these crystals align themselves?
•Grains of magnetite act like small magnets and can align
themselves with the Earth’s magnetic field in molten rock.
•When the rock cools the crystals are ‘locked in’, recording the
Earth’s magnetic field at the time of the rocks formation.
S
N
N
S
then
today
1961 The hypothesis of sea floor spreading
•the mid ocean ridges were areas in the Earth’s crust that was weak &
that magma erupts there to create new oceanic crust.
Evidence for the Sea floor spreading Hypothesis
•The rocks are younger near the oceanic ridge and older as you move
away
•The younger rocks always have present day (normal) polarity
•The stripes run parallel to the ridge and alternate between normalreverse-normal suggesting that the Earth’s magnetic field has reversed in
the past many times.
The sea floor spreading hypothesis gained many converts.
This lead to the “expanding Earth” hypothesis to explain the moving
continents.
This theory was not accepted and so the question became how can the
new crust be added without the Earth increasing in size?
1962 Harry H. Hess reasoned that if the crust is expanding in one
place it must have been shrinking elsewhere.
He theorized that the new crust formed at the mid-Atlantic ridges &
moved away like a conveyor belt to sink back into the mantle at the
oceanic trenches along the rim of the Pacific ocean.
But the evidence to test his hypothesis was still lacking.
Harry’s Idea
Harry Hess
1963 Harry H. Hess / Robert S. Dietz both come up with the
same idea independently.
Submarine mountains are volcanoes
New seafloor is created at the mid-ocean ridge
Moves away from mid-ocean ridge like a conveyor belt
Old seafloor is recycled back into the mantle at oceanic trenches
The sea floor spreading hypothesis
1963 Vine & Matthews explain the magnetic striping pattern.
Earth’s magnetic field reverses over time
The Earth’s crust is divided into a series of plates
This evidence started to convince most scientists.
The ocean floor showed a pattern of magnetic variation that was not
random.
The pattern appeared as a zebra strip of alternating strips of
reversed polarity and then normal polarity .
This pattern became known as magnetic striping .
1960’s Increased monitoring of Nuclear testing helps to locate
earthquake zones
Hugo Benioff suggested that this Earthquake pattern show a plate
subducting (sinking) into the mantle
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
Wilson’s transform faults
Sir Edward Bullard measures ocean temperatures Heat
coming from central ridges
1967
Bruce Heezen hypothesis’s that the Earth is expanding from
the mid-ocean ridges
1968 Ocean floor drilling for Oil
This hypothesis was further supported by petroleum exploration.
Off shore drilling was developed and a research vessel was
designed for marine exploration.
it criss-crossed the ocean floor from South America to Africa. The
samples were dated and did show that the rocks at the ridge were
new crust & got older as you moved away. (Potassium-Argon
dating)
Map showing
Ages of
oceanic crust
1968 Jason Morgan (Princeton) & Dan McKenzie (Cambridge)
independently suggest that the Earth’s crust is divided into plates.
They put the final touches on what is now called the Plate Tectonics
Theory.
1977 Hot springs discovered at the mid-ocean ridges
Black
smokers
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.
The Plates
Pacific Ocean Floor:
The Earth is like a giant Magnet. It magnetic field is
generated by movement in the outer core.