Plate Tectonics

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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?
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Early ideas Continents seem to be able to fit
together!!
1858 Geographer Antonio Snider-Pellegrini made
these maps
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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 its 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.
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Alfred Wegener 1912 Proponent of the
continental drift theory/hypothesis German
meteorologist
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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

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Wegeners evidence for Continental Drift
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Scientific community ridicule Wegeners ideas His
theory is rejected

• Continents Oceans didnt 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!

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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
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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 1950s 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

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Mid Ocean Ridges
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Marie Tharp Bruce Heezen used echo soundings to
build up a picture of the ocean floor
Continental shelf
Continent
Mid-ocean ridge
Sea mount
Central rift zone
Abyssal plain
Oceanic trench
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Ocean floor features
Trenches
Sea mounts
Mountain chains
Fracture zones
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Sediments on the Ocean Floor
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1950s 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
Earths present magnetic field. (The north end of
the rocks crystal points toward the magnetic
pole). Other rocks have reversed polarity.
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• How do these crystals align themselves?
• Grains of act like small magnets
  and can align themselves with the Earths
  magnetic field in molten rock.
• When the rock cools the crystals are locked in,
  recording the Earths magnetic field at the time
  of the rocks formation.

magnetite
S N then
N S today
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• 1961 The hypothesis of sea floor spreading
• the mid ocean ridges were areas in the Earths
  crust that was weak that magma erupts there to
  create new oceanic crust.

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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 normal-reverse-normal
  suggesting that the Earths 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?

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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.
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Harrys Idea
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Harry Hess
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• 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.
• Earths magnetic field reverses over time
• The Earths crust is divided into a series of
  plates
• This evidence started to convince most scientists.

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The ocean floor showed a pattern of magnetic
variation that was not random. The pattern
appeared as a zebra strip of alternating strips
of and then . This
pattern became known as .
reversed polarity
normal polarity
magnetic striping
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1960s Increased monitoring of Nuclear testing
helps to locate earthquake zones
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Hugo Benioff suggested that this Earthquake
pattern show a plate subducting (sinking) into
the mantle
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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
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Wilsons transform faults
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Sir Edward Bullard measures ocean temperatures
Heat coming from central ridges 1967 Bruce
Heezen hypothesiss that the Earth is expanding
from the mid-ocean ridges
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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)
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Map showing Ages of oceanic crust
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1968 Jason Morgan (Princeton) Dan McKenzie
(Cambridge) independently suggest that the
Earths crust is divided into plates. They
put the final touches on what is now called the
Plate Tectonics Theory.
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1977 Hot springs discovered at the mid-ocean
ridges
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Black smokers
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So by the mid-60s 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.
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The Plates
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Pacific Ocean Floor
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The Earth is like a giant Magnet. It magnetic
field is generated by movement in the outer core.