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
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 alive 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
Marie Tharp & Bruce Heezen used echo soundings to build up a picture of the ocean floor
Continent Mid-ocean ridge Sea mount
Oceanic trench
Abyssal plain
Continental shelf
Central rift zone
1950’s More evidence is discovered:
Magnetic strips & Polar reversalsAfter 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.
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magnetiteHow do these crystals align themselves? •Grains of 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.
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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.
•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 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?
Evidence for the Sea floor spreading Hypothesis
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.
1963 Vine & Matthews explain the magnetic striping pattern. Earth’s magnetic field reverses over timeThe Earth’s crust is divided into a series of plates
This evidence started to convince most scientists.
1963 Harry H. Hess / Robert S. Dietz both come up with the same idea independently.
Submarine mountains are volcanoesNew seafloor is created at the mid-ocean ridgeMoves away from mid-ocean ridge like a conveyor belt Old seafloor is recycled back into the mantle at oceanic trenchesThe sea floor spreading hypothesis
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
Hugo Benioff suggested that this Earthquake pattern show a plate subducting (sinking) into the mantle
1963 Tuzo WilsonDeveloped the hypothesis of hot spots to explain islands like Hawaii & a third kind of plate boundary called a transform plate boundary
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 OilThis 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)
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.
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.