So how did the theory of Plate tectonics develop? 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 Continental shelf Continent Mid-ocean ridge Sea mount Central rift zone Abyssal plain 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 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. magnetite S N then N S 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 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?

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 and then . This pattern became known as . reversed polarity normal polarity 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

Theory of Plate tectonics 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 The Earth is like a giant Magnet. It magnetic field is generated by movement in the outer core.