Presentation on theme: "So how did the theory of Plate tectonics develop?"— Presentation transcript:
1So how did the theory of Plate tectonics develop? Just as the theory of evolution underpins BiologyThe theory of Plate tectonics is the basis of Earth ScienceSo how did the theory of Plate tectonics develop?
2Early ideas: Continents seem to be able to fit together!! 1858 Geographer Antonio Snider-Pellegrini made these maps
3The 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 scienceTheories in scienceIt may be a currently accepted theory, widely supported theory, a discarded theory, and a new theory.
4Alfred WegenerProponent of the continental drift theory/hypothesisGerman meteorologist
5The Hypothesis of Continental Drift Continents were once together in a super continent he called PangeaThen broke up into two super continents call Laurasia and GondwanalandContinents can move by plowing through the crust like ships in the seaContinents are still moving today
9Scientific community ridicule Wegener’s ideas His theory is rejected Continents & Oceans didn’t move!Not enough evidenceDoes not make sense in terms of physics, what forces could move whole continents over large distances?Evidence in the southern hemisphere only!
101930Wegener leaves for Greenland to find evidence for his hypothesis.He is never seen againThe last photo himHis theory is hotly debated for the next 30 years
11Ocean floor is mappedBefore 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
161950’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.
17How 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.magnetiteSNthenNStoday
191961 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.
20Evidence for the Sea floor spreading Hypothesis The rocks are younger near the oceanic ridge and older as you move awayThe younger rocks always have present day (normal) polarityThe 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?
211962 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.
241963 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 beltOld seafloor is recycled back into the mantle at oceanic trenchesThe sea floor spreading hypothesis1963 Vine & Matthews explain the magnetic striping pattern.Earth’s magnetic field reverses over timeThe Earth’s crust is divided into a series of platesThis evidence started to convince most scientists.
25The 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 polaritynormal polaritymagnetic striping
261960’s Increased monitoring of Nuclear testing helps to locate earthquake zones
31Sir Edward Bullard measures ocean temperatures Heat coming from central ridges 1967Bruce Heezen hypothesis’s that the Earth is expanding from the mid-ocean ridges
321968 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)
38Theory of Plate tectonics So by the mid-60’s all the evidence came together to form theTheory of Plate tectonicsThis theory states that the Earth crust is broken into a series of plates which are constantly on the move.