Earth Science- The Earth’s Moving Plates The Theory of Continental Drift

The Earth’s Moving Plates In 1912 a scientist named Alfred Wegner proposed the hypothesis of continental drift Wegner proposed that the continents had drifted apart after forming Pangaea- a super continent

The Hypothesis of Continental Drift He supported his theory with the available evidence: Continental shape Fossil evidence Landform evidence Ancient ice age evidence

Continental shape Wegner noted how the East coast of South America fit Africa’s West coast He found other coasts could be fit as well

Fossil evidence Fossils of identical species of plant and animal are found on different continents Since the organisms could not have crossed the ocean, Wegner proposed the continents were joined

Landform evidence Joining the continents lines up ancient landforms Mountain ranges match between: Argentina and South Africa E. USA and Scottish highlands Unusual rock formations are shared between Brazil and South Africa

Ancient Ice Age Evidence Glaciers carve striations into the Earth South Africa and South America have matching striations Glacial remains on South America and Africa support a position of Pangaea around the South Pole

The Theory of Continental Drift Scientists of the day rejected Wegner’s ideas They could not believe continents could move

The Theory of Plate Tectonics 30 years after his death, Harry Hess proposed explanation of more recent evidence supporting a new theory that explains continental drift and ocean movement: The theory of plate tectonics

Review of Continental Drift Alfred Wegner proposed the continents had been joined in a super-continent he called Pangaea He supported this with fossil, continent shape, land form and ice age evidence His theory was rejected because scientists of his day did not believe continents could “drift”  https://play.kahoot.it/#/k/662df51f-df8e-42b5-86f2-714956ea2137

Earth Science 7- The Earth’s Moving Plates The Theory of Plate Tectonics

Moving Continents New evidence explaining continental movement came from: Mapping of the ocean floor Age of Rocks on the ocean floor Magnetic reversals Locations of earthquakes and volcanoes

Mapping of the ocean floor Mapping of the ocean floor revealed deep trenches and huge mountains Trenches run along the edge of the ocean Mountains create a ridge in the middle of the ocean An ancient ocean would be expected to be flat

Age of Ocean Floor Rocks Sediment layers in mid ocean are thin If ocean had remained unchanged for millions of years sediment would be thick Young rocks are on the mid ocean ridges Old rocks are closer to the trenches Conclusion: new ocean crust forms at the ridge

Sea Floor Spreading Scientist Harry Hess named this formation of new crust “sea floor spreading” Magma rises at the mid ocean ridges Magma cools to form new crust Crust expands at mid ocean ridge and moves Oceanic crust sinks back down to the mantle at the trenches

Magnetic Reversals Magnetite is a magnetic mineral In molten rock it aligns with the Earth’s magnetic field When rock solidifies the alignment is fixed

Magnetic Reversals The Earth’s magnetic field reverses, and the pattern of magnetite alignment reverses Strips with alternating patterns of magnetic alignment are found running parallel to the mid ocean ridge This suggests the mid ocean ridge is the strips origin

Volcanoes and Earthquakes Modern technology allows earthquakes to be recorded in the ocean These observations led to the theory of plate tectonics This theory explains the way oceans and continents move, Wegner’s didn’t explain oceans

The Theory of Plate Tectonics The surface of the Earth is composed of ~ a dozen plates The plates are continually moving Oceanic plates have ocean floor on them Continental plates have continents on them

The Theory of Plate Tectonics Plates move at different speeds It is still a mystery why plates move It is believed that plates move on the currents of the mantle below It is also suggested the plates moves because of pull at the subduction zone

Review of Plate Tectonics Plate tectonics is based on evidence from the last 40 years Mapping of the ocean floor Sea floor sediment Magnetic reversals Undersea volcanoes and earthquakes Plate tectonics provides evidence that both the continents and oceans are moving