Continental Drift and Seafloor Spreading Notes Part 1

Alfred Wegener and the Continental Drift hypothesis Up until the early 1900s, long-held tradition in the earth sciences stated that continents have remained in fixed positions throughout geologic time In 1926, Wegener refuted this idea, suggesting that continents once had fit together in a “Super Continent” called Pangaea. Pangaea > Greek for “all land” Wegener’s idea was largely dismissed by the scientific establishment for lack of a mechanism that could drive the movement of continents

Alfred Wegener Video (3:57) http://thinktv.pbslearningmedia.org/resource/ess 05.sci.ess.earthsys.wegener1/plate-tectonics-the- scientist-behind-the-theory/

What evidence did Wegener have to support his hypothesis? 1. Fit of the continents The jigsaw shape of continents seemed to fit together like a puzzle. Africa appeared to tuck in neatly against the coasts of North and South America In fact, all of the continents can be arranged to join with nearly no overlap or gaps.

What evidence did Wegener have to support his hypothesis? 2. Distribution of climates Core samples that showed that climates had changed on continents Amazing Facts: Did you know... ...that India was once in the Southern Hemisphere connected to Antarctica? ...that North America was once surrounded by warm, tropical seas? ...that Africa was once covered by glaciers, which were kilometers in thickness? ...that the Sahara desert was once a tropical rain forest?

What evidence did Wegener have to support his hypothesis? 3. Distribution of fossils Fossils of the same plants and animals that were found on different continents If continents were all joined ~300 Ma (million years ago), then this would have provided an opportunity for plants and animals to disperse among joined continents. Once continents drifted apart, similar species would then be separated by large expanses of uncrossable ocean.

What evidence did Wegener have to support his hypothesis? 4. Matching rock units and Mountain Belts Geoscientists can recognize particular and distinctive rocks that were created all at the same place and at the same time. Wegener recognized several such assemblages of rock, now separated by ocean basins. #1 – in both South American and Africa #2 – in a belt along the east coast of North America and Greenland, and along the west coast of Africa, Great Britain and Scandinavia.

What evidence did Wegener have to support his hypothesis? 1. Fit of the continents 2. Distribution of tropical climates 3. Distribution of fossils 4. Matching of rock units across ocean basins Most scientists at the time did not believe in this theory of drifting continents, as Wegener could not explain what force was actually moving the continents.

Earth’s Seafloor Spreading Theory Hess In the 1960’s, geologist, Harry Hess, and oceanographer, Robert S. Dietz  looked at these features of the seafloor, and reasoned that new oceanic crust was created at ridges and consumed at trenches – this was called sea floor spreading. This theory combined with Wegner’s 1912’s continental drift theory led to the theory of plate tectonics. Dietz Rifting & Sea Floor Spreading Animation…

Sea Floor Spreading Echo sounding (sonar) allowed us to map the ocean floor Scientists discovered that the ocean floor is NOT flat