wegener and the continental drift hypothesis

wegeners continental drift hypothesis

Wegener put his idea and his evidence together in his book The Origin of Continents and Oceans, first published in 1915. New editions with additional evidence were published later in the decade. In his book he said that around 300 million years ago the continents had all been joined into a single landmass he called Pangaea, meaning all earth in ancient Greek. The supercontinent later broke apart and the continents having been moving into their current positions ever since. He called his hypothesis continental drift.

the problem with the hypothesis

Wegeners idea seemed so outlandish at the time that he was ridiculed by other scientists. What do you think the problem was? To his colleagues, his greatest problem was that he had no plausible mechanism for how the continents could move through the oceans. Based on his polar experiences, Wegener suggested that the continents were like icebreaking ships plowing through ice sheets. The continents moved by centrifugal and tidal forces. As Wegeners colleague, how would you go about showing whether these forces could move continents? What observations would you expect to see on these continents? Alfred Wegener suggested that continen- tal drift occurred as continents cut through the ocean floor, in the same way as this icebreaker plows through sea ice. Early hypotheses proposed that centrifu- gal forces moved continents. This is the same force that moves the swings out- ward on a spinning carnival ride. Scientists at the time calculated that centrifugal and tidal forces were too weak to move continents. When one scientist did calculations that assumed that these forces were strong enough to move continents, his result was that if Earth had such strong forces the planet would stop rotating in less than one year. In addition, scientists also thought that the continents that had been plowing through the ocean basins should be much more deformed than they are. Wegener answered his question of whether Africa and South America had once been joined. But a hypothesis is rarely accepted without a mechanism to drive it. Are you going to support Wegener? A very few scientists did, since his hypothesis elegantly explained the similar fossils and rocks on opposite sides of the ocean, but most did not.

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mantle convection

Wegener had many thoughts regarding what could be the driving force behind continental drift. Another of We- geners colleagues, Arthur Holmes, elaborated on Wegeners idea that there is thermal convection in the mantle. In a convection cell, material deep beneath the surface is heated so that its density is lowered and it rises. Near the surface it becomes cooler and denser, so it sinks. Holmes thought this could be like a conveyor belt. Where two adjacent convection cells rise to the surface, a continent could break apart with pieces moving in opposite directions. Although this sounds like a great idea, there was no real evidence for it, either. Alfred Wegener died in 1930 on an expedition on the Greenland icecap. For the most part the continental drift idea was put to rest for a few decades, until technological advances presented even more evidence that the continents moved and gave scientists the tools to develop a mechanism for Wegeners drifting continents. Since youre on a virtual field trip, you get to go along with them as well. Click image to the left or use the URL below. URL:

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instructional diagrams

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questions

based on his polar experiences, wegener thought the continents

a) were joined into a single landmass that was near the north pole.

-->  b) could move like an icebreaking ship through ice sheets.

c) could not have been joined because glaciers would have originated in the oceans if they were.

d) all of the above.

the continents were joined into a single landmass that has since broken apart, which wegener named

a) rhodinia

b) gondwana

c) panthelassa

-->  d) pangaea

many of the best scientists of the day agreed with wegeners hypothesis even though he did not have a plausible mechanism for continental movement.

a) true

-->  b) false

the reason scientists did not accept wegeners mechanism for continental drift is

a) if these forces were strong enough to move continents the planet would spin out of control.

-->  b) they calculated that centrifugal and tidal forces are too weak to move the continents.

c) if the continents were plowing through ocean basins, sea level should be much higher than it is.

d) none of these.

this drives continental movements.

-->  a) convection currents in the mantle

b) centrifugal forces

c) tidal forces

d) horizontal heat conduction.

the heat source for plate motions is the mantle.

a) true

-->  b) false

a convection cell explains a circular motion of

-->  a) warm material rising and cool material sinking.

b) cool material rising and warm material sinking.

c) deep material being squeezed by high pressure to the surface and surface material falling into the deep.

d) surface material becoming dense and sinking due to high pressure and deep material having less pressure and so becoming buoyant.

scientists rejected wegeners theory because

a) it didnt explain the fossil evidence.

b) it didnt explain the puzzle-like fit of the continents.

-->  c) there was no plausible mechanism to explain continental movement.

d) all of the above

where two adjacent convection cells rise to the surface, a continent could break apart and the pieces could move in the opposite direction.

-->  a) true

b) false

the hypothesis of continental drift was ignored until new evidence was found that pointed to a mechanism.

-->  a) true

b) false

diagram questions

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