The evidence for continental drift and plate tectonics

Overview

Continental drift hypothesis, jigsaw fit and magnetic reversal.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What was the continental drift hypothesis, and what did it claim?

   Model answer

   It was the proposal, argued in detail in the early twentieth century, that the continents have not always been in their present positions but were once joined in a single supercontinent, Pangaea, which broke apart with the fragments drifting to their current locations. It explained a great deal of geological and biological coincidence, but it offered no convincing mechanism for how continents could plough through solid ocean floor, and it was rejected for that reason.

2. 2

   What is the jigsaw fit, and why is it stronger evidence than it first appears?

   Model answer

   The coastlines of South America and Africa look as though they would slot together, and the same is true of other continental edges. On its own a coastline match is weak, because coastlines are shaped by present sea level and by erosion. The evidence becomes far stronger when the fit is made at the edge of the continental shelf, at around the 1000 m depth contour, which is the true geological edge of the continent. Matched there, the fit is close enough that it is very hard to attribute to chance.

3. 3

   How does matching geology across ocean basins support the idea that the continents were joined?

   Model answer

   Rock sequences of the same age, type and structure are found on facing continents now separated by thousands of kilometres of ocean. Belts of ancient fold mountains run to the coast of one continent and continue on the other, so that when the continents are reassembled the mountain belts line up as continuous chains. Distinctive rock units, such as matching sequences of ancient glacial deposits, also correlate across the gap.

4. 4

   What do ancient glacial deposits tell us about the former positions of the continents?

   Model answer

   Deposits left by ice sheets of the same age are found in South America, southern Africa, India and Australia, all of which now lie in warm latitudes where such ice sheets could not form. Scratches left on the underlying rock show the ice moved outwards from a single centre when the continents are reassembled. This shows those landmasses were once joined together near the pole, and have since drifted apart and towards the equator.

5. 5

   How does fossil evidence support continental drift, and why is it more persuasive than simply finding the same fossil twice?

   Model answer

   Identical fossil species are found on continents now separated by ocean, and the persuasive cases are those where the organism could not possibly have crossed. A freshwater or land reptile could not swim a wide salt ocean, and a seed-fern with heavy seeds could not have been carried across it. Finding the same species on both sides of an ocean therefore means the land was continuous when the organism lived, not that it migrated.

6. 6

   Why did continental drift fail to convince most geologists at the time, despite all this evidence?

   Model answer

   Because it had no credible mechanism. The evidence showed that the continents had moved, but the explanation offered, that continents ploughed through the ocean floor driven by tidal and rotational forces, was demonstrably impossible: the forces were far too weak and the ocean crust far too strong. Without a plausible driving process the matching coastlines, rocks and fossils could be dismissed as coincidence or explained away by land bridges that had since sunk.

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