Continental Drift (3.1)

Question 1

What are some historical concepts?

Answer 1

• Suggestions that coastlines once fit together made in 1596 as map-making improved.
• C19th view of mountains as wrinkles on surface of cooling, contracting Earth. Continuous crust (Gondwanaland) broke apart as planet shrunk.
• Contracting Earth models discredited C20th, no agreed replacement model.

Question 2

What was the origin of continental drift?

Answer 2

• German meteorologist Alfred Wegener first proposed land masses change position with time.
• Proposed existence of supercontinent Pangaea.
• Suggestion by F.B. Taylor (1910) that lateral motions caused mountain belts at edges of drifting continents.
• Both uniformitarian concepts; drift is still happening.

Question 3

What was Wegener’s evidence?

Answer 3

• Fit of continents: fit not coincidental, shorelines make rough fit, continental shelf edges make even better fit (Bullard, 1965)
• Past glaciations: glacial evidence for Permian age glaciers found on 4 continents, found in regions too warm for glaciers today.
• Palaeoclimatic belts: tropical sediments occur in northern parts of Pangaea.
• Fossil distribution: identical fossils found on separate landmasses, could not have crossed the ocean.
• Matching geological units: fold belts, igneous provinces, metallogenic provinces.

Question 4

What was the problem with Wegener’s proposed theory and evidence?

Answer 4

• He could not explain how/why continents ploughed through ocean floor.
• Suggested motions driven by centripetal force acting on elevated continents due to Earth’s rotation and/or tidal effects due to gravity of Sun and Moon.
• Calculations showed forces too small.
• Wegener’s theory was largely dismissed, he was vilified for scientific method of having hypothesis and then looking for evidence.

Question 5

What was empirical evidence for mantle flow?

Answer 5

• Deduced from active uplift over past 10ka of deglaciated regions in Scandinavia.
• Realised continents move through substrate which they are embedded.
• Arthur Holmes (1928) proposed continents move laterally, driven by convection currents in mantle beneath.
• Mantle does not need to be liquid to flow, can deform and flow as plastic solid (when hot).

Question 6

What was the application of palaeomagnetism?

Answer 6

• 1950s, UK geophysicists Ted Irving and Keith Runcorn realised palaeomagnetism could be used as test of Wegener’s continental drift hypothesis.
• Measured palaeomagnetic inclinations of rocks at different ages in region they could reconstruct any change of latitude with time.
• With any declination change, they could calculate how far away pole had been relative to continent.

Question 7

What was the apparent polar wander?

Answer 7

• Irving and Runcorn found calculated poles for Britain and Europe had shifted systematically through geological time.
• Two interpretations: Earth’s poles moved relative to continents, and land masses moved relative to poles.
• By 1956, had enough evidence to show polar wander paths were consistent with continents, but distinct from continent to continent.
• Show paths for Europe and North America moved parallel together between 500 and ~200 Myr ago.
• Still proposed no mechanism, but this caused a revisit to Wegener’s continental drift concept.