Continental drift is the movement of the Earth's continents relative to each other by appearing to drift across the ocean bed The speculation that continents might have 'drifted' was first put forward by Abraham Ortelius in 1596. The concept was independently and more fully developed by Alfred Wegener in 1912, but his theory was rejected for being incomplete. The idea of continental drift has been subsumed by the theory of plate tectonics which explains how the continents moveThe formation and evolution of the Solar System,

1. Chapter 3
Drifting Continents and Spreading Seas
LECTURE OUTLINE
earth
Portrait of a Planet
Third Edition
©2008 W. W. Norton & Company, Inc.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 3: Drifting Continents and Spreading Seas

2. Drifting Continents and Spreading Seas
Prepared by
Ronald Parker,
Earlham College Department of Geosciences
Richmond, Indiana

3. Continental Drift
 The hypothesis that continents are mobile.
 Proposed by German meteorologist Alfred Wegener.
 The Origins of Oceans and Continents published in 1915.
 Wegener hypothesized a former supercontinent Pangaea.
 Idea was based on abundant evidence.
“Fit” of the continents.
Location of glaciations.
Fossil evidence.
Rock type and structural similarities.
Paleoclimatic evidence.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 3: Drifting Continents and Spreading Seas

4. Continental Drift
 Wegener’s idea was debated and ridiculed.
 Most scientists didn’t believe him.
 Lack of a mechanism for drift a major criticism.
 Wegener died in 1930 at the age of 40.
 Lacking an advocate, the drift
hypothesis faded.
 His idea was revived in the 1950s.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 3: Drifting Continents and Spreading Seas

5. Continental Drift
 Now we can measure drift; Wegener was right!
 Sea-floor spreading.
 Subduction.
 Plate tectonics.
 Why was Wegener’s model dismissed by scientists?
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 3: Drifting Continents and Spreading Seas

6. Before Continental Drift
 What were scientists thinking?
 The oceans and the continents were permanently fixed.
 There were no changes in continents back in time.
 Evidence of drastic changes interpreted as…
 Shrinkage effects.
 Rebound from thick sediment loading.
 The result of “upheaval.”
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 3: Drifting Continents and Spreading Seas

7. Continental “Fit”
 Wegener noted the continents seem to fit together.
 He argued that the fit could not be coincidental.
 Present shorelines make a rough fit.
 The continental shelf edges make a better fit.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 3: Drifting Continents and Spreading Seas

8. Glacial Evidence
 Permian glacial till is found on 4 continents.
 The tills in Africa and India are now near equator.
 A cooler earth? No, tropical plants also preserved.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 3: Drifting Continents and Spreading Seas

9. Paleoclimatic Evidence
 Placing Pangea over the Permian South Pole…
 He correctly predicted…
 Tropical coals.
 Tropical reefs.
 Subtropical deserts.
 Subtropical evaporites.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 3: Drifting Continents and Spreading Seas

10. Fossil Evidence
 Identical fossils found on widely separated land.
 Mesosaurus – A freshwater reptile.
 Glossopteris – Subpolar plant with heavy seeds.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 3: Drifting Continents and Spreading Seas

11. Fossil Evidence
 Identical fossils found on widely separated land.
 Lystrosaurus – A non-swimming, land-dwelling reptile.
 Cynognathus – A non-swimming, land-dwelling reptile.
 These organisms could not
have crossed an ocean.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 3: Drifting Continents and Spreading Seas

12. Matching Geology
 Geologic phenomena match across the Atlantic.
 Geologic structures.
 Rock types.
 Rock ages.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 3: Drifting Continents and Spreading Seas

13. Matching Geology
 Geologic phenomenon match across the Atlantic.
 Mountain belts.
Appalachians.
Caledonides.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 3: Drifting Continents and Spreading Seas

14. Criticisms of Drift
 Why wasn’t continental drift accepted?
 There were no mechanisms for moving continents.
 Wegener suggested that continents plowed through the
ocean crust, much like ice breakers cut through ice.
 He invoked rotational (centrifugal) forces.
 These ideas were easily discredited; so was his theory.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 3: Drifting Continents and Spreading Seas

15. Continental Drift
 When Wegener died, the debate did too.
 A few continued to champion Wegener’s idea.
 Arthur Holmes –Earth’s mantle moved by convection.
 The drift hypothesis awaited the evidence provided
by paleomagnetism (see Interlude A).
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 3: Drifting Continents and Spreading Seas

16. The Ocean Floor
 In the 1950s, ocean bathymetry was mapped by sonar.
 Oceanographers were surprised to discover that…
 The deepest parts of the ocean occur near land.
 A mountain range runs through every ocean basin.
 Submarine volcanoes form lines across ocean floors.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 3: Drifting Continents and Spreading Seas

17. The Ocean Floor
 Sonar mapping delineated bathymetric features.
 Trenches.
 Mid-ocean ridges (MORs).
 Seamounts,
 Guyots.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 3: Drifting Continents and Spreading Seas

18. The Ocean Floor
 Modern views of the ocean floor reveal:
 Mid-ocean ridges.
 Trenches.
 Fracture zones.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 3: Drifting Continents and Spreading Seas

19. Oceanic Crust
 By 1950, we had learned much about oceanic crust.
 Oceanic crust is covered by sediment.
 Thickest near the continents.
 Thinnest (or absent) at the mid-ocean ridge.
 Oceanic crust is mafic (basalt and gabbro).
 No granitic rocks.
 No metamorphic rocks.
 High heat flow characterizes the mid-ocean ridge.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 3: Drifting Continents and Spreading Seas

20. Oceanic Crust
 Belts of concentrated subsea earthquakes were found.
 The earthquakes were surprising. They were limited to…
 Parts of oceanic fracture zones.
 Mid-ocean ridge axes.
 Deep ocean trenches.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 3: Drifting Continents and Spreading Seas

21. Sea Floor Spreading
 In 1960, Harry Hess published his “Essay in Geopoetry.”
 He called his theory “sea-floor spreading.”
 Upwelling mantle erupts at the mid-ocean ridges.
 New crust moves away from ridges, gathering sediment.
 At trenches the sea-floor dives back into the mantle.
 Provided a potential mechanism for continental drift.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 3: Drifting Continents and Spreading Seas

22. Magnetic Anomalies
 Towed magnetometers measure ocean crust.
 Magnetism oscillates perpendicular to the MOR.
 These variations are + and - magnetic anomalies.
 Anomalies are linear belts that parallel MOR.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 3: Drifting Continents and Spreading Seas

23. Magnetic Reversals
 Layered lava flows reveal reversals in polarity.
 Magnetic polarity is the direction of magnetic north.
 When reversed, the north magnetic pole is near the south
geographic pole.
 Reversals are geologically rapid.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 3: Drifting Continents and Spreading Seas

24. Magnetic Reversals
 Radioactivity permits rock age-dating.
 A geomagnetic reversal time scale
has been assembled.
 Reversals occur every 500-700 ka.
 171 are known since the end Cretaceous.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 3: Drifting Continents and Spreading Seas

25. Sea-Floor Spreading: Proof
 Polarity reversals explain magnetic anomalies.
 Positive anomalies – Crust with normal polarity.
 Negative anomalies – Crust with reversed polarity.
 Magnetic anomalies are symmetric across the MOR.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 3: Drifting Continents and Spreading Seas

26. Sea-Floor Spreading: Proof
 Magnetic anomalies mimic layered lava flows.
 Magnetic “stripes” form as lava cools at a MOR.
 Ocean crust spreads away from MOR.
 Reversals are recorded within cooled lava.
 SF spreading is the mechanism of continental drift.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 3: Drifting Continents and Spreading Seas

27. Sea-Floor Spreading
 Ages increase away from the MOR.
 Ages are “mirror images” across the MOR.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 3: Drifting Continents and Spreading Seas

28. This concludes the
Chapter 3
Drifting Continents and Spreading Seas
LECTURE OUTLINE
earth
Portrait of a Planet
Third Edition
©2008 W. W. Norton & Company, Inc.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 3: Drifting Continents and Spreading Seas