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