Chapter 3: Drifting Continents and Spreading Seas Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 3 Drifting Continents and Spreading

Chapter 3: Drifting Continents and Spreading Seas Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak

Chapter 3Drifting Continents and Spreading Seas

©2008 W. W. Norton & Company, Inc.

Portrait of a PlanetThird Edition

earthearth

LECTURE OUTLINE

Drifting Continents and Spreading SeasDrifting Continents and Spreading Seas

Prepared by

Ronald Parker, Earlham College Department of Geosciences

Richmond, Indiana

Prepared by

Ronald Parker, Earlham College Department of Geosciences

Richmond, Indiana


Continental Drift Continental Drift The hypothesis that continents are mobile.The hypothesis that continents are mobile. Proposed by German meteorologist Alfred Wegener.Proposed by German meteorologist Alfred Wegener.

The Origins of Oceans and ContinentsThe Origins of Oceans and Continents published in 1915. published in 1915. Wegener hypothesized a former supercontinent Pangaea. Wegener hypothesized a former supercontinent Pangaea. Idea was based on abundant evidence.Idea was based on abundant evidence.

““Fit” of the continents.Fit” of the continents.Location of glaciations.Location of glaciations.Fossil evidence.Fossil evidence.Rock type and structural similarities.Rock type and structural similarities.Paleoclimatic evidence.Paleoclimatic evidence.

The hypothesis that continents are mobile.The hypothesis that continents are mobile. Proposed by German meteorologist Alfred Wegener.Proposed by German meteorologist Alfred Wegener.

The Origins of Oceans and ContinentsThe Origins of Oceans and Continents published in 1915. published in 1915. Wegener hypothesized a former supercontinent Pangaea. Wegener hypothesized a former supercontinent Pangaea. Idea was based on abundant evidence.Idea was based on abundant evidence.

““Fit” of the continents.Fit” of the continents.Location of glaciations.Location of glaciations.Fossil evidence.Fossil evidence.Rock type and structural similarities.Rock type and structural similarities.Paleoclimatic evidence.Paleoclimatic evidence.


Continental Drift Continental Drift Wegener’s idea was debated and ridiculed.Wegener’s idea was debated and ridiculed.

Most scientists didn’t believe him.Most scientists didn’t believe him. Lack of a mechanism for drift a major criticism.Lack of a mechanism for drift a major criticism.

Wegener died in 1930 at the age of 40.Wegener died in 1930 at the age of 40. Lacking an advocate, the drift Lacking an advocate, the drift

hypothesis faded.hypothesis faded. His idea was revived in the 1950s.His idea was revived in the 1950s.

Wegener’s idea was debated and ridiculed.Wegener’s idea was debated and ridiculed. Most scientists didn’t believe him.Most scientists didn’t believe him. Lack of a mechanism for drift a major criticism.Lack of a mechanism for drift a major criticism.

Wegener died in 1930 at the age of 40.Wegener died in 1930 at the age of 40. Lacking an advocate, the drift Lacking an advocate, the drift

hypothesis faded.hypothesis faded. His idea was revived in the 1950s.His idea was revived in the 1950s.


Continental Drift Continental Drift Now we can measure drift; Wegener was right! Now we can measure drift; Wegener was right!

Sea-floor spreading. Sea-floor spreading. Subduction.Subduction. Plate tectonics.Plate tectonics.

Why was Wegener’s model dismissed by scientists?Why was Wegener’s model dismissed by scientists?

Now we can measure drift; Wegener was right! Now we can measure drift; Wegener was right! Sea-floor spreading. Sea-floor spreading. Subduction.Subduction. Plate tectonics.Plate tectonics.

Why was Wegener’s model dismissed by scientists?Why was Wegener’s model dismissed by scientists?


Before Continental Drift Before Continental Drift What were scientists thinking? What were scientists thinking?

The oceans and the continents were permanently fixed.The oceans and the continents were permanently fixed. There were no changes in continents back in time.There were no changes in continents back in time.

Evidence of drastic changes interpreted as…Evidence of drastic changes interpreted as… Shrinkage effects.Shrinkage effects. Rebound from thick sediment loading.Rebound from thick sediment loading. The result of “upheaval.”The result of “upheaval.”

What were scientists thinking? What were scientists thinking? The oceans and the continents were permanently fixed.The oceans and the continents were permanently fixed. There were no changes in continents back in time.There were no changes in continents back in time.

Evidence of drastic changes interpreted as…Evidence of drastic changes interpreted as… Shrinkage effects.Shrinkage effects. Rebound from thick sediment loading.Rebound from thick sediment loading. The result of “upheaval.”The result of “upheaval.”


Continental “Fit”Continental “Fit” Wegener noted the continents seem to fit together.Wegener noted the continents seem to fit together. He argued that the fit could not be coincidental.He argued that the fit could not be coincidental. Present shorelines make a rough fit.Present shorelines make a rough fit. The continental shelf edges make a better fit. The continental shelf edges make a better fit.

Wegener noted the continents seem to fit together.Wegener noted the continents seem to fit together. He argued that the fit could not be coincidental.He argued that the fit could not be coincidental. Present shorelines make a rough fit.Present shorelines make a rough fit. The continental shelf edges make a better fit. The continental shelf edges make a better fit.


Glacial EvidenceGlacial Evidence Permian glacial till is found on 4 continents. Permian glacial till is found on 4 continents. The tills in Africa and India are now near equator.The tills in Africa and India are now near equator. A cooler earth? No, tropical plants also preserved. A cooler earth? No, tropical plants also preserved.

Permian glacial till is found on 4 continents. Permian glacial till is found on 4 continents. The tills in Africa and India are now near equator.The tills in Africa and India are now near equator. A cooler earth? No, tropical plants also preserved. A cooler earth? No, tropical plants also preserved.


Paleoclimatic EvidencePaleoclimatic Evidence Placing Pangea over the Permian South Pole…Placing Pangea over the Permian South Pole… He correctly predicted…He correctly predicted…

Tropical coals.Tropical coals. Tropical reefs.Tropical reefs. Subtropical deserts.Subtropical deserts. Subtropical evaporites.Subtropical evaporites.

Placing Pangea over the Permian South Pole…Placing Pangea over the Permian South Pole… He correctly predicted…He correctly predicted…

Tropical coals.Tropical coals. Tropical reefs.Tropical reefs. Subtropical deserts.Subtropical deserts. Subtropical evaporites.Subtropical evaporites.


Fossil EvidenceFossil Evidence Identical fossils found on widely separated land. Identical fossils found on widely separated land.

MesosaurusMesosaurus – A freshwater reptile. – A freshwater reptile. GlossopterisGlossopteris – Subpolar plant with heavy seeds. – Subpolar plant with heavy seeds.

Identical fossils found on widely separated land. Identical fossils found on widely separated land. MesosaurusMesosaurus – A freshwater reptile. – A freshwater reptile. GlossopterisGlossopteris – Subpolar plant with heavy seeds. – Subpolar plant with heavy seeds.


Fossil EvidenceFossil Evidence Identical fossils found on widely separated land.Identical fossils found on widely separated land.

LystrosaurusLystrosaurus – A non-swimming, land-dwelling reptile. – A non-swimming, land-dwelling reptile. CynognathusCynognathus – A non-swimming, land-dwelling reptile. – A non-swimming, land-dwelling reptile.

These organisms could not These organisms could not

have crossed an ocean.have crossed an ocean.

Identical fossils found on widely separated land.Identical fossils found on widely separated land. LystrosaurusLystrosaurus – A non-swimming, land-dwelling reptile. – A non-swimming, land-dwelling reptile. CynognathusCynognathus – A non-swimming, land-dwelling reptile. – A non-swimming, land-dwelling reptile.

These organisms could not These organisms could not

have crossed an ocean.have crossed an ocean.


Geologic phenomena match across the Atlantic.Geologic phenomena match across the Atlantic. Geologic structures.Geologic structures. Rock types.Rock types. Rock ages.Rock ages.

Geologic phenomena match across the Atlantic.Geologic phenomena match across the Atlantic. Geologic structures.Geologic structures. Rock types.Rock types. Rock ages.Rock ages.

Matching GeologyMatching Geology


Geologic phenomenon match across the Atlantic.Geologic phenomenon match across the Atlantic. Mountain belts.Mountain belts.

Appalachians.Appalachians.Caledonides.Caledonides.

Geologic phenomenon match across the Atlantic.Geologic phenomenon match across the Atlantic. Mountain belts.Mountain belts.

Appalachians.Appalachians.Caledonides.Caledonides.

Matching GeologyMatching Geology


Criticisms of DriftCriticisms of Drift Why wasn’t continental drift accepted?Why wasn’t continental drift accepted? There were no mechanisms for moving continents. There were no mechanisms for moving continents.

Wegener suggested that continents plowed through the Wegener suggested that continents plowed through the ocean crust, much like ice breakers cut through ice.ocean crust, much like ice breakers cut through ice.

He invoked rotational (centrifugal) forces.He invoked rotational (centrifugal) forces. These ideas were easily discredited; so was his theory. These ideas were easily discredited; so was his theory.

Why wasn’t continental drift accepted?Why wasn’t continental drift accepted? There were no mechanisms for moving continents. There were no mechanisms for moving continents.

Wegener suggested that continents plowed through the Wegener suggested that continents plowed through the ocean crust, much like ice breakers cut through ice.ocean crust, much like ice breakers cut through ice.

He invoked rotational (centrifugal) forces.He invoked rotational (centrifugal) forces. These ideas were easily discredited; so was his theory. These ideas were easily discredited; so was his theory.


Continental DriftContinental Drift When Wegener died, the debate did too. When Wegener died, the debate did too. A few continued to champion Wegener’s idea. A few continued to champion Wegener’s idea.

Arthur Holmes –Earth’s mantle moved by convection. Arthur Holmes –Earth’s mantle moved by convection.

The drift hypothesis awaited the evidence provided The drift hypothesis awaited the evidence provided by paleomagnetism (see Interlude A).by paleomagnetism (see Interlude A).

When Wegener died, the debate did too. When Wegener died, the debate did too. A few continued to champion Wegener’s idea. A few continued to champion Wegener’s idea.

Arthur Holmes –Earth’s mantle moved by convection. Arthur Holmes –Earth’s mantle moved by convection.

The drift hypothesis awaited the evidence provided The drift hypothesis awaited the evidence provided by paleomagnetism (see Interlude A).by paleomagnetism (see Interlude A).


The Ocean FloorThe Ocean Floor In the 1950s, ocean bathymetry was mapped by sonar. In the 1950s, ocean bathymetry was mapped by sonar. Oceanographers were surprised to discover that… Oceanographers were surprised to discover that…

The deepest parts of the ocean occur near land. The deepest parts of the ocean occur near land. A mountain range runs through every ocean basin. A mountain range runs through every ocean basin. Submarine volcanoes form lines across ocean floors.Submarine volcanoes form lines across ocean floors.

In the 1950s, ocean bathymetry was mapped by sonar. In the 1950s, ocean bathymetry was mapped by sonar. Oceanographers were surprised to discover that… Oceanographers were surprised to discover that…

The deepest parts of the ocean occur near land. The deepest parts of the ocean occur near land. A mountain range runs through every ocean basin. A mountain range runs through every ocean basin. Submarine volcanoes form lines across ocean floors.Submarine volcanoes form lines across ocean floors.


The Ocean FloorThe Ocean Floor Sonar mapping delineated bathymetric features.Sonar mapping delineated bathymetric features.

Trenches.Trenches. Mid-ocean ridges (MORs).Mid-ocean ridges (MORs). Seamounts,Seamounts, Guyots.Guyots.

Sonar mapping delineated bathymetric features.Sonar mapping delineated bathymetric features. Trenches.Trenches. Mid-ocean ridges (MORs).Mid-ocean ridges (MORs). Seamounts,Seamounts, Guyots.Guyots.


The Ocean FloorThe Ocean Floor Modern views of the ocean floor reveal:Modern views of the ocean floor reveal:

Mid-ocean ridges.Mid-ocean ridges. Trenches.Trenches. Fracture zones.Fracture zones.

Modern views of the ocean floor reveal:Modern views of the ocean floor reveal: Mid-ocean ridges.Mid-ocean ridges. Trenches.Trenches. Fracture zones.Fracture zones.


Oceanic CrustOceanic Crust By 1950, we had learned much about oceanic crust.By 1950, we had learned much about oceanic crust. Oceanic crust is covered by sediment. Oceanic crust is covered by sediment.

Thickest near the continents.Thickest near the continents. Thinnest (or absent) at the mid-ocean ridge.Thinnest (or absent) at the mid-ocean ridge.

Oceanic crust is mafic (basalt and gabbro).Oceanic crust is mafic (basalt and gabbro). No granitic rocks.No granitic rocks. No metamorphic rocks.No metamorphic rocks.

High heat flow characterizes the mid-ocean ridge. High heat flow characterizes the mid-ocean ridge.

By 1950, we had learned much about oceanic crust.By 1950, we had learned much about oceanic crust. Oceanic crust is covered by sediment. Oceanic crust is covered by sediment.

Thickest near the continents.Thickest near the continents. Thinnest (or absent) at the mid-ocean ridge.Thinnest (or absent) at the mid-ocean ridge.

Oceanic crust is mafic (basalt and gabbro).Oceanic crust is mafic (basalt and gabbro). No granitic rocks.No granitic rocks. No metamorphic rocks.No metamorphic rocks.

High heat flow characterizes the mid-ocean ridge. High heat flow characterizes the mid-ocean ridge.


Oceanic CrustOceanic Crust Belts of concentrated subsea earthquakes were found. Belts of concentrated subsea earthquakes were found. The earthquakes were surprising. They were limited to… The earthquakes were surprising. They were limited to…

Parts of oceanic fracture zones. Parts of oceanic fracture zones. Mid-ocean ridge axes.Mid-ocean ridge axes. Deep ocean trenches.Deep ocean trenches.

Belts of concentrated subsea earthquakes were found. Belts of concentrated subsea earthquakes were found. The earthquakes were surprising. They were limited to… The earthquakes were surprising. They were limited to…

Parts of oceanic fracture zones. Parts of oceanic fracture zones. Mid-ocean ridge axes.Mid-ocean ridge axes. Deep ocean trenches.Deep ocean trenches.


Sea Floor SpreadingSea Floor Spreading In 1960, Harry Hess published his “Essay in Geopoetry.”In 1960, Harry Hess published his “Essay in Geopoetry.” He called his theory “sea-floor spreading.” He called his theory “sea-floor spreading.”

Upwelling mantle erupts at the mid-ocean ridges. Upwelling mantle erupts at the mid-ocean ridges. New crust moves away from ridges, gathering sediment.New crust moves away from ridges, gathering sediment. At trenches the sea-floor dives back into the mantle. At trenches the sea-floor dives back into the mantle.

Provided a potential mechanism for continental drift.Provided a potential mechanism for continental drift.

In 1960, Harry Hess published his “Essay in Geopoetry.”In 1960, Harry Hess published his “Essay in Geopoetry.” He called his theory “sea-floor spreading.” He called his theory “sea-floor spreading.”

Upwelling mantle erupts at the mid-ocean ridges. Upwelling mantle erupts at the mid-ocean ridges. New crust moves away from ridges, gathering sediment.New crust moves away from ridges, gathering sediment. At trenches the sea-floor dives back into the mantle. At trenches the sea-floor dives back into the mantle.

Provided a potential mechanism for continental drift.Provided a potential mechanism for continental drift.


Magnetic AnomaliesMagnetic Anomalies Towed magnetometers measure ocean crust. Towed magnetometers measure ocean crust. Magnetism oscillates perpendicular to the MOR. Magnetism oscillates perpendicular to the MOR. These variations are + and - magnetic anomalies.These variations are + and - magnetic anomalies. Anomalies are linear belts that parallel MOR.Anomalies are linear belts that parallel MOR.

Towed magnetometers measure ocean crust. Towed magnetometers measure ocean crust. Magnetism oscillates perpendicular to the MOR. Magnetism oscillates perpendicular to the MOR. These variations are + and - magnetic anomalies.These variations are + and - magnetic anomalies. Anomalies are linear belts that parallel MOR.Anomalies are linear belts that parallel MOR.


Magnetic ReversalsMagnetic Reversals Layered lava flows reveal reversals in polarity. Layered lava flows reveal reversals in polarity.

Magnetic polarity is the direction of magnetic north. Magnetic polarity is the direction of magnetic north. When reversed, the north magnetic pole is near the south When reversed, the north magnetic pole is near the south

geographic pole. geographic pole.

Reversals are geologically rapid.Reversals are geologically rapid.

Layered lava flows reveal reversals in polarity. Layered lava flows reveal reversals in polarity. Magnetic polarity is the direction of magnetic north. Magnetic polarity is the direction of magnetic north. When reversed, the north magnetic pole is near the south When reversed, the north magnetic pole is near the south

geographic pole. geographic pole.

Reversals are geologically rapid.Reversals are geologically rapid.


Magnetic ReversalsMagnetic Reversals Radioactivity permits rock age-dating. Radioactivity permits rock age-dating. A geomagnetic reversal time scaleA geomagnetic reversal time scale

has been assembled.has been assembled. Reversals occur every 500-700 ka.Reversals occur every 500-700 ka. 171 are known since the end Cretaceous.171 are known since the end Cretaceous.

Radioactivity permits rock age-dating. Radioactivity permits rock age-dating. A geomagnetic reversal time scaleA geomagnetic reversal time scale

has been assembled.has been assembled. Reversals occur every 500-700 ka.Reversals occur every 500-700 ka. 171 are known since the end Cretaceous.171 are known since the end Cretaceous.


Sea-Floor Spreading: ProofSea-Floor Spreading: Proof Polarity reversals explain magnetic anomalies. Polarity reversals explain magnetic anomalies. Positive anomalies – Crust with normal polarity.Positive anomalies – Crust with normal polarity. Negative anomalies – Crust with reversed polarity.Negative anomalies – Crust with reversed polarity. Magnetic anomalies are symmetric across the MOR.Magnetic anomalies are symmetric across the MOR.

Polarity reversals explain magnetic anomalies. Polarity reversals explain magnetic anomalies. Positive anomalies – Crust with normal polarity.Positive anomalies – Crust with normal polarity. Negative anomalies – Crust with reversed polarity.Negative anomalies – Crust with reversed polarity. Magnetic anomalies are symmetric across the MOR.Magnetic anomalies are symmetric across the MOR.


Sea-Floor Spreading: ProofSea-Floor Spreading: Proof Magnetic anomalies mimic layered lava flows.Magnetic anomalies mimic layered lava flows.

Magnetic “stripes” form as lava cools at a MOR. Magnetic “stripes” form as lava cools at a MOR. Ocean crust spreads away from MOR.Ocean crust spreads away from MOR. Reversals are recorded within cooled lava.Reversals are recorded within cooled lava.

SF spreading is the mechanism of continental drift.SF spreading is the mechanism of continental drift.

Magnetic anomalies mimic layered lava flows.Magnetic anomalies mimic layered lava flows. Magnetic “stripes” form as lava cools at a MOR. Magnetic “stripes” form as lava cools at a MOR. Ocean crust spreads away from MOR.Ocean crust spreads away from MOR. Reversals are recorded within cooled lava.Reversals are recorded within cooled lava.

SF spreading is the mechanism of continental drift.SF spreading is the mechanism of continental drift.


Sea-Floor SpreadingSea-Floor Spreading Ages increase away from the MOR. Ages are “mirror images” across the MOR.

Ages increase away from the MOR. Ages are “mirror images” across the MOR.


Chapter 3Drifting Continents and Spreading Seas

©2008 W. W. Norton & Company, Inc.

Portrait of a PlanetThird Edition

earthearth

LECTURE OUTLINE

This concludes the

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Chapter 3: Drifting Continents and Spreading Seas Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 3 Drifting Continents and Spreading