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Sea-Floor Spreading, Subduction,& Plate

Boundaries

Lecture 21

Prop: Test 3 Invitations

Alfred Wegener’s Evidence for Continental Drift

1. Continental “Fit”2. Similar Rocks, Ages

3. Similar Fossils4. Widespread Glaciation

Source:http://vishnu.glg.nau.

edu/rcb/Penn.jpg

Pangaea = Gondwanaland

+ Laurasia

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Alfred Wegener’s Evidence for Continental Drifts Was NOT

Accepted in USA.

Problem: No Mechanismfor Continents to Plow Through Oceanic Crust

Answer lies on & in the Sea Floor - Ch. 18 + 19 in Text

WW II

Submarine Science

Mapping of Ocean Basins

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Seismic & Sonar Study Reveals

Sea-Floor TopographyGeophysicists’sAnswers Life’s

Important Questions

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Know these mega-scale Landforms• Continental Shelf• Continental Slope• Continental Rise• Abyssal Plain• Mid-Ocean Ridge (~ Mid-Ocean Rise)• Trench

Know differences between Oceanic Crust and Continental Crust

Ocean Basin BathymetrySee Figures 18.5 to 18.16 in Plummer & Others 10th ed.

Ocean Basin Bathymetry See Figures 18.5, 18.15 in Plummer & Others 10th ed.

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Cold WarMore Submarine Science

Magnetic Anomalies in Ocean Basin

Deep-Sea Magnetic

Anomalies

Deep Sea Drilling Project (DSDP)Studied Geology of Ocean Basins

Continental Record = IncompleteDeep Ocean Basin Record Was Thought

to be More CompleteGlomar Challenger Harry

Hess

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Magnetic Anomalies: Atlantic Ocean South of IcelandNew Ocean Crust Created at MidNew Ocean Crust Created at Mid--Ocean Ridges: Spread CentersOcean Ridges: Spread Centers

DSDP Surprise Finding:

Ocean Basins are <200,000,000 y OldMuch Younger than the Continents

Very Young Sediments at the Mid Ocean Ridges

Age of the Ocean Basins

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Spreading at the Red Sea: Break-Up of Asia and Africa

Note a “Failed”Arm of Rifting in

East Africa

Birth of an Ocean Basin?

Break-Up of a Continent and

Origin of an Ocean Basin

Problem:

If New Crust is Created at Mid-Ocean Ridges,

Where is Old Crust Consumed to Balance the New Crust?

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Where is Old Crust Consumed to Balance the New Crust?

Answer Lies in Deep-Focus Earthquakes.

Earthquakes Occur in Brittle

(Elastic) Lithosphere

LithosphereAsthenosphere

70 70 -- 100 km100 km

Distribution of All Earthquakes(Deep and Shallow Focus)

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Subduction Zones: Ocean-Ocean Margin

Subduction Zones: Ocean-Continent Margin

Deep-Focus Quakes Only in

Subduction Zones

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Isotherms in Subduction Zones

400o

C

600o

C

Japan Trench &Volcanic

Arc

Back-Arc and Fore-Arc “Basins”See Figure 19.35 in Plummer & Others 9th ed.

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Subduction Zones &

Volcanoes

Spread Centers Feed Subduction Zones

Missing Mechanisms for Continental Drift:Sea Floor Spreading & Subduction

Continents are Passive “Boats”

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Himalayas

How Are Mountains Built?First a Brief Review of Sea Floor Spreading

Break-Up of a Continent and

Origin of an Ocean Basin

Missing Mechanisms for Continental Drift:Sea Floor Spreading & Subduction

Continents are Passive “Boats”

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Spread Centers Feed Subduction Zones

Subduction Zones: Ocean-Ocean Margin

Subduction Zones: Ocean-Continent Margin

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Orogeny(Mountain Building)

in the Himalayas

Continental Lithosphere

Will NOT Flush Down Subduction

Zones

Orogeny in Himalayas

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Orogeny in Himalayas

Orogenyin the

Himalayas

Orogeny - Mountain Building - Sediment Supply

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Orogeny - Mountain Building - Glaciation

Plate Margins:

• Spread Centers (Mid-Ocean Ridges)• Subduction Zones (Trenches)• Transform Faults

Plate Margins:

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Transform Faults & Fracture Zones

Transform Faults

San Andreas Fault:Right-Lateral

Transform Fault

Left-Lateral Offset!!Why?: Fault is

Produced by Offset, vs. Offset Produced by Fault

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Transform Faults:Three

CausalScenarios

Plate Motion

on a Sphere

Plate Motion on a Sphere Requires

Transform Faults

Because of Different Rates of Motion

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The Plates

The Plates: Pacific-Centered View(Distorted by Mercator Map Projection)

What Drives Plate Tectonics?

1. Convection Cells in Mantle 2. Sinking of Cool, Dense Plates3. Mantle Plumes - Hot Spots

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Convection Cells

Convection Cells in MantleConvection Cells in Mantle

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Mantle Plumes

Mantle Plumes Under Continents

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• Known “Hot Spots”

Hawaiian Islands

Mantle Plumes - Hot Spots

•

Numbers = Age of Rockin Millions

of Years

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Sinking of Cool, Dense Plates

Seamount

Volcanoes Surrounded by Fringing Coral Reefs

NASA

Atolls: Submerged Volcanos Support ReefsOnly in Warm Clear Seas

Tuamoto Islands