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 Fossils 4. Widespread Glaciation
Pangaea = Gondwanaland + Laurasia
Source: http://vishnu.glg.nau. edu/rcb/Penn.jpg
1 Alfred Wegener’s Evidence for Continental Drifts Was NOT Accepted in USA. Problem: No Mechanism for Continents to Plow Through Oceanic Crust
Answer lies on & in the Sea Floor - Ch. 18 + 19 in Text
WW II
http://www.geocities.com/Pentagon/2560/bowph1.jpg http://www.spe.sony.com/movies/da sboot/ Submarine Science Mapping of Ocean Basins
http://members.microdsi.net/shirschm/Welcome.index.html
Seismic & Sonar Study Reveals Sea-Floor Topography Geophysicists’s Answers Life’s Important Questions
Spongebob Theme Song Lyrics
Are ya ready kids? Aye Aye Captain! I CAN'T HEAR YOU! AYE AYE C APTAIN! ohhhhhh!!!!
Who lives in a pineapple under the sea? SPONGEB OB SQUAR EPANTS! Absorbant and yellow and poreous is he SPONGEB OB SQUAR EPANTS! His nautical nonsense be somethin you wish SPONGEB OB SQUAR EPANTS! Then drop on the deck and flop like a fish! SPONGEB OB SQUAR EPANTS! Ready?
SPONGEB OB SQUAR EPANTS! SPONGEB OB SQUAR EPANTS! SPONGEB OB SQUAR EPANTS!
SPONGEYB OB SQUAR EPAAAANTS
2 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 Bathymetry See 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.
3 Cold War More Submarine Science Magnetic Anomalies in Ocean Basin
Deep-Sea Magnetic Anomalies
Deep Sea Drilling Project (DSDP) Studied Geology of Ocean Basins Continental Record = Incomplete Deep Ocean Basin Record Was Thought to be More Complete Glomar Challenger Harry Hess
4 Magnetic Anomalies: Atlantic Ocean South of Iceland New Ocean Crust Created at Mid-Ocean Ridges: Spread Centers
DSDP Surprise Finding:
Ocean Basins are <200,000,000 y Old Much Younger than the Continents
Very Young Sediments at the Mid Ocean Ridges
Age of the Ocean Basins
5 Birth of an Ocean Basin?
Spreading at the Red Sea: Break-Up of Asia and Africa
Note a “Failed” Arm of Rifting in East Africa
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?
6 Where is Old Crust Consumed to Balance the New Crust?
Answer Lies in Deep-Focus Earthquakes.
Earthquakes Occur in Brittle (Elastic) Lithosphere
Lithosphere 70 - 100 km Asthenosphere
Distribution of All Earthquakes (Deep and Shallow Focus)
7 Subduction Zones: Ocean-Ocean Margin
Subduction Zones: Ocean-Continent Margin
Deep-Focus Quakes Only in Subduction Zones
8 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.
9 Subduction Zones & Volcanoes
Spread Centers Feed Subduction Zones
Missing Mechanisms for Continental Drift: Sea Floor Spreading & Subduction
Continents are Passive “Boats”
10 How Are Mountains Built? First a Brief Review of Sea Floor Spreading
Himalayas
Break-Up of a Continent and Origin of an Ocean Basin
Missing Mechanisms for Continental Drift: Sea Floor Spreading & Subduction
Continents are Passive “Boats”
11 Spread Centers Feed Subduction Zones
Subduction Zones: Ocean-Ocean Margin
Subduction Zones: Ocean-Continent Margin
12 Orogeny (Mountain Building) in the Himalayas
Continental Lithosphere Will NOT Flush Down Subduction Zones
Orogeny in Himalayas
13 Orogeny in Himalayas
Orogeny in the Himalayas
Orogeny - Mountain Building - Sediment Supply
14 Orogeny - Mountain Building - Glaciation
Plate Margins:
• Spread Centers (Mid-Ocean Ridges) • Subduction Zones (Trenches) • Transform Faults
Plate Margins:
15 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
16 Transform Faults: Three Causal Scenarios
Plate Motion on a Sphere
Plate Motion on a Sphere Requires Transform Faults Because of Different Rates of Motion
17 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 Plates 3. Mantle Plumes - Hot Spots
18 Convection Cells
Convection Cells in Mantle
19 Mantle Plumes
Mantle Plumes Under Continents
20 • Known “Hot Spots”
Hawaiian Islands
Mantle Plumes - Hot Spots
•
Numbers = Age of Rock in Millions of Years
21 Sinking of Cool, Dense Plates
Seamount
Volcanoes Surrounded by Fringing Coral Reefs
Atolls: Submerged Volcanos Support Reefs Only in Warm Clear Seas Tuamoto Islands NASA
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