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.spe.sony.com/movies/da
sboot/
http://www.geocities.com/Pentagon/2560/bowph1.jpg
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 y a 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!
Absorb ant 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 MidMid-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 MidOcean 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
Asthenosphere
70 - 100 km
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
22