Chapter 3
Plate Tectonics
Theory & Evolution
Assignment 1 – Summary of Why?
• Class results: 35 enrolled students
• 22 Seniors
• 33 Non Science majors
•
•
•
•
7 Fear/Hate math
Interests (80% - say need Q GE) to finish or other
Alt. E w/ Algae
Most “Love the Oceans/Water” want to learn more
OUTLINE
•
•
•
•
•
A Theory of Earth
Historical Development of Plate Tectonics
Earth’s layers
Study of Plate Motions
Summary of Plate Driving Forces
A Theory of Earth
• What is the Theory of Plate Tectonics?
– Earth crust divided into plates, move independently
• In response to?
– HEAT FLOW thru crust
What is continental drift?
The coming together or drifting apart
of Continents on the face of earth
OUTLINE
•
•
•
•
•
A Theory of Earth
Historical Development of Plate Tectonics
Earth’s layers
Study of Plate Motions
Summary of Plate Driving Forces
~1900
MOST Scientists considered
Earth to be rigid, Stagnant . . .
Wegener’s idea hinted that:
Crust (continents) floated in molten fluid
• Idea:
– Earth’s rigid crust underlain by weak plastic interior. . .
RECALL
(HOT-molten fluid)
((HOT))
5500*C
6600*C
Using Wegener’s idea – Arthur Holmes (1928)
proposes:
Mantle convection - serves as driving mechanism
for continental drift
• What is Mantle convection?
– Materials rising from mantle – spread out sideways (laterally)
and pulls continents w/ it.
Continents move
Hot air/liquid rises. . .
crust
Think of Boiling Pasta
MANTLE
1960s New Technolgies
• Sonar of ocean shows:
– Mid ocean ridges (mtn ranges)
Warmcool
cool Warm
cool
cool Warm cool
Warm
cool
cool
Heat flow=
Greater at
Mtn. Ridges
At odds w/ idea
Of Static earth. . ..
With Pangea/Mantle Convection idea
Seafloor Spreading concept emerges. . .
Dietz/Hess (1960s)
What is the idea of Seafloor Spreading?
Earth’s crusts being driven apart (laterally) by convection currents. . .
Submersibles confirm
Mid ocean ridge
lateral
movement
Plate Tectonics/ Seafloor Spreading
Submersibles confirm
Awesome Biological discoveries – mid ocean Ridges (hydrothermal vents)
lateral
movement
1970s
http://www.youtube.com/watch?v=XotF9fzo4Vo&NR=1
2003
http://www.youtube.com/watch?v=AlHJqA8YkoI
Seafloor Spreading
Direct evidence was lacking., . .
Paleomagnetism
(study of magnetic properties in rocks)
RECALL
+
-
-
(Iron)
Over (geologic time-100 Mya)
Poles reverse (Polar Reversal)
+
+
+
How does this help confirm the
theory of continental drift?
Curie Point – at certain temp. magnetic properties (elements) of rock solidify
Align with earth’s current magnetic field. . .
Can determine Position of the “Poles” when the
Rock formed. . .
Very Old
Older
Old
Vine-Matthews-Morley
Hypothesis
Using Magnetometer
Discovered “Zebra stripes”
Indicating polar reversals
in seafloor and age distances
From mid Ocean Ridge
Mid ocean Ridge (Mtn)
Where is the seafloor oldest & youngest above?
New
Newest
Plate Tectonics confirmed
-What’s really going on. .
Mid ocean ridge
SeaFloor spreading -makes new ocean crust (basalt)
Subduction- removes ocean crust. . .(melts  forms coast Mtns)
zone
•Earth’s layers
shifting “Lithospheric plates”
That move atop “plastic”
. . .Asthenosphere
Lithosphere:
Oceanic Crust vs Continental Crust
• What is their composition?
• Which is more dense/why?
Granite/quartz
(less dense~ 2.7 g cm-3)
Mostly Silica/less minerals
Mid ocean ridge
Basalt
(more dense~ 3 g cm-3)
Lots of minerals/Iron, nickel etc
OUTLINE
•
•
•
•
•
A Theory of Earth
Historical Development of Plate Tectonics
Earth’s layers
Study of Plate Motions
Summary of Plate Driving Forces
The Major Plates
both continental and oceanic
How many plates are recognized by the Geophysicists?
= 14 (see above)
GPS now tracks Plate Movements
(via Mtn tops etc.) all moving in diff. directions
Types of Plate Boundaries
• What are the three types?
1.
Peru/Chile trench
2.
3.
Convergent boundary
(oceanic crust vs oceanic crust)
e.g. Marianna Trench
• both
Forms Mariana Is.
1 crust
Subducts
Marianna
Convergent boundary
(oceanic crust vs continental crust)
e.g. Peru/Chile & the Andes Mtns
Formation of
Andes Mtns
S. America
Plate
Melting of Subducted
oceanic crust
Nasca
Plate
Convergent boundary
(continental crust vs continental crust)
e.g. the Himalayas Mtns
Mt. Everest
http://www.youtube.com/watch?v=xL6sKtG2UVs
How can Rigid plates can move about
a sphere?
• Because of the existence of . . .
– Transform Faults
Relieves built up tension in spreading plates. .
Transform Boundaries (faults)
-offsets the stress of expanding plates
Mid ocean ridge
Transform Boundaries (faults)
-offsets the
stress of expanding plates
Hot Spots & Mantle Plumes
• What are hotspots?
– Stationary locations in the asthenosphere
where magma breaks through
Diamond Head
Hawaii
http://www.youtube.com/watch?v=hOCfb9ox_90
http://www.youtube.com/watch?v=GrONBPEgxTc&feature=related
e.g. Hawaiian Islands
Or Tahiti (Bora bora)
Hot Spots (Literally) Tahiti
Opening and Closing of
Ocean Basins (over geologic time)
• The Wilson Cycle
– Seafloor spreading
Formation of an Ocean Basin
e.g. the
Atlantic
(stable crust)
(formation of . . .. . .Mid Ocean ridge) rains fill in etc.
Opening and Closing of
Ocean Basins (over geologic time)
Closing of an Ocean Basin
e.g. N. Pacific ~ Eventually
Ocean Seafloor age
N. Pacific basin eventually swallowed
Old
New
OUTLINE
•
•
•
•
•
A Theory of Earth
Historical Development of Plate Tectonics
Earth’s layers
Study of Plate Motions
Summary of Plate Driving Forces
Plate Driving Forces
(i.e. the “forces” that “move” the “plates”)
• What are the two major Models?
1) Convection model
Convection currents
w/i asthenosphere
drive plate motions
Magma rises
b/c hot
Less/dense
Magma
Sinks b/c
cool/dense
2) Ridge-Push, Slab-Pull Model
(plain)
(plain)
hot
New
crust
cooler
old
crust
Major Concepts
END
on a Nice Note: Tahiti –Bora Bora
GPS now tracks Plate Movements
(via Mtn tops etc.)