Plate Tectonics
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Transcript of Plate Tectonics
Plate Tectonics
• It’s a pretty zany idea
• What brought it about?
• Geologic observations made by Wegener and first suggested in 1915
• Why is it important?
Geographic EvidenceFossil Evidence
Types of Evidence for Continental Drift
Types of Evidence for Continental Drift
Evidence from Glacial Deposits Evidence from Coal Deposits
Magnetic Evidence
Patterns of reversal are nearly mirror image
The Earth’s magnetic field can be:
Normal: Compass points North
Reversed: Compass points South
The direction of the magnetic field is recorded by rocks as they form
AnimationFrom Smith and Pun
Sea Floor Spreading andSea Floor Magnetism
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Whole Ocean Magnetics
How Plates Behave (Bean pot analogy)
Smith and PunPlate Motions Through Time
The PlatesThe plates are essentially RIGID pieces of the Earth’s crust
They move due to convection of the mantle
They can contain both OCEANIC and CONTINENTAL crust
Passive Margin
Active Margin
(Plate Boundary)
Isostacy
D=kT Depth below waterlineH=(1-k)T Height above waterlinek= B/W Fraction of block below water line
H
D
T
Thick Pine Thin Pine Thin Oak
Where does continental crust come from?
Continental Crust
Three Basic Types of Plate Boundaries
• Divergent - plates moving away from one another (Tension)
• Convergent - plates moving toward one another (Compression)
• Transform - plates moving past one another (Shear)
! Most plates include con/divergent and transform components !
Smith and PunCorrelating Processes and Plate
Boundaries
Smith and Pun
Sea Floor Spreading and Sea Floor Magnetization
Motion at Divergent Boundaries
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Formation of Oceanic Crust
Pillow lavas
Sheeted dikes
Sediments
Gabbro
Peridotite (Mantle rock)
Life at Midocean Ridges
midocean RIDGES
HotterLess Dense
Higher
CoolerMore Dense
Lower
CoolerMore Dense
Lower
Smith and PunForming a Divergent Boundary
ANIMATION20 - Plate Divergence
Smith and Pun
Motion at Convergent Boundaries
Convergent Boundaries
• Oceanic crust is denser and “rides lower” than continental crust
• Expect different things to happen– Ocean-Continent Collisions – Continent Continent Collisions
TrenchesMountains
Animations
• Subduction– Animation 20- Ocean-Continent
– Animation 20- Continent-Continent
Transform Boundaries
• San Andreas Fault in California– Animation 20-Transform Boundaries
• Transform Faults in Midocean Ridges– Motion at a transform plate boundary
The Complex Nature of Plate
Boundaries
Hotspots
– Smith and Pun: Hotspots and Plumes
What drives the plates?
What drives the plates, really?