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Plate Tectonics
Living with a Dynamic Planet
Alfred Wegener:
a man before his time
• Meteorologist
• Research on arctic climates
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– Greenland
Evidence for past climates
preserved in rocks
A super-continent: Pangea
Pangea broke apart 200
million years ago
Continents drifted to their
present locations
Wegener’s Continental Drift
Evidence for Pangea: glaciation
Evidence for Pangea:
paleontology (fossils)
How Science Works:
Continental Drift Disputed
• Lack of Mechanism – what makes
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continents move?
Alternate explanations possible
Anti-German sentiments in the U.S.
What are the credentials of Wegener?
Continental Drift largely dismissed until
1960’s
Oceanography: a new science
and new evidence
• Following WW II – a convergence of
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questions, technology, manpower and
money
Defense needs – mapping the seafloor
Anti-submarine technology
GI Bill – a wave of scientists and engineers
Booming postwar economy and support for
science
A new view of the Seafloor
• Completely surprising!
• Mountains on the sea floor
• Trenches – near some continental edges, near
islands
More puzzles: Magnetic stripes near
the Mid-Ocean Ridges
A new hypothesis: Sea-Floor Spreading –
Making New Oceanic Lithosphere
Continental Drift Revised:
the New Global Tectonics
• Earth’s lithosphere is made of crust and
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uppermost mantle
Lithosphere is broken into plates
Plates float on moving asthenosphere (upper
mantle)
Plate motion driven by heat in mantle
Plates interact – especially at edges
Lithosphere and Asthenosphere
Plates
• Made of Continental and Oceanic Crust –
attached to uppermost part of mantle
• Plates move
• Plate interactions produce specific features (e.g.,
mountains, etc.) and processes (e.g.,
earthquakes)
• Motion is also related to heat dissipation and
density differentiation
Balancing the System
• Sea Floor (oceanic lithosphere) is created at
MOR (divergence)
• Either Earth is expanding (no evidence!) or
• Sea Floor must be destroyed somewhere –
where plates converge
Subduction – destroying sea floor
The sinking of cold, dense,
oceanic lithosphere into
the asthenoshere,
beneath an over-riding
plate.
• Associated: Trenches,
Volcanoes, Mountains,
Earthquakes
Convergence Brings Continents into
Collision – Forming Mountains
• Subduction of oceanic
lithosphere brings
landmasses together
• Collision of continents (or
continent/island arc)
• Due to low densities,
neither can subduct
• Crash!!
TRANSFORM BOUNDARIES
– Two Plates Passing
• Transform boundaries
link other types of plate
boundaries:
• Transform boundaries
allow a single plate edge
to have multiple types of
interactions w/other
plates
• Transforms are major
Faults – Earthquakes!
The Big Picture of Plate
Interactions
Plate Tectonics Explains:
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Formation of Oceans
Formation of Mid-ocean Ridges
Formation of Trenches
Formation of Mountains
• Formation of Metamorphic Rocks
• Formation of Sedimentary Rocks as mountains
erode
• Earthquakes: cause and distribution
• Volcanos: cause, types and distribution
• Formation of Igneous Rocks