Transcript Plate Tectonics

Plate Tectonics
Chapter 3 (p 61-82)
A New Understanding of Earth
• Earth has a geologically active surface
• How do internal layering and heat
contribute to mountain building, the
arrangement of continents, the nature of
the seafloors, and the wealth of seemingly
randomly distributed geological features
found everywhere?
• Patterns?
The Age Debate
• Know to be 4.6 billion years old
• Late 1700s most scientists believed that
earth was about 6,000 years old
– Based on interpretation of the Bible
• Uniformitarianism – earth processes
happening today are identical to those in
the past
– Going on for a very long time based on
current rates
The Age Debate
• Catastrophism – biblical catastrophic
events shaped the young earth
– Flood
– Explained mountain-top fossils
• Natural selection
– Time was needed for many species to exist
Age-of-earth arguments led to discoveries
during the 1800s  ancient earth
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A Puzzling Fit
• Fit of South Atlantic continents
– Evidence for single large landmass
– Fossil evidence
– Fit of continental shelves
• All by chance?
• No mechanism for continental
movement suggested
Continental Drift
• 1912 – Alfred Wegener
– Pangaea (pan = all, gaea =
earth)
• Single supercontinent
–Pieces broke apart 200
million years ago
–Still moving today
• Evidence
–Fit of shorelines
–Fossils of tropical plants
in Antarctica
Seismic Events
Seafloor Spreading
• 1960 - Mid-Atlantic ridge suggested as
origin of new seafloor (spreading center)
– Explains “fit” of continents
– Mechanism for movement – convection
currents in mantle
– Then ridges should be hot  they are
– New crust should become more dense over
time  it does
– Crust furthest from ridge should be oldest  it
is
Where does the old crust go?
• Subduction zones
– Crust plunges into the mantle in the Pacific
– Creates a balanced system
Plate Tectonics
• 1965 - John Tuzo Wilson suggests that
there are 12 plates that make up Earth’s
lithosphere (crust)
– These plates float on the asthenosphere
– Moved by hot mantle becoming less dense 
rising
• Lifts and cracks the crust = plate edges
• Avg. movement ~ 2 in per year
Plate Tectonics
• Plate movement caused by two forces:
– Plates form and slide off the raised ridges of
the spreading centers
– Plates are pulled downward into the mantle by
their cool, dense leading edges
• This theory explains many previously
unanswered questions
Major Plates
Plate Boundaries
Divergent Plate Boundary
• Line along which 2 plates are moving apart
– Mid-Atlantic Ridge
– Cooling magma creates new crust
Convergent Plate Boundary
• Areas of violent geologic activity where
plates are pushed together
– Oceanic crust is destroyed
Convergent Plate Boundary
• What happens when 2 convergent plates
are of equal density?
Transform Plate Boundary
• Plates moving
laterally past one
another
– Necessary since
Earth is a sphere
– Does not create or
destroy crust
– Creates
earthquakes
Transform Plate Boundary