1. 11
Plate TectonicsPlate Tectonics
Chapter 9Chapter 9

2. 22
Rigid Earth TheoryRigid Earth Theory
• It was once believed that Earth’s crust was
hard and brittle and could not bend
• Plasticity
–We now know that Earth’s crust can bend (like a
tough plastic) before breaking

3. 33
IsostacyIsostacy
• Maintaining equilibrium in the crust
• Addition or removal of crustal material causes a sinking or
rebounding of crust
–A glacier growing or remelting, crust eroding off the
surface, sediment deposits, water bodies on land, esp.
those created by dams, etc.

4. 44
Alfred Wegener andAlfred Wegener and
His Continental Drift TheoryHis Continental Drift Theory
• German meteorologist, 1920s
–Pangaea (Gk. “whole land”)
“The present continents were originally
connected as one enormous
landmass that has broken up and
drifted apart over the last few 100
million years. The drifting
continues….”
What evidence led him to this theory?

5. 55

6. 6666

7. 77
Wegener’s Lines of EvidenceWegener’s Lines of Evidence
• Similar geology (rocks and rock structures)…
• …petrology (rock chemistry),
• …matching glacial features (U-shaped valleys, glacial
deposits, etc.) on continents separated by oceans,
• …continent shapes that seem to fit together,
• …patterns in the locations of volcanoes,
• …paleontology (fossilized plants and animals)...
Ex.: S. America/Africa, Madagascar/India, Australia/Antarctica

8. 88
……but no one bought it.but no one bought it.
What do you mean,
“The continents are
floating???”
So why don’t we
see the crust
ripping apart
right now?
And hey, what’s the
power source driving
these movements of all
the land masses,
anyway???
The
crust is
too
rigid!
What a
knucklehead.

9. Then along came OceanographerThen along came Oceanographer
Harry Hess in the 1960s…Harry Hess in the 1960s…

12. The evidence continued to mount…The evidence continued to mount…
• Military seafloor mapping: Seafloor geology—structure,Military seafloor mapping: Seafloor geology—structure,
chemistry, and agechemistry, and age
– Oceanic crust: only 100 m.y.oOceanic crust: only 100 m.y.o
– Continental crust: 4.1 b.y.o.Continental crust: 4.1 b.y.o.
• Core samplingCore sampling
• Seafloor sedimentSeafloor sediment
• Rigid Earth folks retired—paradigm shift to plasticityRigid Earth folks retired—paradigm shift to plasticity
• Geologists, geophysicists, seismologists,Geologists, geophysicists, seismologists,
oceanographers, physicists, and paleontologists alloceanographers, physicists, and paleontologists all
agree the theory fits the evidence gathered within theiragree the theory fits the evidence gathered within their
respective fieldsrespective fields
• Convection currents as mechanism/power sourceConvection currents as mechanism/power source

13. 1313
Convection CurrentsConvection Currents
• Slow-moving
convection currents
within the mantle
transfer heat from
the outer core to the
upper mantle
• Finally, there was a mechanism and a
process for moving all those pieces of crust!

14. 14141414

15. 1515
The Theory of Plate TectonicsThe Theory of Plate Tectonics
Tectonic (crustal) plates
• Pulling apart (diverging)
= Spreading centers
• Slamming together, sinking (converging)
= Subduction zones
• Sliding laterally (sideways)
= Transform fault boundaries

16. 1616
African Rift ValleyAfrican Rift Valley
1616

17. 1717
Divergent Plate BoundariesDivergent Plate Boundaries
• Spreading centers
–Crust pulling apart, magma rising to the surface

18. 1818
Convergent Plate BoundariesConvergent Plate Boundaries
• Subduction zones
–Crust being forced together
–Lightest material rises (mountain-building) while the
heaviest stuff sinks (pushed back into the mantle)
–Remelting (mostly from friction) creates volcanoes
–Intense, deep-focus earthquakes

19. 1919

20. 2020
Three Types of Subduction ZonesThree Types of Subduction Zones
1. Continental crust meets oceanic crust
–Oceanic crust sinks
–Big trench offshore
–Volcanoes on the continental margin
–Big earthquakes (potential for tsunamis)

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Continental-Oceanic SubductionContinental-Oceanic Subduction

22. 2222
Three Types of Subduction ZonesThree Types of Subduction Zones
2. Oceanic crust meets oceanic crust
–The older and colder crust will probably sink
–Big earthquakes and volcanic islands (called
“island arcs”)
–Deep ocean trench
–Potential for tsunamis

23. 2323
Oceanic-Oceanic SubductionOceanic-Oceanic Subduction

24. 2424
Three Types of Subduction ZonesThree Types of Subduction Zones
3. Continental crust meets continental crust
–Too light to subduct
–Mountain-building
–Big earthquakes
–Little if any volcanism (mostly intrusive)

25. 2525
Continental-Continental SubductionContinental-Continental Subduction

26. 2626
Transform Fault BoundariesTransform Fault Boundaries
• Tectonic plates slide past one another
–Earthquakes are less intense than subduction
–No volcanoes
–Little or no mountain-building

27. 2727
““Hot spots”Hot spots”
• Also called magma plumes
• Generally occur some distance from any other
type of plate boundary
• Unrelated to convergent, divergent, or
transform boundaries
• Anomalous (odd) “balloons” of rising magma
–Hot spot stays in one position as the moving, island-
covered crustal plate rides away from it

28. 2828

29. 2929

30. 3030
Accreted TerranesAccreted Terranes
• A moving continent may
pick up new land material
as lighter (felsic) material
scrapes off of a subducting
plate

31. 3131
Accreted TerranesAccreted Terranes
• A moving continent may
pick up new land material
as lighter (felsic) material
scrapes off of a subducting
plate

32. 3232
CratonCraton
• These terranes were added to the original
material first formed from magma that rose out
of Earth’s earliest crust
–Craton--the name given to these ancient proto-
continents
3232
cratons

33. 3333
Continental ShieldsContinental Shields
• More magma material was added to the
cratons, forming continents.
–Continental shields: Where the earliest
continental material still exists intact and is
exposed at the surface.
3333

34. 3434
TopographyTopography
• Right from the very beginning, the crust was
affected by stresses and strains that caused
crustal deformations
• Over time, the crust has continued to be
folded, faulted, broken, eroded and further
built upon, creating the topography, the ups
and downs of land relief, that we see today
3434