Rigid Earth Theory. Plasticity. Isostacy. Alfred Wegener and Continental Drift. Wegener's lines of evidence. Harry Hess and more evidence. Power source = convection currents in the mantle. Theory of Plate Tectonics. Plate boundaries: Divergent (spreading centers), Convergent (subduction zones), Lateral (transform faults). Three types of subduction zones. Hot spots. Accreted Terranes. Cratons. Continental Shields. Topography. (maps for lab)

1. Plate TectonicsPlate Tectonics
Chapter 9Chapter 9

2. 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.

5. 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?

6. 66

7. 77

8. 88

10. 1010
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

11. 1111
……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.

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

15. 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

16. 1616
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!

17. 17171717

18. 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

19. 1919
African Rift ValleyAfrican Rift Valley
1919

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

22. 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

23. 2323

24. 2424
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)

25. 2525
Continental-Oceanic SubductionContinental-Oceanic Subduction

26. 2626
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 (“island arcs”)
Deep ocean trench
Potential for tsunamis

27. 2727
Oceanic-Oceanic SubductionOceanic-Oceanic Subduction

28. 2828
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)

29. 2929
Continental-Continental SubductionContinental-Continental Subduction

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

34. 3434
““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

35. 3535

36. 3636

37. 3737
Accreted TerranesAccreted Terranes
A moving continent may pick up new land material as
lighter (felsic) material scrapes off of a subducting plate

40. 4040
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
4040
cratons

41. 4141
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.
4141

42. 4242
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