1. At some margins, the plate are diverging (moving apart)
-- Called rift zones on continental plates
-- Called midocean ridges on oceanic plates
New seafloor is being created at the
midocean ridges
This is the answer to Wegener’s dilemma..

2. The Mid-
Atlantic
Ridge
D. S. Kelley et al.,
Nature (2001)

3. A mid-ocean ridge vent (“black smoker”) On-axis vent (~400oC) Black material is FeS (iron sulfide)
Kump et al (2000)

4. Lost City ventfield Off-axis vent (~60oC) Carbonate chimney (8 m)
Aragonite and brucite
flange
Lost City
ventfield
SEM picture of ‘b’
Filamentous microbial
communities
Kelley et al. (2001)

5. How do we know that the seafloor is
spreading?
Answer: We look at the magnetic patterns
on the seafloor

6. Earth’s magnetic field -- Dipole field (N and S poles), like a bar magnet
Kump et al (2000)

7. Where else does this happen?
The magnetic field is not always oriented
in the same direction. Every once in awhile,
it reverses polarity, i.e. the north pole
becomes the south pole, and vice versa..
Where else does this happen?

8. On the Sun.. Picture of the Sun’s corona showing the influence of
magnetic field
lines

9. The Sunspot Cycle Sunspot number goes from low to high and back
again every 11
years
Solar magnetic
field reverses
polarity on this
same time cycle
Magnetic field is
created by the
solar dynamo

10. “Butterfly Diagram
From G. Albetti, The Sun, 2nd ed. (1969)

11. Earth’s magnetic field reversals occur
over longer time scales and are not as
regular
Earth’s field is generated by the
geomagnetic dynamo
Definition: dynamo—generation of a
magnetic field by motions within a
convecting, conducting fluid

12. Question: In what parts of the solid Earth
does convection occur?

13. What parts of the solid Earth convect?
The outer core
This is where the magnetic field is generated
The mantle
This is what drives plate tectonics

14. Geomagnetic Field Reversals Normal polarity Reversed polarity Brunhes
Matayama
Normal polarity
Reversed polarity
Gauss
Gilbert
5 Ma
(Epochs)

15. The geological record of magnetic reversals from lava flow on land
Kump et al (2000)

16. Sea-floor spreading as indicated by magnetic stripes
Kump et al (2000)

17. Internal structure of the Earth, the top 300 km
Kump et al (2000)

18. Three types of plate boundaries
(a) divergent
(discussed already)
(b) convergent (discussed next)
(c) transform fault (discussed at the end)
Does it make a difference what kind of plates are converging?
Kump et al (2000)

19. Three types of convergent plate boundaries
oceanic-continental:
- oceanic lithosphere is denser.
- subduction of a slab.
- deep sea trenches and earthquakes
(..Japan)
oceanic-oceanic:
important dehydration and
decarbonization reactions.
cycling of water and carbon (more
later)
continental-continental:
continental crust is too buoyant to
be subducted.
crust crumbles and forms
mountains
Kump et al (2000)

20. Transform faults
Danger:
Earthquake zone!
Kump et al (2000)

21. Final concept: The Wilson Cycle
Supercontinents (like Pangea) form and break
apart over about a m.y. cycle
Asia
Pangaea
(250 Ma) NA
Africa SA

22. Final concept: The Wilson Cycle
Supercontinents (like Pangea) form and break apart over about a m.y. cycle
Sequence of events:
Supercontinent forms from collisions of smaller continents
Heat builds up beneath the supercontinent, causing it to rift apart
Small pieces of the supercontinent drift off in all directions
Eventually they run into each other and form a new supercontinent