Download Chapter 7_Part 2

• 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..
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The MidAtlantic
Ridge
D. S. Kelley et al.,
Nature (2001)
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A mid-ocean
ridge vent
(“black
smoker”)
On-axis vent
(~400oC)
Black material is
FeS (iron sulfide)
Kump et al (2000)
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Carbonate
chimney
(8 m)
Off-axis vent (~60oC)
Aragonite and brucite
flange
Lost City
ventfield
SEM picture of ‘b’
Filamentous microbial
communities
Kelley et al. (2001)
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How do we know that the seafloor is
spreading?
Answer: We look at the magnetic patterns
on the seafloor
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Earth’s
magnetic
field
-- Dipole field (N
and S poles),
like a bar
magnet
Kump et al (2000)
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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?
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On the Sun..
Picture of the
Sun’s corona
showing the
influence of
magnetic field
lines
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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
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“Butterfly Diagram
From G. Albetti, The Sun, 2nd ed. (1969)
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• 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
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Question: In what parts of the solid Earth
does convection occur?
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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
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Brunhes
Matayama
Geomagnetic
Field Reversals
Normal polarity
Gauss
Reversed polarity
Gilbert
5 Ma
(Epochs)
14
The geological record of magnetic
reversals from lava flow on land
Kump et al (2000)
15
Sea-floor spreading as
indicated by magnetic stripes
Kump et al (2000)
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Internal
structure
of the
Earth,
the top
300 km
Kump et al (2000)
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(a) divergent
(discussed already)
Three
types of
plate
boundaries
(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)
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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)
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Transform faults
Danger:
Earthquake zone!
Kump et al (2000)
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Final concept: The Wilson Cycle
• Supercontinents (like Pangea) form and break
apart over about a 400-500 m.y. cycle
Asia
NA
SA
Africa
Pangaea
(250 Ma)
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Final concept: The Wilson Cycle
• Supercontinents (like Pangea) form and
break apart over about a 400-500 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
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