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Natural Science II – ERTH 1040
Plate Tectonics
J. D. Price
Mid ocean
Ridge
Again – our understanding
of the earth falls off with
depth.
In the 1920’s, Alfred
Wegner cited
evidence that the
continents were joined
(as recently 60 Ma).
The coastlines of
South America and
Africa match. Rocks
and fossils show
similar environments
across several
continents.
© Wiley, 2002
19-385
Bathymetry: mapping the topography of the sea
floor. Initial done with ship-board sonar soundings,
now complimented by satellite measurements.
Q: What prompted mapping of the ocean floors?
The Mid-Atlantic Ridge
The Mid-Atlantic Ridge
Roughly in the center
of the Atlantic ocean,
there is a topographic
rise. This is an area
marked by volcanism
(mostly subaqueous,
with exceptions like
Iceland)
Figure 19.22
19-384
Radiometric dating reveals the youngest crust (red)
is at the ridges. The oldest crust (blue) is only 160
Ma and is furthest from the ridges.
While observing bathymetric
measurement, scientists
found that the rocks on the
seafloor recorded a series of
reversals of the earth’s
magnetic field. These roughly
parallel ocean ridges.
U.S.G.S.
The earth's
magnetic field
The movement of the core
and interaction with the
mantle is thought to make the
planet behave like a giant bar
magnet.
It protects the
surface from the
solar wind
Atoms produce magnetic fields
But atoms are randomly oriented – so fields cancel
each other. In some materials, the atoms form
domains, which may align in a field.
Basalt
A rock containing
abundant Fe, Mg, Ca, Si, O
Univ. North Carolina, Atlas of rocks,
minerals, and textures website
Magnetite (Fe3O4)
A magnetic mineral
abundant in basalt.
http://webmineral.com/specimens/Magnetite.jpg
Q: How is the Earth’s magnetic
field recorded in rocks?
Magnetic field recorded
The earth’s field flips
orientation irregularly
(10 K years to 1 M
years). Magnetic
north moves south.
Flips may take 1 K
years in some cases.
Magnetite grains in
lavas preserve the
earth’s field at the
time they crystallize
and cool.
Figure 19.21
19-383
Reflection of seafloor spreading
Orientation
Figure 19.23
N
S
N
19-385
Q: Why is the magnetism recorded
on both sides of the ridge?
S
The major plates of the
lithosphere
Source:After W. Hamilton, U.S. Geological Survey
Q: What are the motions of the plates?
Direction and magnitude of motion
for the plates
Arrows indicate direction and relative
magnitude of each plate’s motion
If the lithosphere (crust & uppermost mantle) is in
movement, but all of the fragments are moving in
different directions with different accelerations
then
•Some pieces are moving apart from each other
Divergence - extension
•Some pieces are moving into each other
Convergence - compression
•Some pieces are moving obliquely to each other
Transform - strike slip
Divergent Boundary Oceanic Rift
Q: What is a probable cause of
spreading?
Divergent Boundary Continental Rift
http://jsc.nasa.gov
STS-32
Q: What will happen to East
Africa with time?
Example: the East African Rift
Hot Spot
Pacific plate
motion
Example: Hawaiian chain
Hawai’i
Transform margin
Example: San Andreas Fault
© Cornell Geology
Continental collision
Example: the Himalaya
Q: Why doesn’t one of the
plates subduct?
Subduction: Continental Arcs
Example: the Cascades
Q: What is the deepest part
of the oceans?
Important point - the subducted slab doesn’t melt to produce
the volcanism seen in the arc. It simply doesn’t get warm
enough.
Melting under subduction volcanoes is most likely related to
fluid release deep in the mantle. Fluids move through
networks to flux mantle, promotes melting at 1200ºC.
Subduction: Island Arcs
Example: Indonesia
Gunung Semaru and
Bromo Caldera, Java
Q: What causes mantle melting
in Island Arcs?
If the materials are able
to move, convection
may occur
Note: this works for most
materials for most relative
heats
Cold lithosphere eventually sinks under its own
weight. The pull drags plate, extends spreading
center. A deeper mechanism is probably the ultimate
source of motion - convection.
Plate tectonics results from heat transfer
The brittle lithosphere plates of the Earth “floats”
on the hotter and more ductile asthenosphere
Plates are constructed by volcanism at divergent
margins
Plates slide past each other along transform
margins
Oceanic plates cool, becoming heavier, and sink
at convergent margins.
Important Question: Are continents new or old?
Mid ocean
Ridge
The upper mantle has a
“depleted” composition. Crust +
depleted mantle is roughly that of
the lower mantle and chondrites
The lanthanides (La-Lu) are also known as the rare
earth elements.
A
G
E
S
Shield
Platform
Orogen
Extended
Basin
oldest rocks, 2.5 - 3.9 Ga
oldest sediments, 2.5-0.6 Ga
compressed,< 0.6 Ga
<0.6 Ga
<0.2 Ga